But it is the "volatility" of the gasoline that allows the space to fill with vapor that replaces/displaces the liquid. Yes, the pressure in a "closed" system is equal at all points. And, the vaporizing or evaporating process that takes place when the temperature of a volatile liquid is raised, occurs uniformly from one end of the fuel hose to the other end, including the filter AND the tank. When vapors (or "evaporates") are driven out of the liquid, they will rise to the highest point(s) available. You can't see the vapor pocket that is at the highest point in the fuel line near the carb or fuel pump (unless you have clear fuel hose) but it's there. You can't detect the vapors that form and make their way back into the tank, but they do. You might detect some of the vapors that escape through the tank vent (by smell). The vapors that occur in the fuel filter cannot escape the filter (unless the inlet and outlet of filter are arranged in the vertical) and thus will collect as a "pressure head" above the liquid. This pressure head will at that point exibit a property that all gases have (known as "specific pressure"). The specific pressure of a gas (or vapor) increases with the temperature of the gas (or vapor). The fuel line scenario does not represent an actual "closed system" because a vent to atmosphere is included as part of the design. In this case, the vapor pocket above the liquid in the fuel filter can develope a head of the specific pressure of gasoline vapor at a given temperature above the vaporization point for gasoline. If the specific pressure that developes reaches a pressure high enough to overcome the weight of the fuel between the filter and the tank, this pressure will push the fuel back into the tank (the carb needle valve is resistant to flow or movement toward the carb). At these temperatures, it is possible for the gasoline to continue evaporating in the line and filter until the filter is near empty in appearance (if the outlet of filter is higher than the inlet). Given enough time at high enough temps, the entire contents of the fuel tank would evaoporate. Because there is a vent in the tank, a pressure head of vapor will not take place there (if the vent is open). If the tank was unvented (truly closed system) a pressure head would develope there too and the pressure in the entire tank, fuel hoses, and filter would be equal. In this scenario, the fuel would be pressurized above atmospheric pressure and would attempt to flow in the direction of least resistance (as is always the case with fluids under pressure). If the pressure reached a level greater than what the carburetor needle valve could hold back or resist, the fuel would FLOW into the carburetor and on into the engine as long as the high temps and pressures persisted. The fuel pump (if present) would offer no resistance to flow as the inlet and outlet check valves (or "flaps") are oriented so the fuel would flow right on through). If an inline cutoff valve is present in the fuel hose and is in the off (closed) position, a true closed system is created, but only the cutoff valve, fuel hoses, filter and fuel pump (if equipped) are in this closed system. If your machine has a cutoff valve, and the valve is closed in very hot weather, the pressure head of vapor that developes in this closed system could very well reach a level that could unseat the float needle valve and allow some fuel to enter the carb. Realistically speaking, it is unlikely to unseat. In reality, what happens is that as the ambient temperature increases, the pressure in that section of the fuel circuit increases, but the needle valve, cutoff valve, hoses and hose clamps are able to contain the pressure with no leakage. When the ambient temperature decreases, the pressure in that section of the fuel circuit decreases and any fuel vapor that formed due to the higher temperatures, will now "condense" and become liquid fuel again.

Mownie, I need to think about this more.

It seems to me that the space in which the vapor forms initially is created because not enough pressure is present to force the fuel in the fuel filter higher than the fuel line. When fuel is first added to the system, the "pocket" is immediately filled with fuel vapor because the liquid fuel is too heavy to fill it at the current pressure. In other words, the fuel vapor doesn't create the "pocket" it simply occupies it. However, I agree that once fuel vapor is present in the "pocket" the vapor will expand and contract with changes in temperature. Even so, I believe that the fuel filter could be "bled", similar to a brake line, if higher pressure was applied.

Like I said, let me think about it more. In the meantime, maybe someone else can provide an additional perspective.

-Deerslayer

I have the same trouble with a beer glass ... it seems to always be empty! But, yet, I can feel the effects of it. .. "fluid dynamics".

I have observed this phenomenon for decades now. First saw it on gasoline fueled trucks. Have seen it on all types of vehicles. I and others have attempted to eliminate this vapor pocket by "bleeding" or "purging" but given a bit of time and high ambient temperatures, the vapor pocket always returns. In fuel systems where the filter is situated between the fuel pump and the carburetor, the vapor pocket will not grow to be as large because the fuel pump with it's check valves blocks flow in that direction and the carburetor needle valve blocks flow in that direction, thus allowing the pressure to be contained or held. With the pressure contained in that section of the fuel system, evaporation (which creates the vapor pocket) slows or halts completely. As the ambient temperature decreases, the vapors will condense back to liquid and the pressure in that section of the fuel system will diminish or disappear. If the filter is located between the suction side of the fuel pump and the tank, the vapor pocket will grow larger and when the specific pressure of the vapor pocket exceeds the pressure exerted by the weight of the fuel between the filter and the fuel tank, fuel will be displaced back toward the tank. If the evaporation continues for a long enough time, fuel will continue to be displaced back toward tank until the vapor reaches a point in the fuel hose where the route of the hose takes an upward turn to meet the tank. At this point the vapors will lose the ability to push the fuel further because the vapors themselves will "bubble" up into the tank and rise to the surface of the fuel where they "burst" and become part of the vapors or fumes in the tank. At high ambient temperatures there is a steady "plume" of gasoline vapor emanating from the vent of the gasoline tank due to the "dynamics of evaporates" from a highly volatile liquid. To gain a clearer idea of evaporation, condensation, head pressure and specific pressure of gases, a person can study the principles of refrigeration. That field is based almost entirely on the behavior and properties of gases, vapors, and liquids at various pressures and temperatures in CLOSED LOOP systems. While it may be difficult to draw a parallel between the "partially empty" (or is it partially full?) fuel filter and your home air conditioner, the similarities are there.

