Not enough info. what motor size ? What is your definition
of " sludge " ? That car will have a 3 lt. or 3.8 lt.
The 3.8's are BAD for head gaskets. What is your definition
of " two much pressure "? Normal pressure in 5 minuets will
make the upper rad hose so hard you can't squeeze it. You
might want to replace the rad and heater core and have the block flushed. If there is that much sludge in the rad
your transmission will over heat. Not good. No offence but if your husband is a mechanic he should know about the 3.8
and when you pop the rad cap off any heated engine the
boil over is normal. Again no offence but he may have been
a mechanic more off than on. The rust you see is from the
block. You have an aluminium rad. If there is enough rust in the system it will insulate the temp. sensors and never show an over heat condition. Prime example. Remove all antifreeze and run the engine until it seizes from over heating. Your temp gauge will not come off cold. No antifreeze to transfer heat to the sensor. Argo no temp reading. If the antifreeze has never been changed or flushed every 3 years and the motor HAS over heated you have a problem. Good luck. Maybe someone else will help you with a very simple repair. I've given you the worse possible reply. ( can't spell senario ). Again, good luck.

Thanks for your reply...

The motor is a 3lt. with 91,000 miles. I have only had the car for 2 years, so I don't know what prev. owners have done though I have done the "usual maintness". The sludge looks like fudge, but when smoothed out or rubbed between fingers, looks more rusty red brown in color. No smell and no seperation (like oil and water). And you mentioned no temp reading, I do remember when I lost heat the first time, the temp needle never moved off cold and the rad place I took it to couldn't figure out why as they said everything was working properly. Though since the first flush, the temp gage has been working. Out of the last 15 yrs, my husband has been a mechanic for about 8 of those years and is pretty knowledgeable with cars, but (as most people we have talked with) has never seen sludge boil and overflow out of the reservoir. The few people we spoke to, one of which has over 35 yrs of continual exp said it sounded like to much pressure, to change the rad cap. So we did and still no change. Again, thanks for the reply and hopefully the additional info will help to further figure out my issue.

That's too much sludge buildup between cleaings. There has to be a cause. Possibilities:

1. Head gasket leak - blowing combustion products into the coolant.

2. Air is emtering the "closed" coolant system. A major cause of corrosion in the coolant system is admission of air. Check out the tubing/hose that connects the overflow bottle to the system. Look at the underside and at bends. Air can be drawn into the system upon cooling. As the pressure falls in the engine and goes negative (relative to the atmosphere), coolant normally flows back into the engine/radiator, but if there is an air leak in the connecting tubing, air will be drawn in instead, and the engine will develop hot spots the next time it is run.

An improper or ill fitting cap can cause problems.

Get a cooling system pressure tester. That will tell you if you have any leaks. Test or replace the radiator cap. With it full of coolant, start and observe coolant with the radiator cap off. A steady stream of bubbles that doesn't stop indicates a head gasket leak. Get a thermometer, gage you can temporarily install in a water jacket, or one of those point and shoot type heat detectors. The object of those suggestions is to determine if you have any leaks and if the engine is hot cold or normal temperature. Your story contains a variety of symptoms, some of which sound like a normal conditions to me that you've concluded is part of the problem. Need to separate what the normal conditions are from the abnormal.

Flush and refill using green or yellow antifreeze. Stay away from the orange dex-cool stuff. It has a reputation for causing the kind of problems you describe. Make sure all the air is out of the cooling system. Air gets trapped in some engines and is difficult to remove. I've found it helpful to drill a small hole in the flange of the thermostat to let the air out so the system can be filled.

Personal observation from years gone by, often times cooling systems in Ford products got full of rust a lot quicker than most for some reason. Some I used to flush about every year to keep reasonably clean for some reason. There may have been a cause, but neither I nor anyone else in the shop I worked in at the time knew what it was. jmo

You are describing a leaking head gasket. Also as Gary
said that engine, and i have 3, are very prone to air locks
as the coolant is refilled. It could take 1/2 hour to bleed
off the air and it isn't fun. you DO NOT put the rad cap on and hope for the best. It will boil over. are you sure you are not getting transmission oil in the rad ? I have seen head gaskets go in that type of engine. Pretty rare.
antifreeze that comes out brown is usually burnt and VERY
high in acid. when you plug in your block heater it will
short out as the antifreeze burns through it and burn the
antifreeze again. A domino effect. Have them do an acid test on your antifreeze and a " hot engine " pressure test. Good luck.

Thank you for all the responses!!!

I know this was from 2007. I had put prestone radiator sealant in radiator. Messed up thermostat. Blew hose . And water kept bubbling out but with a spoiled milk look to it. Replaced thermostat and flushed it. Water kept boiling. Well after 5 hours of that. It came down to science. Water boils around 200o and my fan wasnt turning on till 222o so we raised boiling point. If you add 50/50 antifreeze , you will need two jugs. Or the super strength and add the rest water. It worked. Note: we had a chrystler pacifica 160,000 miles base. 2006. We also had the computer to watch temp because chry....doesnt have temp gauge.

That is very good, but this goes into the extend life coolants issue. I am just beginning to attempt to get a handle on this as there are many combinations of situations and coolants. In short, from what I can tell, use the manuf's. recommended type, change coolant every two years. Not the over-anticipated 5 yr./ 150,000 mi. claim.

Everybody is passing the buck, aftermarket manuf's. directing to the manuf's., manuf's. are silent and understating, much conflicting and bad info. on the internet, manufacturer not acknowledging end users' claims, and end users mixing coolant types.

I ha e a 2004 poniac grand prix v6 3.8l engine i have put a water pump on it a thermastat in it and my coolant keeps disappearing or boiling out of the coolant over flow tank whenn it reads hot dont know if there is a bad thermastat or heatercore clogged up or blowed head gasket i keep my heat on to try a d cool it off that dont always work theheatis hot when its hot when the temp is normal but it will run hot sometimes and blow out cold air any suggestions would help thanks and forgot to mention that the antifreeze looks brown