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Intermittent Overheating

Kingkong0192

NAXJA Forum User
Location
New Milford, CT
1999 Jeep Cherokee XJ.

Thing runs/drives/feels fine.

At random (like every 5-10 minutes or so) the temperature gauge will shoot up from around 220 (it sits 1 line over 210 so i think that's 220) all the way to totally pegged at 260 degrees.

After about 30 seconds or so the gauge drops back down to 220 and just stays there until it decides to shoot back up to 260 again and repeat the process.

I figured it was a faulty thermostat so when the temperature spiked to 260 degrees i popped the hood and shot the thermostat housing with an IR gun and it did in fact confirm that it was 262 degrees.

Bad radiator cap? Air in the system? Any ideas?
 
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How is your coolant level? Low coolant at the ECT can cause a spike like that. You might also check the connection at the ECT. I had corrosion on mine and the gauge would cut out from time to time. A little filing and some dielectric grease and its been good.

Squeeze both radiator hoses. If they're both hot at running temp, then the thermostat isn't stuck closed. Probably a good idea to get the cap and system pressure tested.

If you're at 262 at the tstat housing then i'd be concerned. Check the weep hole at the water pump to look for any signs of coolant. If any, replace the pump.

Is the electric fan running constantly? I believe spec on it is something like 225 degrees.
 
Coolant is normal.

Didn't squeeze the radiator hoses yet, but i did put the thermostat from one of my other jeeps into it and took it for a drive. It sat at around 220 and didn't spike this time. Not sure if i just got lucky, or that fixed the problem.

Didn't check the weep hole or see if the fan was running either.

However, i did notice that the oil pressure gauge is also reading some weird numbers. That gauge sits at just about maxed out at times as well (i think 90PSI) and then when i let off the gas it drops to 80PSI.

Sometimes drops down to 50, then back up, down, etc etc.

Will look into this further tomorrow. After all it is my brothers jeep and i'm just trying to help him out.
 
Typically when you see those symptoms, the radiator cap has failed.

XJ standard 16lbs pressure on the cooling system raises the boiling point of the coolant. If the cap is weak, it will cause a pressure loss, the coolant will boil, and the temperatures will spike as the coolant flashes over to steam.
 
I suspected you're tstat from Texas start. Hope that's it.

My oil pressure read exactly like yours. Replaced 're sending unit. Made readings correct.

Me too. Just had a 0630 head crack last year on another one of my cherokees. Don't really feel like dropping a new head on this one as well.

We will see if that holds. Going to replace all the cooling system components tomorrow as everything closed today as it's Thanksgiving.

Don't have a mechanical oil gauge either so i might just buy the sending unit and swap it out and see if that fixes that problem.

Typically when you see those symptoms, the radiator cap has failed.

XJ standard 16lbs pressure on the cooling system raises the boiling point of the coolant. If the cap is weak, it will cause a pressure loss, the coolant will boil, and the temperatures will spike as the coolant flashes over to steam.

That makes total sense. Will report back once i replace all of the cooling components.
 
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