Intermittent overheating problem - belt/tensioner/AC pulley??

[AVR2]

My 2005 KJ Limited has developed a weird intermittent overheating problem. The temp gauge usually sits smack in the middle no matter what, but over the last month or so, roughly once a week it'll start to climb after a period spent idling or moving slowly in traffic if you have the AC on. You know it's about to happen because the AC suddenly starts blowing warm air.

Turn the AC off and it goes away, and it'll then be fine for another week or so - you can have the AC on full while idling for long periods and the air stays cool and the gauge will stay rock-solid in the middle. And then one day it'll happen again.

Water pump, belt tensioner and belt are all less than a year old. Coolant level is normal, nothing's leaking anywhere.

Could it possibly be the AC pulley or clutch starting to clam up and causing some kind of drag (and thus extra heat) in the system? Or could it be the tensioner or belt? I had to pull away up a small hill earlier today, and I think I might have heard the belt slipping when I hit the gas. Would a loose or slipping belt be consistent with the AC going warm at the same time as the coolant starts to heat up?

 

[diyman]

It quite well could just be the fan relay, cheap fix. Just don't forget to use some contact cleaner
when installing new one. Relays are one of the most overlooked parts on a vehicle...
Diagnostics could require an advanced scanner which should force the 'relay out' of the PCM high,
thus allowing You to hear if fan is working. Sometimes relays seem to work ( they 'click' ) but their
contacts don't...

Guess electric fan is ok, ....isn't it?! Also, always carry a couple of relays in the tool or glove box..

Just my 0.02...

 

[AVR2]

I have the max cooling package, so electric fan plus mechanical fan. You can't see the electric fan and it's a PITA to get to it because you have to remove the hood latch and crossmember and then disconnect the mechanical fan drive first.

 

[diyman]

Wait AVR2, most Jeeps have the HD cooling here ( fan clutch etc....) but I've just helped a friend
whose WK had the same issue as Your KJ and it was a damn relay. No need to disassemble anything.
Relay is in the fuse/relay box IIRC.
Electric fan should be 'heard' , not 'seen' working..., so no need to perform dangerous manouvers in
the engine compartment.
If no such scanner is available, You could try opening the hood, starting engine, and have someone
turn A/C on and off while You try to determine if fan is kicking in (when A/C is on).
Fan noise can be heard over engine noise, if You listen carefully.

This is IMHO, more experienced members chime in if You will....

Let us know.

 

[AVR2]

I'd really really like it to be the fan relay because it's such a cheap and easy fix, but do relays go intermittently bad? Like once a week?

And does HD cooling absolutely depend on both fans running, even while idling? The mechanical fan is so much bigger and beefier than the electric fan that you'd think that you simply wouldn't notice if the electric went out, unless you were making really heavy demands of the engine - certainly not while idling in traffic.

 

[rockymountain]

I'd really really like it to be the fan relay because it's such a cheap and easy fix, but do relays go intermittently bad? Like once a week?

And does HD cooling absolutely depend on both fans running, even while idling? The mechanical fan is so much bigger and beefier than the electric fan that you'd think that you simply wouldn't notice if the electric went out, unless you were making really heavy demands of the engine - certainly not while idling in traffic.

yea sometimes they start out getting flaky and if you wait long enough they simply stop working altogether gradually. USUALLY. I'd have to agree with diyman since it is so intermittent.

 

[AVR2]

yea sometimes they start out getting flaky and if you wait long enough they simply stop working altogether gradually. USUALLY. I'd have to agree with diyman since it is so intermittent.

OK, I'll check it out. It still seems weird that having that thin plastic fan stop working could cause the coolant to start heating up on idle when that big heavy mechanical fan is there.

 

[rockymountain]

OK, I'll check it out. It still seems weird that having that thin plastic fan stop working could cause the coolant to start heating up on idle when that big heavy mechanical fan is there.

They run hot as it is at 200 degrees F most of the time while driving. They heat up quick too when you come to a stop. The mechanical fan just doesn't move enough air while sitting. Mine will run at 200 after moving for a while and then when I stop it gets up to about 204. Then I take off and it goes to 206 - 208 and then comes down. The fan comes on at 220. IIRC


btw I don't have HD hd cooling and I got those temps when the AC was off.

 

[LibertyFever]

You can test the operation of the electric rad fan by jumpering 12v to the connector at the fan. It should be a three pin connector, a common and Hi & Lo speed. Beware, the jumper wires will get hot, the fan draws a lot of current.

If you try to spin the mechanical clutch rad fan while the engine is hot you should feel a resistance.

How clean is your rad? I've had mine get plugged with mud causing the engine to easily overheat. A simple flush with a garden hose cleaned it all up.

If all else fails it could be your thermostat sticking closed. My golden rule is to replace the cheapest things first.