Muncher

I just bought my first jeep in November and about two weeks afterwards it overheated. I took it to the shop and they said because they couldn't duplicate the problem they could not fix it. They did however replace the thermostat.
I thought it was fixed but three weeks later it overheated again. I took it back to the shop and they did a pressure check of the cooling system and also tested for bad head gasket.
I cannot for the life of me figure out what is wrong. Any help here would be greatly appreciated.

tjwalker

We need more details. A few questions for you to answer that will help us help you....

*How long does it take to overheat?
*Did you verify the overheat with a laser thermometer pointed directly at the thermostat housing? (some overheats are electrical failures because of faulty sensors or gauges, not true overheats)
* Does it overheat at idle in your driveway?
*Does it overheat while driving at highway speeds?
*How many miles on the engine?
*Are you having to add coolant to the coolant reservoir?
*Any evidence or documentation of other cooling system components ever been replaced (radiator, radiator cap, water pump, cooling fan shroud)
*Does it still have an intact cooling fan shroud?
*How did they test for the bad head gasket? Did they sniff the coolant for presence of hydrocarbons or did they perform a compression test?

Muncher

It varies but usually quite some time to overheat
It was boiling water into the overflow reservoir
Doesn't overheat at idle in driveway
Doesn't overheat at highway speeds
122000 miles on the engine
Yes I have to add coolant to the reservoir
No evidence of other replacement parts
Yes fan shroud is intact
Compression test

BruceB

could be as simple as a worn out radiator cap......

Muncher

I was thinking of replacing the radiator cap because it is a cheap fix. Thank you for the information
Well I replaced the radiator cap but it overheated again today. It had been awhile since it happened. Still haven't figured out the root cause.
The shop said they checked the head gasket, the radiator and cannot find any thing wrong. Is there a way to check the water pump?? If it was the water pump wouldn't it overheat more often??

third coast

Go to an auto parts store that rents tools and get a combustion gas tester - also called block tester. You will need to buy the test fluid for under $10. Follow instructions to sample the gasses from the top of the radiator. Testing at the coolant overflow tank is an option if you are getting bubbling there. Flush the tester thoroughly first and keep coolant out of the tester. The fluid in the tester will change color if it is exposed to combustion gas in the radiator. If there is a color change that indicates a leak into the cooling system. That could be due to a bad head gasket or a warped or cracked head.

Sirsyc0

It’s been awhile but when I purchased my XJ the coolant system was rusty. Previous owner had been adding straight water for quite awhile. After cleaning the system up, I had similar symptoms. It wouldn’t over heat at idle or on the highway. I couldn’t figure out why it would get warm in certain situations. I went ahead and pulled everything out except the heater core. I found the water pump impeller was half gone from pumping rusty water.

third coast

Another few additional thoughts:
* Force the electric fan to always run and see if that makes a difference. Do a search on this site on how to do the bypass. If you have a faulty temp sensor or bad electric fan this could result in loss of cooling air and explain why at highway speeds with plenty of forced airflow through the radiator it seems fine.
* I assume you checked that the external radiator fins are not packed with debris or badly damaged. How about the air conditioning condenser that sits in front of the radiator?
* A bad thermostatic fan clutch mechanism (what the belt driven fan attaches to) can result in reduced airflow when needed.
* It's possible that you may have some plugged tubes in the radiator as a result of the previous disintegrated water pump.

MrTodd

I ended up replacing the radiator as it was full of rust and clogged passages.

Todd

Muncher

I found a leak on the round plastic part that protrudes from underneath the radiator filler cap. I also wanted to clarify that when I said my jeep was overheating I meant that it was at about 220 degrees but the coolant in the reservoir was boiling. My best guess is that the small leak caused the coolant level to drop this causing the overheating. All comments and recommendations are appreciated.

third coast

I don't know how accurate your temperature gauge is but I would not expect it to be exact. An infrared thermometer aimed at the top of the radiator might give you a better idea but still there is room for error. Some physics: distilled water boils at 212 degree F at sea level. Altitude will effect this. For every 500' increase in elevation the boiling point drops by a bit under 1 degree. (this is what makes high altitude mountain driving tougher to maintain temperature) A 50/50 antifreeze mix increases the boiling point (increase is better) to about 223 deg F at sea level pressure. Now pressurize the system and the boiling point increases further. Again this is good. The radiator cap allows pressure in the cooling system to build until it reaches a point established for that particular cap design. Usually the pressure setting is marked on the cap. Your year probably came with a 16 psi cap which would raise the cooling system pressure by 16 psi over outside air pressure. The result of a 50/50 antifreeze mix and a 16 psi radiator cap would be a boiling point around 267 degree F at sea level. You can see the importance of a proper antifreeze mix and cooling system pressure.

You mentioned a leak in the area of the cap. If this was preventing the system from building pressure and if the antifreeze to water percentage is low this could explain boiling at around 220 degrees F. You need the system pressurized to get satisfactory results. This requires a proper cap and seal to the radiator with no leaks.

Getting a 50/50 mix is not easy on this system if you started with 100% water due to the amount of fluid that remains in the block even if you drain the radiator. Use an antifreeze checker to determine when the mix is correct by remove cooling system fluid and adding 100% antifreeze.

This cooling system does have a tendency to trap air that will slowly be expelled as seen by bubbling at the expansion tank as the system heats up.

Muncher

Well it's been awhile but it overheated again. I took it back to the shop again I wasn't there for a half hour and they say "you're all set to go.". I asked what it was and said the electric fan was not working properly. But it only seems to come on when you turn on the AC. At what temperature should the electric fan come on?? I'm not convinced that fixed the problem but I will drive it and see. I appreciate all the information and tips I have received from everyone here.

MrTodd

for a stock system in a 1999 XJ, the electric fan will be powered when the coolant temperature reaches approximately 218° F and then will shut off when that temperature drops below 209° F according to the factory service manual.

Todd