Mownie, you appear to know what you are talking about and what you say makes sense. I was trying to abstractly solve the problem. You, on the other hand, have decades of actual experience. Decades of actual experience almost always wins.

Thanks for saving me some time. I no longer need to think about it.

-Deerlsayer

Mownie -- I have looked into this subject further (I don't want to say researched) and find you are absolutely correct. You have also provided the answer I have been looking for, for a number of years. Thank you for your explanation.

Dang-it, Mownie! I can't stop thinking about this! I agree with everything you said except how the vapor pocket initially forms. You state:

"I and others have attempted to eliminate this vapor pocket by "bleeding" or "purging" but given a bit of time and high ambient temperatures, the vapor pocket always returns."

I believe the pocket returns because:
1. The system isn't closed as you pointed out.
2. The weight of the fuel in the fuel filter above the fuel line forces fuel out of either or both ends of the fuel line.

The resulting vacuum in the filter is then immediately filled by fuel vapors. Once the fuel vapors are in the pocket, changes in temperature will expand and contract the space occupied by the vapors like you stated. Other than the cause of the initial pocket formation, I agree with everything you said. Also, if the above is true, it is consistent with my initial explanation.

BTW, thanks for confirming that the vapor pocket can be purged. I had no first-hand knowledge that it was possible.

I do understand the principles of refrigeration which are based on Boyle's law (pv=nrt). However, I have no experience with the finer points of fuel systems like yourself.

-Deerslayer

Bill, can you post a link to the information you viewed?

-Deerslayer

All this explination is great, but it doesn't solve my problem of running out of gas while the tank is full. Tank is vented, Carb has been off and cleaned twice, fuel lines and filter changed. This is a gravity feed system, no fuel pump. After awhile the clear filter is empty and the mower coughs and stalls.

TPaul, there are two types of tractor fuel filters. One is for gravity feed systems the other is for fuel pump systems. Most (if not all) clear filters are used with a fuel pump. They are more restrictive. The gravity feed fuel filters that I have seen are red/orange.

-Deerslayer

Deerslayer: I read extensively in "Marks' Standard Handbook for Mechanical Engineers," a McGraw-Hill published book. I was most interested in the chapter/section on vapors.

I am now confident that Mownie is entirely correct. My bumper sticker version of this phenometer is: when the vapor pressure of the gasoline within the filter exceeds the head pressure of the gasoline from the gas tank, some of the gasoline within the filter will be displaced or pushed back into the fuel tank via the fuel line. Let us remember to see if the filter has more gasoline in this winter.

Bill

Mownie, I have a few observations, theories, and experiments that I would like to run by you. First the observations.

Observation 1
GTs and LTs usually use a mechanical fuel pump that is located on the engine. The fuel pump pulls fuel through the fuel system causing negative pressure in the system.

Observation 2
Most modern cars and trucks use an electric fuel pump located in the gas tank. The fuel pump pushes fuel through the fuel system causing positive pressure in the system.

Observation 3
When you fill a tractor or a car/truck with fuel for the first time, a "pocket" in the fuel filter forms immediately.

Theory 1
Since the "pocket" forms immediately when the fuel system is filled, the pocket is not caused by fuel evaporation.

Theory 2
(Tractor) If you fill a fuel filter will fuel, insert it in the fuel line then start the engine, the fuel will be quickly consumed. The "pocket" that forms in the fuel filter is not caused by fuel evaporation because it occurs quickly. Since a tractor fuel system has negative pressure, the fuel that is higher than the fuel line is consumed before any fuel is drawn from the tank.

Theory 3
(Auto/truck) If you fill a fuel filter will fuel, insert it in the fuel line then start the engine, the fuel maintains its level in the fuel filter and a "pocket" does not immediately form. Since an auto/truck fuel system has positive pressure, the pressure in the fuel system supports the weight of the fuel contained in the upper half of the fuel filter.

Over time, fuel in the fuel filter will evaporate and cause fuel vapor to accumulate in the top of the fuel filter. However, this does not occur immediately. It may take days for the pocket to form. The higher the ambient temperature, the quicker the pocket forms.

Experiments
I initially planned to prove Theory 2 by actually trying it on my GT. However, before writing this post, I reread the thread and noticed that Bill had already experimented with a full fuel filter and he said that it was quickly consumed when he started the engine. I believe that Theory 2 has been proven. However, you may disagree with the conclusion.

In a previous post you indicated that you have proven Theory 3 and came to the same conclusion.

I think a number of us have witnessed the first half of Theory 1 (Observation 3). However, Theory 1 contains a major conclusion that you may not agree with.

Mownie, what do you think?

-Deerslayer

Bill, thanks for the source. It appears that I began writing my last post shortly before your last post was posted.

