One detail that should not be overlooked on the subject of "what temperature the gauge is indicating": If the gauge pointer just barely moves off the peg (cold side) when the engine is running with no load on a cool day, but moves more to the right when the load or ambient air temperature increases.......it may indicate the thermostat is stuck open (or is absent altogether...was removed ). If the pointer does not move off the peg at all: The sender unit wire is disconnected/broken, or the sender is defective, or the gauge is defective, or the gauge has no power input (from ignition switch output). If you touch the wire that attaches to the sender unit to a ground and the pointer moves all the way to the right (max temp), the sender is defective.If you touch the wire to ground and the pointer does not move all the way to the right, you need to check that the gauge feed wire is delivering 12VDC to the gauge before you arbitrarily replace the gauge. These gauges utilize an internal (usually internal to the gauge itself) "constant voltage regulator" to assure that the displayed information does not fluctuate with minor changes on battery (system) voltage in normal operation of the machine. What that translates to is this: Using a DC volt meter, the gauge input wire (the wire from ignition switch) will show battery voltage but the gauge output wire (the wire that runs to the sender)will show something less than battery voltage (like 8V or 6V). Make sure the wire is not connected to sender nor grounded before testing input & output voltage of gauge (that would cause invalid readings). Never rule out the chance of having a broken wire going from gauge to sender. If the gauge shows good input and output voltage readings but the guage will not max out when the sender wire is grounded, try using a "jumper wire" to test for broken wiring. Touch the exposed end of a short wire to the output of the gauge (the side with less than battery voltage) and the other end of the wire to a ground. If the gauge now shows max temp, the wire from gauge to sender is faulty. If a fuse "pops", you were touching the wrong side of the gauge wiring. Chances are good that simply grounding the sender wire will reveal a defective sender unit but in case that turns out wrong, you have been given the advanced test procedure as well.

It's possible that there's still air trapped in the block. When the engine is cold, remove the radiator cap and start the engine. As the engine warms, rev it up a bit while filling the radiator with coolant. Easier if you have a helper. When it's as full as you can get it, replace the cap before letting the engine return to idle. Also helps if you can park where the front of the vehicle is on an incline. Could still have a leak at the cap or in the line between the radiator and the recovery tank that will allow coolant to be pushed out one way, but allow air to be sucked in the other. Just a couple ideas fwiw.

I don't think you should automatically take it to another shop. You're already into it for $300 with these guys who thought they fixed it but didn't. Don't know what your engine looks like or what they did for sure so I don't have an opinion as to if $300 was reasonable or not. Could have happened like this. Your vehicle had a coolant leak and overheated for whatever reason. They fill it up and pressure test. Find that leak, repair it. Pressure test again, no leaks. Run it for a while, gets to normal temp and holds there. They conclude...maybe falsely, the leak was the only problem, that it overheated because because the coolant level was too low because of that leak they repaired. Under what conditions does it overheat and how long does it take? Idleing in traffic, or cruising down the road? AC on or off? Only when it's hot outside, or does it matter? Overheating usually isn't that tough to diagnose if given enough details. The coolant going away due to a leak would certainly be one of the causes. Other possibilities would be the thermostat not working, electric cooling fan(s) or fan clutch not working will do it. Radiator plugged up inside or a bunch of junk restricting air flow on the outside will do it. Even the cap not holding pressure can cause it. The shop you took it to is guilty at most of not being thorough enough. That would be easy to do if you came in and said something like you were just driving along and all of a sudden it overheated. jmo Glad to see you back earthworm. I was afraid something had happened to you.

I suspect that the system was not full, possibly air space in the heads. I don't know about the location of an air-bleed on the 3.1 L V6, but I think is is similar to the 2.8 L V6. My 2.8 l V6 did not have an air bleed. the drill was to fill it as much as possible with a top most hose removed and when coolant started flowing out the hose bib, re-attach the hose and fill as much as possible. Pre-mix the antifreeze and water, usually half and half (unless you purchased the pre-mixed stuff). Be on your toes when you purchase antifreeze. The 100% antifreeze and premix often occupy the same shelf space and it easy to get these mixed up. Use this mix to add coolant to the system. Do not try to mix in the radiator since it is too difficult to get the desired mix ratio. Fill the overflow bottle with coolant to a level above the full mark. With the heater control set to high heat, radiator cap off, start the engine. Add coolant as the level sinks (for short period of time) and then put on the radiator cap. Don't remove this cap again during the filling process. Don't run the engine no more than 5 minutes because there is probably air in the heads. Stop the engine and allow to cool. You should see coolant being drawn out of the overflow bottle. Check the level in the bottle and start the engine. Allow it to run long enough to open the thermostat. The upper radiator hose has a significant rise in temperature when the thermostat opens. Stop the engine and allow to cool. As the coolant cools, it will suck more fluid from the bottls, that is, if the fill cap is in good condition and there are no air leaks in the line running from under the cap to the overflow bottle. You can hasten the first two cooling periods by running some water over the radiator core. Check the fluid level in the bottle. Always add coolant to the bottle, never at the radiator cap. The cap must remain in place to keep from admiting new air into the system. After these first 2 minor thermal cycles, there should be enough coolant in the system to run the car for a short time. Drive the car until the thermostat opens and the temperature comes to near normal. Park it and allow to cool. Resupply the overflow bottle and after this 3rd thermal cycle, there should be enough fluid in the system to drive the car at mild service conditions. Watch the coolant level in the bottle for the next 2 to 3 cool downs, add coolant as needed, and when the level stabalizes, you may go back to the normal check schedue. Because the intake manifold was disturbed, keep an eye on the oil level and condition over the next few days. We're looking for possible coolant leaks into the crankcase. On this engine, the intake manifold sits between the heads, acts as the valley cover, and has coolant passages. A rise of oil level and color change may indicate coolant leak into the crankcase. Oil contaminated with ethylene glycol will have a brownish color and may...