The most complicated Overheating problem yet

[Runnin'OnEmpty]

You've been through several water pumps, but are you sure they were the right ones? Seems to me I recall reading around here years ago that the pump for the 4.2 bolts up and looks right, but the impeller's configured to be spun the opposite way (due to different serpentine belt routing compared to the 4.0).

That's right, the 4.0 serpentine drive is reverse rotation.

The only way to check this is by removing the waterpump and eyeballing it.
(Or possibly observing the impeller through the lower inlet hose opening.)

 

[richardN]

UPDATE:
I got a chance to do a couple things. I put a smoke machine on it to detect any intake leaks or anything. The test was normal. Everything was air tight.
Then I got to thinking about That header I have on there. So I took it off and put on he stock exhaust manifold. No change
It always runs the same , Its fine for about 10 min of driving . any kind , then it starts climbing all the way up to the red.
I have 4 water pumps for it. they are the correct direction impeller.

 

[RCP Phx]

Have you done a compression test? Have you done a "leak-down" test? Have you done a pressure check on the cooling system?

 

[my2monkeys]

I suggest T stat. Have you tried running without a tstat and see if it overheats.

Good luck, and make sure there's no air bubble in the system, which is usually purged out after a few cycles.

 

[Unclewolverine]

I suggest T stat. Have you tried running without a tstat and see if it overheats.

Not recommend in any instance.

 

[SlowXj]

So once I had a locked air pocket in a 4.0 grand.... I had to take the stat out and fill the block with water and it fixed my problem. It would run close to ten minutes before it got hot

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[Anak]

An air pocket could be the issue.

My technique is to remove the heater hose from the T-stat housing and fill the radiator as high as possible before the coolant leaks out of the fitting. Doing that ensures the water level is up to the T-stat.

Certainly a cheap thing to check.

 

[techno1154]

I suggest T stat. Have you tried running without a tstat and see if it overheats.

Not recommend in any instance.

Yes, not recommended but I have done that in the past,...without thermostat and with a 180 thermostat. The 4.0 did not self destruct. Running without a thermostat could be a very useful diagnostic (so to speak) tool.

 

[8Mud]

Have you done a compression test? Have you done a "leak-down" test? Have you done a pressure check on the cooling system?

A tiny leak or crack in the head gasket or the head can cause overheating. I've had my experience with this situation. When the engine was running there is a high pressure in the cylinders that drives exhaust gas into the cooling system. The exhaust gas eventually finds it's way to the top and into the coolant canister that is vented. The exhaust gases can form air pockets in the engine slowing coolant flow and/or just pumping very hot exhaust gases into the coolant causes an excessive temperature rise. The tiny leak only passes a little coolant into the cylinders after shutdown. Deposits on a plug or two is a tip off, check them after the motor has been running a short while. When the motor gets up to operating temperature for very long it is likely to burn the deposits off the plugs. A few other tips offs are a miss at cold start idle, excessive amounts of steam or droplets coming out the exhaust or bubbles coming to the top of the radiator with the cap off on a warm (themrostat open) motor. Running the motor with the radiator cap off is likely to drive soem coolant out of the radiator filler, it usualy isn't excessive. The tests are described in the quote.

 

[SlowXj]

A tiny leak or crack in the head gasket or the head can cause overheating. I've had my experience with this situation. When the engine was running there is a high pressure in the cylinders that drives exhaust gas into the cooling system. The exhaust gas eventually finds it's way to the top and into the coolant canister that is vented. The exhaust gases can form air pockets in the engine slowing coolant flow and/or just pumping very hot exhaust gases into the coolant causes an excessive temperature rise. The tiny leak only passes a little coolant into the cylinders after shutdown. Deposits on a plug or two is a tip off, check them after the motor has been running a short while. When the motor gets up to operating temperature for very long it is likely to burn the deposits off the plugs. A few other tips offs are a miss at cold start idle, excessive amounts of steam or droplets coming out the exhaust or bubbles coming to the top of the radiator with the cap off on a warm (themrostat open) motor. Running the motor with the radiator cap off is likely to drive soem coolant out of the radiator filler, it usualy isn't excessive. The tests are described in the quote.

I have had this issue in my daily for 75k miles... I fill the overflow about 3 times a year. I do have to flush my coolent every year as it gets disgusting quickly. I know it would get warm on the trail, but it stays around 210 with ac on in stop and go traffic with the efan cycling. I see no need to fix it honestly until it becomes a bigger issue of course

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[8Mud]

I have had this issue in my daily for 75k miles... I fill the overflow about 3 times a year. I do have to flush my coolent every year as it gets disgusting quickly. I know it would get warm on the trail, but it stays around 210 with ac on in stop and go traffic with the efan cycling. I see no need to fix it honestly until it becomes a bigger issue of course

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I ignored mine also for a long while until one day it leaked enough coolant into a cylinder to hydrolock and turn a valve into an upside-down mushroom. A five-cylinder 4.0 actually runs pretty well.

 

[JeffDB]

I didn’t know exactly where to post my issue but thought it might be ok on this thread. The last couple of weeks I noticed my engine temp running a bit high on occasion. Around 220. Didn’t think much of it. Yesterday wife and I hit the trails and I noticed my temp had dropped to 150-160 range. Strange as we were crawling through the woods with air temps in the 80’s. Thinking my thermostat was stuck open I wasn’t too worried but kept an eye on it anyway. After getting back on highway heading home temp suddenly went back to normal. Then about 90 miles later check gauge light came on and temp was almost in the red. Pulled over and opened hood and overflow was boiling and overflowing. Is this a thermostat issue or something more involved like a head gasket? I appreciate any suggestions. I’m going to replace thermostat but not sure how to burp air out afterwards.

 

[RCP Phx]

I didn’t know exactly where to post my issue but thought it might be ok on this thread. The last couple of weeks I noticed my engine temp running a bit high on occasion. Around 220. Didn’t think much of it. Yesterday wife and I hit the trails and I noticed my temp had dropped to 150-160 range. Strange as we were crawling through the woods with air temps in the 80’s. Thinking my thermostat was stuck open I wasn’t too worried but kept an eye on it anyway. After getting back on highway heading home temp suddenly went back to normal. Then about 90 miles later check gauge light came on and temp was almost in the red. Pulled over and opened hood and overflow was boiling and overflowing. Is this a thermostat issue or something more involved like a head gasket? I appreciate any suggestions. I’m going to replace thermostat but not sure how to burp air out afterwards.

I would start with a new Mopar temp sensor and a Stant thermostat. There isn't anything to "burp" on a '98.

 

[CJR]

JeffDB,

I've had my XJ coolant system pass a "static pressure test" but fail a "dynamic pressure test"; i.e. when I accelerated hard on a hot day, I saw a puff of white smoke out of the exhaust. When I let up on the gas the white smoke disappeared. Obviously, some head bolts were acting like "relief valves", i.e. loosening the clamping force on the head gasket to allow coolant to flow into the combustion chamber and out the exhaust. Then when the combustion pressure dropped (foot off gas), the head gasket resealed and stopped coolant flowing into the combustion chamber and out the exhaust. My fix was to pull the head, pressure-test for cracks ,Magnaflux, resurface the head, reinstall with "reusable" ARP head bolts and a quality head gasket.

My suggestion is to pull the head and correct the problem ASAP before this overheating issue gets real expensive ($$$$$$$$$$$).

Best regards,

CJR