This is clearly an interesting problem.

Please reread my last post. I'm interested in your input also.

-Deerslayer

Regardless of "why" the fuel filter looks almost empty, is it correct that only a fraction of the filter medium is being used? Seems to me that the majority of the filter never actually filters anything.

Focus on the fact that ALL liquids can be forced to change to a vapor if enough heat is applied to the liquid. The pressure being exerted on the liquid (be it atmospheric or artificial via "pressure vessel") has a direct effect as to the amount of heat necessary to "boil" the liquid. It is generally accepted that water will "boil" at 212 degrees F. at a barometric pressure of 14.7 PSI. If you build a camp fire to make a pot of coffee on the beach near one of the great oceans, that water will begin "boiling" at 212 degrees F. If you travel to the mountains and climb to 7,000 feet above sea level and make a pot of coffee, the water will begin to "boil" at about 198 degrees F. "Boiling" is the process of runaway evaporation due to raising the temperature (adding heat) of a liquid to the point that molecules of the liquid are able to exert enough pressure to to expand to the vapor state spontaneously in spite of being in contact with other molecules of the same sustance which are still in the liquid state. Evaporation of a liquid begins at a much lower temperature than "boiling", so don't think they are one and the same. You can increase the evaporation rate of a liquid by raising the temperature of the liquid, or by reducing the pressure being exerted on the liquid. If you place a quantity of water in a sealed vessel (of which you can regulate the "head pressure") you can raise the "boiling point" of water to a temperature higher than 212 degrees F. On the other hand, if you attach a vacuum pump to the sealed vessel and begin removing (evacuating) the contents of the head space (air and water vapor initially) you will increase the evaporation rate of the water. If your vacuum pump has sufficient capacity to out run the evaporation rate, you can incite the water to "boil" at normal room temperatures. How does all this relate to the liquid vs. vapor ratio of a fuel filter? If you were to put water in a pressure vessel and purge or bleed all the air from the head space so that the vessel held only liquid water, you could add heat to that vessel and raise the temperature of the water and never observe the formation of a vapor pocket because as the temperature of the liquid increases, so does the pressure being exerted on the liquid which decreases (or halts) the evaporation rate (raises the boiling point). Because the vessel holds the water molecules under pressure (hydrostatic pressure at this time) the molecules can never (in theory) reach the energy level at which they can expand and change to the vapor state. Instead of being able to change to the vapor state, they are forced to continue to absorb more heat which in turn increases the pressure of the vessel which raises the boiling point ad infinatum (in theory). Now assume that this same pressure vessel were given a means of venting or shedding pressure from the BOTTOM and into a separate vessel or tank that is open to atmosphere. Assume that the level of water in the open tank is greater than the highest elevation of the sealed vessel.We begin with the sealed vessel COMPLETELY filled with liquid water and no vapor pocket above the liquid. Assume that the vent provision in the bottom of the vessel has a valve which is closed. Assume that the open vessel was filled to it's current level AFTER the vent valve was closed. Now we will open the valve and the 2 vessels become joined via the connecting hose. The first change to take place will be that the hydrostatic pressure of the sealed vessel will increase due to the weight of the water in the open vessel exerting it's force on the water in the sealed vessel. Any increase of barometric pressure on the open vessel will result in an increase of hydrostatic pressure in the sealed vessel as well. If we leave this apparatus for a while at room temperature or higher for a while, we will begin to see a small head of water vapor form at the highest point in the sealed vessel. As the vapor pocket forms, it will exert the specific pressure for water vapor at that temperature and displace an identical volume of liquid back into the open vessel. As water is displaced back into the open vessel, the elevation of the water and the volume of water in the open vessel increases and thus the weight of the water in the open vessel. As the weight of water in the open vessel becomes greater, it is able to exert greater pressure on the sealed vessel and at some point a state of equilibrium will be reached balancing the tendency for more water to evaporate against the tendency for the water vapor to condense back to liquid. From this point we can add more heat the the water in the sealed vessel which will increase evaporation and cause the vapor pocket to grow larger, exerting more specific pressure against the water and thus displacing more water back to the open container. If, instead, we cool the sealed vessel (extract heat), we would observe that the water vapor that comprises the pocket will lose some energy (heat) and that will cause some of the vapor to condense back into liquid water. This will result in water moving from the open vessel back into the sealed vessel. Cool the vessel enough and you will see it return to the condition of being completely full of liquid water. Cool it to a temperature higher than that and you will see some of the vapor pocket remaining in the top of the vessel. You could add enough heat to the sealed vessel to cause the water in it to boil and you would reach a point where the water vapor in the sealed vessel would exert enough specific pressure to completely displace all the water in the sealed vessel back into the open vessel. At this point, if you stopped ADDING heat but maintained the heat at that temperature, you would have reached a state of equilibrium where not water could flow from the open vessel back to the sealed vessel, but neither would any molecules of water vapor leave the sealed vessel to move to the open vessel. If you now begin extracting heat from the sealed vessel, liquid water will flow toward the unsealed vessel. Continue extracting enough heat and once again you fill see the sealed vessel completely filled with liquid water and no vapor pocket. The volume of water in the open container will be less that what you started with at the beginning of the process because while you were boiling the water, the vapors being displaced into the open vessel had enough energy to allow them to evaporate completly into the atmosphere. Now to apply this to a clear fuel filter in a gasoline powered vehicle you just substitute the sealed vessel for the fuel hoses and filter, substitute the open vessel for the gasoline tank, and substitute water for gasoline. The reason that the vapor pocket persists in the fuel filter is because gasoline is a blend of many chemicals, some of which are volatile enough to remain in the vapor state to very low temperatures. Because gasoline is always evaporating at normal ambient temperatures, there will always be vapors emanating from the liquid. If you provide a vessel to trap these vapors (head space in a fuel filter), you will observe the formation of a vapor pocket even at some extremely low (by human standards). If the temperature is cold enough, evaporation will halt (or reverse) and no vapor pocket will form. At temperatures we consider to be "normal" especially in the hotter months, we will always see some vapor pocket in a clear gasoline filter unless it is oriented to the vertical to allow the vapors to escape the filter (but they will still form a vapor pocket somewhere if a head space exist for the pocket to form in). And once this pocket forms, it will persist to some degree because at "normal" ambient temperatures, the vapors will not condense back into liquid.

