Hey guys this is my first xj its a 1990 4.0 with 135k miles on it. picked it up for a steal for 2k. everything is in great condition. I bought it an hour from where i live and it drove in just fine. As i got it home though it kinda heated up a bit. The guy explained that it did heat up with the ac on at an idle. SO i replaced coolant resevoir, thermostat, and water pump. I flushed the system 3 times. The fan is on a switch so i can control it from the dash, but it also comes on with the ac. so here is the question, now that i have replaced all this I just overheat all the time. what the heck!! also after i fixed everything I can drive for 5 minutes and the jeep will stall. so im really disappointed. i havent even been 4x4ing yet and im kind of afraid to until this thing is running like a champ. I have read so many threads and it all says the same thing. "I had this problem once, not sure what fixed it, but i know it was a pain in the ***" Great! Doesnt really answer the question. also one final side note, engine idles really low like 500 rpms, Cuz the guy said he set it that way. Please help me!!!!!!!!

 

search key is your best friend on this site...so many threads about xj's overheating almost all of them have or do at one point or another...

 

Have you checked for any leaks? Also could of caused some problems when you changed all that stuff but not sure what excatly. Could be ur fuel pump maybe going bad too!!

 

I had the exact same issue with my 1990, What solved the issue? Well a bit of everything. I just like you had no idea what to do about it. I tried replacing the coolant bottle, then the t stat, then the radiator and it did nothing, then I tried a new E-fan still nothing, then the water pump and ding ding ding problem solved. Essentially I had to replace every part of the cooling system but it was worth it as I have not crept over the 210 mark in 2 years.

 

Just throwing a parts grenade at the thing is not the answer. Well I suppose it could be but that's expensive. First things first I would take a note on how much coolant it is using. If you are having to put in coolant every so often then you have a leak. Coolant doesn't burn up so if you are losing any then it's leaking from somewhere. And that will cause it to overheat. Good Luck

 

Ditto on the search idea, there are a million threads on overheating XJ's, many of the other problems as well

Also, get a hold of a clymer or a haynes manual, they are an amazing source of information on the truck. It will walk you through the factory specs of doing stuff

How hot is it overheating?

If it has changed since you messed with it, then make sure you did everything right, is the water pump in right, did you put the thermostat in the right direction?? is there enough coolant in the system? I am not saying that to insult your mechanical skills, it never hurts to double check. In my house we call those "pretty" moments

I dont know about this whole low idle thing, is the 90 4.0 carburated?? Mine 98 sure isnt. Anyway a low idle can cause a crap load of problems, over heating and stalling to name a few

it might be worth your while to start with good old tune up stuff. Take a look at your plugs, cap and rotor, if they look old get new ones. Oil, air filter, CCV valves can all impact how the truck runs.

Clean out your throttle body, unless you have a carb, and then adjust it and clean that too.

If you can get the jeep running smooth then the rest of problems may get better. At the very least if the truck is idling better, the other problems may be easier to identify

 

Okay guys lots of great responses and I love how quick everybody threw in the knowledge and here is the continuation.... my buddy did the work with me and we double and triple checked everything from the tstat to serpentine belt tension. yes I have a haynes manual, but its only helped me install and remove... not super amazing for troubleshooting. Searched lots of threads and one that helped was the mounting bolts for the ecm. if they are not grounding properly it can cause the stall. so following that guys directions i had absolutely no stalling since. Thank you forums!!! but now we are still overheating. This 90 is not carb and yes i have changed spark plugs and wires and oil.... oh and the temperature sending unit.... now here is a big culprit and most arrows point to this. when i bought the jeep the coolant in the resevoir was dirty looking, but no water in the oil so it ruled out crack in the block. upon flushing system and removing hoses, coolant is a red rust color and lots of red rust color stuff in the well tsat housing and on water pump. which can obviously cause a blockage, should i just get the coolant system vaccummed at a jiffy lube? also failed to mention guy i bought this jeep from was as honest as the day is long and told me he replaced the radiator and it looks brand new so i believe him. just to rule out more things that could/have been replaced. thanks for the help and brainstorming!!!!

 

Replace the radiator, it could be 21 years old and no amount of flushing is going to remove rusty, scaly build-up........sounds like that XJ has had some serious neglect and the owner lied his *** off. Go thru it bumper to bumper.

 

This is just a shot in the dark... but at least it's easy...
I'm wondering if the low idle has something to do with the overheating...

I'd pull a couple spark plugs and see how they look. I'm wondering if somehow you're running lean. I'm not sure this would cause an overheating problem, but I always thought lean-running engines ran hot. And I'm not sure why a lean engine would idle low, but like I said, it's easy to check - just pull a couple plugs and compare to various plug pictures.