overheating at idle

hello everyone i have an 89 xj 4.0. as of today (or at least as far as ive notice, just got the jeep) its overheating at idle, like when im sitting at a light or something. but when i start moving it drops right back down to normal.

also when i got home and turned it off, i turned the key back on the look at the temp and the gauge was still going up (with engine off). so now im not sure if it is even overheating, could it just be the gauge? assuming it is actually overheating what would cause it to slowly rise up at idle but go right back down to normal in about 3 seconds once moving? just the fact that it goes back down to normal so quick makes me think its the gauge... any help please!

Sticky thermostat? The increased off-idle water pump flow pushes it back open- seen that a few times. Electric fan coming on?

Check your fluid level. You didn't mention it yet and no else said it! Funny things can happen when the fluid is low. Also, unplug your wire to the temp sensor (for yours it's on the rear of the head, sticking up on the driver side) and clean it and check it again.

there is fluid in it, i have a leaky heater core so i gotta top it off. as far as the electric fan, I'm not 100% on this but as far as ive seen the electric fan only comes on with the AC. obviously the mechanical fan is working though.

but i dont quite understand why the thermostat would make it overheat at idle but go right back down once i start moving, thats why i was thinking its a fan issue. so the electric should come on as well (not just with ac)?

but with that being said has anyone else had an issue like this? specifically with the gauge still going up with the engine off

Temperature climbing at idle/stop and go, drops back down when you get moving--classic fan clutch failure.

The coolant temp will rise after shutting the engine down. Once you shut off the motor, the coolant stops flowing and will absorb heat from the block/head, causing the temp to rise. That's nothing unusual.

As for the overheating issue, it sounds like you have air in the system. You may want to cap off the heater core until you get it fixed and make sure you bleed the air out of the cooling system. Do a search for how to burp the system, it may help.

Of course Joe is right, a fan clutch would be a likely culprit as well...

ok couple question, first off how would i go about capping off the heater core? just disconnect the heater hoses and run a hose between the 2 bypassing the heater core?

next as far as the fan clutch, is there a way to test it? and also how complicated is an electric fan conversion.

and last i did a little research on "burping the system" and i couldn't really find anything definitive, i saw something about removing the temp sending unit and let some coolant come out, but that doesnt seem like it would do very much. does anyone know what the procedure in the FSM is?

ok, i was just out looking at my rig, and this might be a kinda stupid question, but should the mechanical fan be spinning at all times? from the moment i turn the car on the fan is spinning, but my old car did this too. so let me know if im wrong but my guess is when it's freewheeling its still spinning but when the clutch engages it spins faster (with the engine speed)?

I didn't read everyone's posts, sorry if I repeat advice. One thing i have learned while helping people out on naxja is that Joe Peters knows his stuff. If I recall he was a factory tech? Take his advice when you can. I too am a Chrysler certified tech, but id listen to him over me haha.
This definitely sounds like an airflow issue. If your xj has ac, and it doesn't work. remove your a/c condenser. this will help airflow more then you know. also, check your fan clutch. **WHILE THE ENGINE IS OFF** spin the fan. It shoulnt "free wheel" which means it should stop quickly after you spun it. If it freewheels, replace it.

now, the fact that your heater core is leaking most likely has a lot to do with this problem. there is probably air in the system. you need to bled off that air.

To bypass your heater core connect the two hoses that go into the bulkhead "firewall".

Do you still have the pressure bottle on your passenger side of the engine bay? If so I highly suggest you search how to do the cooling system conversion, it will save you lots of time later and in my opinion is more reliable then the jeeps factory system of the early xjs. I don't have anything against pressure bottle cooling systems, but the old jeep bottles have a tendency to crack(leak). so I do hate those.

and for what it's worth, i don't have the factory electric fan installed on my jeep. I have no a/c condenser, just the good ole mechanical fan and just run the factory 195* thermostat. my jeep doesn't get hot at all. not even on the trails does it get hot.
A good reliable jeep is a great jeep. I hate it when I brake down wheeling because my jeep wasn't mechanically sound before I went. Always fix your jeeps problems before lifting it or buying any after market stuff. This makes for fun wheeling, camping trips when you don't break down.
alright I'm done. lol
----- Chris