The vapor pocket forms almost immediately due to evaporation. The evaporation process does not begin at the moment the fuel enters the fuel line or the moment the fuel is dispensed into the fuel tank. It does not begin when the gasoline tanker truck fills the storage tanks at the local gas station. The evaporation process begins as soon as the refining process ends and the products of refining head off the market in unsealed vessels. The most volatile components are the first one to evaporate out of the blend, simply because they are more......volatile....and evaporate more readily at lower temperatures. It is these volatiles that make gasoline eager to burn if oxygen is available (after ignition of course). The initial heat produced by these volatiles in turn encourage the lesser volatile components of the gasoline charge to combust and so on until even the most reluctant to oxidize components of the gasoline have been combusted (or leave unburned in the exhaust). Though the evaporation of the gasoline volatiles is always occuring, we tend not to recognize it because the process results in an invisible product (vapor). We "see" these vapors in the head space because they displace the liquid underneath them. The evaporation of the volatile components in gasoline is what renders gasoline useless if allowed to age in an unpressurized vessel. Some degradation occurs due to certain of the chemical additives breaking down and changing but the bulk of it is because the most volatile components simply evaporate away. Without these "most flammable" components, the combustion process is harder to, or impossible to initiate. That's why you might not get your engine started if you have "old gas" in the system. My apologies to all, especially to tpaul, for being the long winded, excessively wordy person that I am. I won't add anything else to this thread as I have exerted all the pressure I can muster. If all this causes people to ponder things.....great. If it put you to sleep, well.......you probably needed the rest ! Glad to oblige.

Mownie, I agree with all that. You can find much of that information in any good encyclopedia. Let's cut through the techno-speak.

It takes time for fuel to evaporate. It is not immediate. In fact, I believe that there are formulas that will predict evaporation time given the appropriate variables.

Please comment directly on my theories and indicate what you think is either correct or incorrect for Theories 1 & 2. If you want to revisit Theory 3, that's okay too but I think we already agree on that one. Also, please limit your comments to no more than several sentences for each theory.

Everyone, I would like to make a correction to an earlier post. I stated that Boyle's Law is pV = nRT. That is incorrect. Boyle's Law is pV = k. The Ideal Gas Law expands k to nRT resulting in pV = nRT. Where
p is the absolute pressure [Pa],
V is the volume [m3] of the vessel containing n, moles of gas,
n is the amount of substance of gas [mol],
R is the gas constant [8.314 472 m3Pa·Kâ1·molâ1],
T is the temperature in kelvin [K].

I learned those equations over 30 years ago and don't remember actually using them since college so please cut me some slack. 8^)

-Deerslayer

Mownie, I just want to clarify that I didn't see your Theory 1 post before I posted my last post. In case it happens again, the last post that I currently see was authored by me.

I agree that evaporation is always occurring. Maybe I oversimplified my statements for the sake of the readers. I should have said that the head space will not be immediately created through evaporation. I agree that, as stated in Theory 3, evaporation will eventually create fuel vapor that results in head space. You indicated yourself that this process can take days.

Too bad you are checking out on this problem. I enjoyed the conversation.

Does anyone want to pick up where Monie left off?

-Deerslayer

Interesting theories but, in my opinion, you're looking WAY too deep.

The reason the clear filters appear to be "empty" is that the outlet pickup tube is down towards the bottom of the filter, if you stood it up. If you lay it on its side, it will only fill with fuel until the pickup tube is covered and then it's sucking/pushing fuel. As the filter gets dirty, the level of fuel will rise towards the top because the bottom part of the filter media is dirty and has a restricted flow.

Cut one open and you'll see.

Machiem, your first comment is consistent with Theories 1 and 2. We're trying to explain why the fuel doesn't go higher than the pickup tube.

Your point on restricted flow is interesting. I need to think about that.

Thanks for posting.

-Deerslayer

It will only suck/push "air" until it starts getting liquid. Once it starts getting liquid, the liquid blocks off the remaining "air" from being sucked/pushed out. Kind of like putting an empty glass upside down under water. The "air" is trapped until it is displaced by liquid. The bottom of the pickup tube represents the bottom of the glass when underwater.

