xeph

1999 Jeep Grand Cherokee Laredo 4x4 6cyl Automatic 100k mileage

As the topic said, my jeep is being very fickle about overheating. It might overheat one day and not the next, driving under the same conditions, distance, time of day etc...

Initially, I thought it was the cooling fan because the A/C would run cold when moving and hot when idle. It was of course overheating at an idle when this occurred as well.

The day I took it to the mechanic, it was just perfect, cold A/C at an idle, did not overheat. My mechanic kept it for two days, said he tried like heck to get it to overheat, no dice. So I picked it up, went home, all seems fine. Next day, overheating.

Any idea what might be causing this flip-flop inconsistent overheating and then not overheating on any given day?

Thanks for reading.

Pelican

Did your mechanic verify temps with an IR thermometer?

GarrettJ

Does the temp level out or just keep climbing?

steelybill

Maybe a sticky thermostat ? I've had that happen twice on other cars.

Rockhead

I don't know if the coolant pump could be intermittent like that, I don't think so. I agree with the thermostat acting up. Replace the thermostat and have the system flushed. Good luck.

GarrettJ

100k miles and 15-16 years old..might want to flush and replace to cooling system..duno about your vehicle but everyone seems to be replacing the entire XJ cooling system for $250 or less..worth the investment in my eyes

JEvans

On an 01 GC, there's a relay for the electric fan on the radiator. My daughter's Jeep would run hot for no reason, this relay was the culprit. Worse thing was the location ( boxed in under right headlamp assy. No easy access).

dave1123

Yes, this is a "pulse-width modulated" relay controlled by the PCM. It is located underneath the right headlight mounting bracket which requires removing the bumper fascia or entire front plastic mounting panel to get to. There are posts on this site about how to cut a hole in the headlight mount to get to it instead of removing most of the front end. The dealer tells me the reason it's mounted there is to isolate it from engine heat, but I don't see why. Why not mount it in the fender well behind the liner? After all, that's where the horns are.

The PCM controls the fan to turn on and changes it's speed when necessary. The higher the needed speed, the faster the pulse rate. Mopar couldn't make it simple, could they?

xeph

GarrettJ - when it does overheat, it will steadily climb especially if at an idle. If I let it, it will redline eventually.

Pelican - not sure if he used an IR therm, but I will make a note of that on my next trip to him.

Thank for you all the helpful replies. I am taking notes of your suggestions, and will consider the suggestion to flush the cooling system, I know that has not been done in at least 10 years.

GarrettJ

If hasn't been flushed in 10 years and its just a steady climb up it could be a restriction.. Sounds like the coolant isnt moving fast enough

xeph

Just an update, if you're curious. I took it back to my mechanic when it was overheating today, turns out the cooling fan wiring was damaged and the plug had actually melted. Plug replaced, damaged wiring replaced, running good so far.

On another note, when I picked it up, I told my mechanic I had not had it flushed in probably 10 years, he looked at me like I was crazy, made me promise to bring it in soon to get it done.

Thanks for all the advice!