Hi. About 2 months ago, we sat in the Jeep with it idling for about 20 minutes. The temp went way up, and the check gages light came off. Shut it off immediately. Noticed the fan was not coming on. Let it cool down a bit, poked around. Unplugged the fan, plugged it back in. Had to drive home. Didn't overheat at all on the ride home. Noticed the fan was now running. The problem never reappeared until yesterday. Did not drive it today.
I did a search, and I am leaning towards the relay being bad. One question I have is, can the relay go bad intermittantly, or is it either dead or not dead? Also wondering if the connection is bad. i am going to look at it again after work. I found this http://www.lunghd.com/Tech_Articles/..._Diagnosis.htm and was going to use it as a guide.
Thanks.

Does the aux fan come on with the AC?
If so, I'd look at the temp switch. I'm not familiar with your jeep, but I believe it's either on the thermostat housing or on the head near the firewall.
*Does your jeep have a fan clutch (fan on engine)? Usually overheating while idling is due to a bad fan clutch.
*On your relay, I think many of the relays are interchangable.. you could try swapping in a different one. Might as well clean 'em up while you're at it. Hope this helps.

With electrical gremlins ya never really know. Yes I have seen relays go bad intermittantly. Something about heat and metal expanding yada yada. Anyhow, just replace the relay.

I ran the Jeep for about a half hour last night. The fan seemed to come on as it should. The temp never went over 210. Is the relay a solid state relay? Can solid state relays work intermitantly? I'd hate to spend the money and not need it, but then again, better to be safe I guess.

hey watch your fan turn on in the dark. look at the motor it might spark on the back side. that would mean it is shorting out some times witch would allow it to over heat at idle. i recomend replacing th fan and the relay switch

Solid state relay

If it were me, I would replace the relay first. At $20-$30 it is cheap insurance and peace of mind. It may also be your fan. It's 13 years old and may just be time to replace it. If costs are an issue you can try going the junkyard route.

I may just replace it for peace of mind. My girlfriend works at a Chevy dealer, which also owns a Jeep dealer, and they told her it would cost her $60 with her discount (it's actually her Jeep).
I will take a look at the fan motor in the dark too. Thanks for the advice.

Take your hand to the fan blade, see if it spins freely or with any resistance. Resistance would indicate issues with the fan motor. This is how it was explained to me recently..

Relay was my issue on the wife's DubJ

I just checked that link you posted.. you need to read this thread here.

http://www.jeepforum.com/forum/f310/...-relay-420117/

I added the pics showing where you need to cut the bumper cover beneath the pass side headlight.

Happened again today, sitting in the drive thru. Temp spiked up, no fan. Popped the hood, and after a couple seconds the fan came on. Shut it off immediately. Waited a while for it to cool down. Drove it home, about 8 minutes, didn't overheat again. The fan spins freely by hand, so I am pretty sure it's not siezed. I am back at work now, and my local aftermarket supplier says $7 for a relay. That kind of sounds low, so I said send it, and i will look at it. Are aftermarket relays ok? I am getting a t-stat and gasket too, just because. Coolant too.
Is there a regular relay too? Ugh....now I am getting confused. the thing that gets me is that it is intermittant.

Pulled the headlamp, the relay has been changed before, looks like an aftermarket one. Think ill pick up a relay tomorrow. I am going to replace the thermostat and coolant, then cross my fingers.

I think you should be good by changing the relay and the T-stat. The fan seems to be working its just not getting the signal.

I agree. I have the parts. Any special instructions for bleeding the air from the system?
Thanks.

On the 4.0 there is no bleed screw/bolt.

http://www.wjjeeps.com/service/cooling.htm#DRAIN40

Have to use Mopar coolant?