Here's a picture of what I'm talking about. As the filter gets dirty, the fuel will go further up the filter and the "air" will start to dissappear.

{{gwi:318620}}

wheelhorse_of_course's first answer is correct but i would like to elaborate on this.

The pocket of air in the filter is air that is saturated with gas vapor. Look at the weather report where they say humidity is 100% is that air at X temp can hold Y amount of water vapor. Warmer air = capable of more humidity. Same is happening inside the gas filter.

As for the air pocket itself, a common liquid bonds together and with the wall of the container holding it. Just like a shorter bridge can support more weight, the smaller the opening at the filter the more pressure it would take to get the gas to separate to allow the air to pass. If the fuel line was 1/2" diamiter, there would be no air pocket and air would exit replaced with gas.

Take a McD's straw Vs. a coffee straw. submerge each straw in soda then plug the end with your thumb and pull the straw up out of the drink. The coffee straw is small enough to hold the soda because the small area of adhesion between the soda and the wall of the coffee straw is strong enough to hold up the soda. The McD's will give way - the soda will move to the side swap space with the air and drip back into the cup because the soda pushing down is enough to overcome the bond.

Depending on the plastic but i think the gas is bonding with the clear filter wall and flowing. With fuel sticking to the wall and spread out it would appear as if there is no gas in the filter. Now if the filter material is too thick thus the above adhesion kicks in making the gas not flow through the filter unless u have more pressure to push the fuel through the filter.

I know I said "Done", but durn it!!!!!!If anyone wants to test the theory of how the vapor space forms from evaporation, try this. Attach two 1 foot lengths of fuel hose to a clear fuel filter (a used filter will be fine as the dirt and sediment plays no role in this). Plug the end of one hose temorarily. While holding the open hose vertically, fill it with gasoline until both hoses and the filter are full of gasoline. Move the filter until there is not a trace of air visible anywhere in the filter. Lift the end of the plugged hose up until the filter has assumed a horizontal orientation (inlet and outlet are horizontal. Now, secure the hoses so the ends are higher than the filter and remove the plug from that hose. Wait a while and you will see a vapor head form in the top of the filter. The greater the ambient temperature, the quicker the vapor head will develope. This will take place without any sucking or pushing of the fuel and with or without dirt in the filter.

Mownie, if I performed your experiment with a low volatility liquid, a vapor head should not immediately appear, correct?

-Deerslayer

deerslayer: True, but with a caveat. The "low volatility" liquid can't be carbonated or otherwise "effervescent". Liquids that fit that definition have gases dissolved or entrained in them that can come out of solution and form a vapor head. That is not the same as a vapor forming from the same liquid due to a change of state. Let me cite an example from my own experience that I did not think of earlier. I have a "water filled level finder" device. This device has a battery powered sensor unit with 30' of clear plastic tubing connected to the sensor unit. The thing works by filling the plastic tubing with water to a certain level. Then you secure the sensor box in place at the elevation you want to duplicate at another point (roughly within 29' of the sensor unit). Then you take the other end of the tubing (it's open to atmosphere, as is the sensor unit). When the water level in the end you are holding in your hand (the remote end) matches the level in the sensor unit, an audible tone is sounded by the sensor unit. I've had it for years. This thing is stored for long periods by simply coiling up all the tubing ang hanging the whole shebang on a nail so that the sensor is higher than the tubing. Now this tubing is completely full of water with no vapor space showing at the top of the 15 or so coils of tubing formed when I roll up the tubing for storage. Every time I get the thing down again after several months hanging up.......there is a vapor space in each of the coils with water resting in the lower 3/4 of each loop or coil. I consider this to be a good example of how an evaporating liquid can form a vapor head with no influence other than temperatures great enough to allow evaporation (even though much slower than grass can grow).

Mownie, that's great information. It sounds like I can use plain water. I was planning to use antifreeze because it has the lowest volatility of easily obtainable liquids that came to mind.

-Deerslayer

Of course you all realize that John Deere's fuel filters are much better than Sears' fuel filters...

How about a metal filter so you cant see it, and there for, "out of sight, out of mind" ??????? joking of course

johninmd, though you are joking.........you are absolutely right about "out of sight-out of mind". How many times have we seen posted something like "my clear fuel filter seems to be only half full"? I've never seen a thread that states "there seems to be a pocket of air in my black rubber fuel hose where it hooks to the carb." Because THAT pocket of vapor can't be seen, nobody ever mentions it.... but it is there (if that point is higher than the hose leading to it) If all fuel systems had clear hose instead of opaque hose, these "empty" pockets would be seen in more places than just the filter. And......if people try (in vain) to eliminate them....they could end up with some other "empty pockets".

Mownie, before I purchase the materials and run the experiments, I thought I would try to explain my theory one more time.

I believe that if I use a low volatility liquid such as water in your experiment, it will behave the same way as gasoline. If it does, it will disprove your fuel vapor theory.

Imagine your experiment with the plugged hose and the fuel filter in the horizontal position. The other hose is vertical. Clearly, when you unplug the horizontal hose, the liquid in the filter above the hose will drop to the level of the horizontal hose (same level as filter output tube). It doesn't matter if the liquid is water or gasoline. BTW, the vertical hose will drain also but liquid will remain in the bottom half of the fuel filter because it is lower than the filter output tube.

