My wife was driving our jeep and ended up idling in the drive thru for about 5 min when the temp shot up to around 260-normal driving the gauge reads 210..what should the first thing I check--thermostat? or fan motor-and how would I check these? Jeep has just over 126 K miles.

 

"Jeep has just over 126 K miles"

Thermostat prevents under temperature operation, can't control overheating.
What kind of maintenance has this 13 year old vehicle had:
everything?
nothing?

 

Yes the thermostat does control under heating but it can fail and shut close, preventing the coolant from flowing out/in of the engine. You can test the thermostat by putting it in a pot of water and bring it up to a low biol and see if it opens, if not then that's it.

Could have a clogged radiator, electric fan may not be working, thermostat, water pump, etc... Got to tell us more so we can help

 

Start simple. More than likely, you were low on coolant and that could be from a variety of things. One, a leak. Check all hoses and pull away any under shield that could be collecting spilled liquid that would normally leave a puddle.

If you're leak free, than perhaps you have air trapped in your coolant system. You can "burp" the coolant system to get all the trapped air out.

If that doesn't do it, then check the electric fans. If that doesn't do it, check the fan clutch.

Then you can move on the thermostat or other things if the problem continues.

 

I own a 2000 WJ with the 4.0 6-cylinder. If you haven't had any problems with it before this, I'd bet it's the electric fan. Start it up in the driveway and wait for it to overheat. The fan should kick in at about 214*. I'd be willing to bet it's the fan relay, which I'm told is UNDERNEATH the headlight panel on the passenger's side. The EASIEST way to get to it is cut a hole in the panel. This is described elsewhere in this forum. You can check the fan's function first by hot-wiring it to see if it runs. Mine is a single speed fan (at least it only runs in one speed) so it should be easy to check.

The fan is turned on by the PCM (or computer) so that's not usually the problem. I've sat for hours in the winter waiting for the newspaper truck, idling to keep warm. When the fan kicks in, there is a slight throb in the steering wheel and floorboard. I think it's slightly out of balance. My Jeep has NEVER overheated in 135K miles. It only has the electric fan.

 

I'll check the fluid level when she gets home-we've only had this for a few days--bought as-is-no warranty-normal driving it stays around 210 on the gauge-I haven't seen anything leaking on the garage floor. I'll let post when I know what was causing it.

 

Yeah id check fan motor, if fan runs off hotwire but doesnt ever switch on would be relay

 

It was the fan motor--got it replaced today---everything else was ok. Thanks for all the info.

 

Not a problem, glad to help. Good you caught it before it toasted the motor!