Overheating & Next Step?

[Torpedo01]

Howdy,

Took my 98 4.0 auto jeep up in the mountains to do some fly fishing and there were some low to moderately steep inclines along the way. While I’ve always felt it’s been way underpowered since I got it in 2020, (I put 4.10 gears in but I do have 33” tires), I had the gas floored but it couldn’t maintain speed limit over very gradual inclines.

Pulled over to get some gas and the coolant overflow was bubbling like crazy. The temp gauge didn’t show it as overheating but kept an eye on it. Drove it to my spot and let it cool for like 5 hours.

On the highway drive home, temp gauge shot all the way into the red so I pulled over and got a tow back home to be safe. The overflow tank was bubbling so hard it was spraying out into the engine bay from the hose that went to it.

Oil dipstick looks fine, no smoke out the tailpipe. I ordered a new radiator cap as a cheap first step, but is there anything else I should be doing beyond that?

 

[macleanflood]

Compression test.

I'd probably throw a head gasket at it. But I'd look at your bearings and cylinder walls and pistons.

-Mac

 

[TheBoogieman]

Stuck thermostat.

 

[JKP]

Flush system, replace thermostat and radiator cap, fill with fresh coolant and go from there. Be sure the pump is circulating the coolant as well.

Check for clogged cat(s).

 

[Torpedo01]

Thanks for the tips! I’ve ordered the cap and a Robert Shaw thermostat. The fluid is very rusty/brown and the radiator seems original or at least replaced with an OEM over the years. No leaks from what I can tell, so guess I’ll try just flushing it all and seeing how that goes - or maybe it’s worth replacing too?

Clogged cat, hmm, I didn’t think about that being a possibility regarding the power. It’s always been a dog but I never really had a baseline to feel what it should feel like since it’s always been that way.

 

[B1Toad]

Like @JKP said, flush, fresh coolant, stat and cap. If the cap is weak it will boil over. Be aware that a new cap may result in higher pressure which may expose the next weakest link. Do you have grill inserts by chance?

 

[JKP]

I would keep the radiator, if OEM, until it fails. Assuming the tubes look OK after flushing.

Also be sure the lower hose is not weak and collapsing.

A buddy of mine bought a '97 brand new and never changed the coolant. When it started overheating, he pulled the water pump and the impeller was nearly gone. Almost no flow.

 

[Unique Username]

Compression test.

I'd probably throw a head gasket at it. But I'd look at your bearings and cylinder walls and pistons.

-Mac

I’m with you on this one. The only thing that would cause the pressure would be 1) compression coming out or 2) it was completely empty and the cooling system was steam trying to escape. The cause of being empty could be it boiling off from a bad thermostat or a leak. Testing would be needed. It’s sounds like it was holding a consistent pressure inducing the cap was restricting something.

Maybe confirm the system has coolant in it after it cools as step 1.

 

[Torpedo01]

Like @JKP said, flush, fresh coolant, stat and cap. If the cap is weak it will boil over. Be aware that a new cap may result in higher pressure which may expose the next weakest link. Do you have grill inserts by chance?

Nope, no grill inserts. Good tips though thanks.

 

[Torpedo01]

I’m with you on this one. The only thing that would cause the pressure would be 1) compression coming out or 2) it was completely empty and the cooling system was steam trying to escape. The cause of being empty could be it boiling off from a bad thermostat or a leak. Testing would be needed. It’s sounds like it was holding a consistent pressure inducing the cap was restricting something.

Maybe confirm the system has coolant in it after it cools as step 1.

Aw man, I was hoping since there was no milky oil or exhaust smoke, it may not have been a head gasket issue. I’ll do a compression test first.

They don’t seem to be that awful to replace, but I'm not sure what I'm going to find when I pull the head.

 

[Link]

Aw man, I was hoping since there was no milky oil or exhaust smoke, it may not have been a head gasket issue. I’ll do a compression test first.

They don’t seem to be that awful to replace, but I'm not sure what I'm going to find when I pull the head.

No need to pull the head unless you find a leak. Test the cooling system for exhaust gasses.

An exhaust back pressure gauge isn’t expensive. Get one to test your catalytic converter. Excessive back pressure indicates a clogged or partially clogged converter.

 

[B1Toad]

Old school way of checking for compression leaks into the cooling system is to squeeze the top hose after startup. If compression is leaking into the cooling system the hose will usually get hard soon after starting, before the temperature is up.