The farther you raise the plugged hose above horizontal before unplugging, the smaller the fuel filter pocket that forms. If the hoses were long enough to support a column of liquid that was equal to the weight of the liquid in the top half of the fuel filter, a pocket would not form.

The above is true regardless of the liquid used and was what I tried to explain in my initial post and thereafter. Think about it. I bet you'll agree.

The above does not disprove the fact that fuel can evaporate in the fuel system and cause vapor pockets. Vapor pockets caused by fuel evaporation indeed occur. It simply explains why there is a pocket in fuel filters which was the original question.

-Deerslayer

deerslayer, I agree with the scenario in your last post, but I thought I said to raise BOTH hose ends to the highest level possible while orienting the filter so that the inlet & outlet are horizontal. Then remove the plug from the one hose. Now, both hoses are open and higher than the filter. Since you intend to use water, you could just hold a thumb over the end of one hose until you get the apparatus filled and purged of all air. If you want to use something between water and gasoline (on the VOC scale) you can use rubbing alcohol. As to WHY there is a vapor pocket in a fuel filter: It's there because we don't intentionally orient the filter with the inlet and outlet in the vertical order. Most all filters end up lying on the horizontal (or something close) because that is how the fuel hose they connect to situates. Point A fuel tank and Point B carb or fuel pump are usually some HORIZONTAL distance apart, so the fuel hose and hence the fuel filter end up being horizontal too. If a fuel filter is mounted with the inlet & outlet vertical, no vapor pocket can become trapped there because no head space is created to collect the vapors, the vapors will then rise out of the filter to collect somewhere else. But, if the hose leading upward out of the vertical filter is very short, after some time, the vapor pocket that forms in the hose will displace fuel and eventually you will observe that the filter is less than full (again).

Mownie, you did say raise both ends for your experiment. I understood that. I was just using a slight variation of your experiment to illustrate what I think is obvious.

Please reread my last post. What did I write that you don't believe?

I did notice one oversimplification that I made. I said weight of the column of water and weight of the liquid above the outlet tube. I should have said "pressure" in both cases. Pressure is weight divided by cross-sectional area.

Does anyone understand my last post? If you don't, which part can I make more clear? Bill, we haven't heard from you lately.

-Deerslayer

Deerslayer - I am confident that Montie is correct. I included (except I spelled phe·nom·e·non incorrectly -- where is Wally2g when we need him) all my thoughts in my "bumper sticker" statement. I repeat it below, slightly modified:

My bumper sticker version of this phenomenon is: when the vapor pressure of the gasoline within the filter exceeds the head pressure of the gasoline from the gas tank, some of the gasoline within the filter will be displaced or pushed back into the fuel tank via the fuel line due to the vapor pressure exceeding the head pressure and the needle valve, in the carburetor, effectively sealing that hose. Let us remember to see if the filter has more gasoline in it this winter.

Wow!!! I'm beginning to feel like Copernicus must have felt when he tried to convince people that the earth rotates around the sun.

I agree that once the pocket has formed, it fills with fuel vapor like Mownie has said. Furthermore, I bet that if you put away your tractor when it's 40F then don't start it until its surrounding temperature is 75F, more than half of the fuel filter will be empty. The fuel vapor effect that Mownie has described is the reason for this.

Please specifically address the conclusions or processes in my "experiment post" dated Sep 18, 07 at 13:43.

-Deerslayer

When I was in high school, I graduated valedictorian and hung out with some fairly brainy guys. I was also co-captain of our football team. The combination was a bit unusual.

One day after our routine lunch break chess game, one of my buddies stated that the shape of an egg is so perfect that it cannot be broken by squeezing it between the palms of your hands. I basically said that his statement was BS (I wasn't as PC then). He then bet 10 cents that he was right. I took him up on it.

The next day he brought a raw egg to school and a shiny new dime. At lunch time, among a crowd of friends, he handed me the egg. I took the egg and cupped it between the palms of my hands and began to squeeze. Lo and behold, the egg burst as I had predicted and blew its contents all over my shirt and the top of my pants. A roar of laughter came from the crowd. My buddy(?) then promptly handed over his shiny new dime.

I'm beginning to think that I'm in the same type of argument here. If anyone wants to make the effort to run the experiments outlined, please post your results. If I'm wrong, I'll publicly admit it.

I believe that the thinkers in this forum already know the outcome. To the rest of you, God Bless.

-Deerslayer

"I agree that once the pocket has formed, it fills with fuel vapor like Mownie has said."
deerslayer, those are your words. In reading them I infer that you don't grasp that there is no "once the pocket has formed, it fills with fuel vapor." The pocket is fuel vapor. It is the vapor state of liquid gasoline (or the vapor state of whatever liquid that happens to be in the system). The vapor essentially "out gasses" from the liquid and that is what fills the head space where head space is present. This is not like the medieval belief in "spontaneous generation". This is simply understanding that volatile liquids are eager to change from the liquid state into a vapor state and once they have done so they behave somewhat differently than when they are liquid.

Mownie, my point was that gravity forms the pocket not the fuel vapor. Gravity pulls the liquid down that is in the fuel filter above the hose. As the pocket forms by gravity, fuel vapor occupies the space.

It is the same when you partially fill a container with gasoline. The head space exists before you put the fuel in the container. Once the fuel is in the container, fuel vapor immediately fills the head space. Obviously, the fuel vapor does not create the head space. Likewise, fuel vapor does not create the pocket in the fuel filter...gravity does.

Perform your experiment with the two hoses and fuel filter but use water. You will have the same result as with gasoline. This proves that the fuel filter pocket is the result of gravity not fuel vapor.

-Deerslayer

Let's use 2 hollow, transparent, straight, vertical columns to illustrate the "dynamics" of the "vapor forms directly from the liquid due to change of state" process. Our columns are tall enough to allow the influence of gravity to play a significant role in this scenario. Column # 1 is closed on one end (which will be called the top) by a combination "vacuum/pressure gauge diaphragm which visually displays any changes of pressure or vacuum without providing a vent. (Assume that we are doing this at 1 standard atmosphere of barometric pressure.) The other end (bottom) will have an opening just large enough to allow the column to be filled with a liquid when the column is inverted, and a conduit which will connect the bottom of this column to the second column, which is twice as tall as column # 1, and is open to atmospere above the liquid therein. After completely filling column # 1 with a "non-volatile"....liquid mercury, (considered to be at room tempertures but nothing is truly "non-volatile"), and connecting the "conduit" which allows freedom of exchange between the 2 columns, turn column # 1 right-side-up. Secure the column in the vertical. At this point, gravity is certainly acting upon the mercury, and the diaphragm gauge will register negative pressure (because of atmospheric pressure trying to push the diaphragm inward) but you could watch this column for millenia on end and never see the formation of mercury vapor (at this temperature). If we begin adding heat to the mercury in column # 1, we will observe SOME displacement of mecury from column # 1 into column # 2 AND we would see the gauge move from negative pressure toward the zero mark (equilibrium). This displacement occurs due to thermal expansion and the guage moves to show less vacuum because the total mass of liquid in column # 1 has decreased while the total mass in column # 2 has increased. We have moved toward equilibrium by adding heat and displacing some liquid but we still have not seen any vapor form in the top of column # 1. Continue adding heat to # 1 and at a temperature of roughly 356 degrees C the gauge will read zero because we have added enough heat to displace (thermal expansion) liquid mercury from column # 1 into # 2 that the mass of liquid in # 2 is sufficient to balance the gravitational "pull" of the mass in # 1. Add more heat and at a temperature of roughly 356.5 degrees C the mercury will begin to evaporate and we will observe the formation of a "pocket" of mercury vapor. As this pocket grows in volume, it will displace an equal volumn of unvaporized, liquid mercury from column # 1 into column # 2. As the mass of mercury in column # 2 increases, we will observe that the pressure gauge in column # 1 no longer registers zero (equilibrium), it is now showning positive pressure. If we continue to add heat to column # 1, we will eventually cause the formation of enough mercury vapor to completely displace ALL the liquid from column # 1 into column # 2. At that point, column # 1 might appear to be empty but it is not "empty". It is filled with molecules of mercury that have been raise to an energy level (the adding of heat) at which they can exert enough "pressure" (occupy space) to displace the rest of the liquid mercury into column # 2. The pocket of vapor is now as large as it can get in this "gaseous state". We have added the maximum heat that the molecules can absorb and still remain in the vapor state. (if more heat is added now, the molecules would eventually reach the plasma state of matter) I used mercury in this scenario because it probably has the lowest volatility of any liquid (it has a melting point of -38.87 degrees C) Obviously it would strain the resources (and patience) of all but science laboratory personnel to conduct the demonstration I have described herein. If this doesn't graphically illustrate the process of vaporization, I am at a loss to try further. I agree that gravity plays a part in the process (but not the lion's share). It is primarily because volatiles can be coerced into changing from liquid to vapor by raising the molecules of said liquid to a higher energy state by adding heat (energy). Any help from gravity in the case of a vapor pocket forming in a gasoline filter on a typical tractor would, at best, be minor or insignificant.

Are we still talking about air in a fuel filter?

I'm sticking with my theory of a "trapped air pocket". It gets there because the fuel drained out or because of air bubbles trapped in the fuel that escape at the filter.

As for your clear hose coils, my thought is that the water at the ends evaporated, air was sucked into the hose as it "gurgled" to reach another state of equilibrium so the water level in each hose was equal. You weren't there to see it happen. If there's a small amount of air at the top of each coil, it's probably the air that was trapped (dissolved) in the water.

Way too much thought put into this stuff...

Mownie, the more that I think about this, the more I realize that the point that I was trying to make isn't that important.

Here's another way to look at it. When you change a fuel filter, initially the filter contains air. Assume that the filter is installed horizontally. The air in the lower half of the filter is displaced by fuel. The air that is trapped in the upper half of the filter is displaced by fuel vapor. The same thing happens with a vented fuel container when fuel is initially added to the container. The air that was originally in the container is displaced by liquid fuel and fuel vapor.

The size of the fuel vapor pocket in the fuel filter will expand and contract with changes in ambient temperature as you have said.

Whether the fuel vapor creates the pocket or displaces trapped air in an air pocket isn't that important when explaining the overall behavior of fuel vapor in a fuel filter. This is because either way, fuel vapor ends up occupying the top portion of the fuel filter.

Here's a neat video demonstration that I found while researching this.

Fuel Vapor Demo

The video links are at the bottom of the web page.

-Deerslayer

I just put a Purolator glass fuel filter with a replaceable element on. It does not fill with gas. Is there any reason that this type of fuel filter would be a problem on a typical mower engine with a pulse/vacuum Mikuni fuel pump?

The only problem with filters is to be sure that the filter you have elected to replace the OEM filter is intended for use on the SUCTION (inlet) side of the fuel pump. Most inline gasoline filters are intended for use on the suction side of pump. This means that the filter is located between the fuel tank and the carburetor. Filters that are intended to be used on the pressure (outlet) side of fuel pump are usually found on fuel injected engines and will filter out much smaller particles of debris. For this reason, if a pressure type filter is used on the suction side of a fuel pump........the restriction factor of the filter media could actually reduce or prevent fuel flow. The reason is because ANY fuel pump can develope a higher positive pressure (outlet fuel pressure)to PUSH fuel than it can develope negative pressure (on the suction side) to PULL fuel. If the Purolator assembly you have is meant to be used on the suction side of pump, you will likely not have problems, except if it is a very large filter...you may need to prime the carb a few times to get the filter purged on the initial start up. And if the filter bowl looks half full.........don't worry.

If it's like the glass bowl filter on the suction side of my small diesel tractor, there should be one or two bleed screws on top of the metal casting to let the air out.

Mownie said: "Most inline gasoline filters are intended for use on the suction side of pump. This means that the filter is located between the fuel tank and the carburetor."
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Mownie, just to clarify, you mean or meant to say that the suction side is between the fuel pump and the carburetor, correct? In my set-up, I have a long fuel line (4 feet appx.) from an under-seat 4 gallon tank up to the front and then to the Mikuni pulse/vacuum fuel pump, then to the carb. First, I do not know if this fuel filter was designed to be used on the suction side (between the tank and the filter) because I got it an an auto store. How would I determine that? Second, even if it is normally OK to use an auto fuel filter on a tractor, do other factors effect the rate of fuel that can be drawn from the tank by these little pulse fuel pumps, i.e.,,,,the length of the fuel line from the tank to the fuel filter; gravity,if the fuel pump is higher than the fuel tank outlet or fuel line; or, the pressure in the fuel tank if the vent cap is plugged or if the hole in the cap is too large (in my case I stuck a pin through the hole in the vent cap to make sure it was venting). I know I can easily take off the fancy glass fuel filter and get the typical plastic/paper type, but I just thought to ask you before I did that.

and one other question that came to mind from reading your posts. Assuming that this glass fuel filter is designed to work on the pressure side of the fuel pump,,,would that fuel filter work better if I switch the lines (so that the line from the tank goes directly into the pulse pump, then through the glass filter, then to the carb) or is that going to mess up or clog the fuel pump or create some other issue?

heri_cles, the sentence should have been "This means that the filter is located between the fuel tank and the fuel pump". I'm sorry for the error. A fuel pump has 2 "ports" through which liquid flows. One is the suction or inlet, this port must be connected to a hose that leads to the fuel tank. The other port is the pressure or outlet, this port connects to a hose that leads to the carburetor. Vacuum or pulse operated pumps also have the third port for the pulse hose but the part about the 2 liquid ports is true for ALL fuel pumps no matter if they are mechanically, electrically, or vacuum operated.
I would bet that the filter assembly you have is meant for use on the suction side (between fuel pump and tank). Most of the filters I have seen that are for the pressure side of pump are constructed of metal. As to your other concerns: Length of hose.....With a run of about 4 feet and an inside diameter of 1/4" (and assuming the horsepower of the engine to be less than....say....50HP) your fuel pump would not even know it was drawing through a hose. At the low rate of fuel consumption of your engine...the fuel line might as well be considered as an "extension of the fuel tank". As to the height of the pump above the fuel tank....this is always a consideration BECAUSE these small diaphragm pumps are very weak in the suction aspect of pump performance....but in the application on a lawn or garden tractor you would really not have a problem because in most cases the pump only has to "lift" the fuel something less than 10". As to the size of tank vent........so long as the vent allows air to enter the tank at a rate that matches (or exceeds) the rate at which the fuel is being drawn out of the tank....increasing the vent size will not increase pump performance. Having too large a vent can lead to dirt and debris entering tank or fuel sloshing out of tank. I visited the website of your link and the fact that they included a warning not to use this filter with fuel injection systems supports the idea that it is a suction filter. I noticed that it has 5/16" hose nipples, does your tractor have 5/16" hose? Lastly, you DO NOT want to situate your filter between the pump and carburetor because then you lose the benefit of keeping the "crud" out of the fuel pump.

mownie:
I replaced the fuel line all the way from the tank with a fuel line I purchased from an auto store a while beck. I think it was 1/4" but I honestly do not recall. If it was 1/16" smaller than the fuel filter nipples, it seems to have gone on pretty easy anyway.
{{gwi:346959}}

Yep, that looks about as full as it can get in hot weather. By the way, judging from the elevation of the fuel filter in regard to the location of the oil filter I would guess that the pump is not having to lift the fuel very much. Is your tractor performing satisfactorily??

I now wonder if the oil filter is full or half empty. LOL

All right now Steveski...