Ok for starters its a 98 XJ 4.0L Auto A/C. It got hot on me while wheelin a while ago and I blasted the mud off the exterior of the radiator and didnt have anymore problems, for a while. Then a few weeks ago the temp red lined at a stop light.Sick of the BS I replaced the water pump,serp belt,temp sensor, thermostat, and cooling relay. All NEW parts not reman. Now, if I dont leave my heater on HI she will over heat for no apparent reason while crusin down the freeway at 65,idling in the driveway,at a light etc. And whats REALLY annoying is its not consistant,some days its fine some days its not. Outside temp doesnt really seem to matter,it overheated today and the outside air temp was about 35*F. Any ideas?

And yes I searched before posting this...

 

electric fan, relay, broken wire in the fan circuit. fan clutch (if you have one). mine did it to me a few weeks ago and it was my fan clutch and the electric fan quit working at the same time. let me know how you make out.

also, you may want to look into a FAILSAFE t-stat. they lock open when you hit 250 I think to eliminate the pressure and let the coolant flow freely. It's what I use in mine , just a thought.

 

Radiator.

 

I did the failsafe T-stat already no luck. When I changed the water pump I checked the radiator,it appears clean inside and water flows freely from hose outlet to hose outlet. Elec fan works and I put a switch on it to be sure. Clutch fan appears to be good. Also no oil in anti-freeze and no anti-freeze/water in the oil. Has plenty of anti-freeze in the radiator and in the coolant resevior. Im stumped...

 

Have you back flushed the heater core? With all that you have done, and you claim the fan clutch is good, that would be the last thing I would do. After that if its still overheating you have either a cracked head or blown head gasket. Just because theres no antifreeze in the oil doesnt mean the head didnt crack or the gasket didnt blow. trey the heater core flush first. could be a minor stoppage of flow that only acts up occasionally. What about your rad cap? have u replaced that?

 

Could have water jackets plugged in the heads(I think that's what they are... the ports in the heads) my wifes grand am did that on engine number 2

 

Only thing u haven't replaced is the radiator & fan clutch. Shouldn't need any fans to adequately cool at hwy speeds. U can have scale build-up inside the rad that will greatly reduce its ability to transfer heat. Sounds like your heater is acting as your radiator.

 

Ya the rad cap was the first thing I tried.What would be the best place to back flush the heater core from? The heater/AC assembly on the 98's is a giant enclosed piece of plastic that goes thru the firewall to behind the dash,there is no easy access like the older ones. I havent paid to have the cooling system cleaned/descaled, Im a low budjet jeep junkie, but that may be where this is leading. I was just hoping the wisdom of the masses would point out something stupid I missed.

 

you say you flushed the radiator, did you flush through the condensor coil? Sometimes crap builds up between them. Coach

 

My 99 will sit at 212 all day long till I start pulling a hill. Then suddenly will shoot over close to the red line and the dash light "Check Guages" will illuminate. If I turn off the air (if its running) and crank the heater then after a few minutes it drops back to 212 like nothing ever happened, no matter the outside temp.

If this were coolant overheating the gauge should rise steadily and a tad slower, and then drop at the same pace back to a cooler indication after some time of change in driving. For this reason I believe an additional temp switch is involved. Ive already changed the sender and t-stat. Radiator flows well.

Anyone know if there is another temp sensor on maybe the trans or possibly the oil system showing in the manual? I havent had time to look in a manual but Ive come across others on the net that have run into this same issue and they have tried replacing the radiator as well as everything else in the cooling system with very limited luck.

My electric fan comes on and runs fine.

My belief though is that the trans temp has something to do with this. Im doing a rebuild soon as I carry a lot of weight and have signs heavy wear in the pan. I will be comparing before and after.

 

x2 I didn't think to mention that. It comes as second nature any time I flush or clean anything I do that. Sorry blonde moment lmao

 

New rad available for +/-$125.

 

Radiator cheaper yet:
http://cgi.ebay.com/ebaymotors/New-1...Q5fAccessories
or 1 800 Radiator around $100

 

In the event that anybodys desperatly trying to solve a similar problem, it ended up being the radiator. The radiator had a cracked seam that only leaked under extreme pressue and was full of scale. $100 bucks later (new radiator from Amazon) and my jeep wont even hit 210*.

 

Nailed it!!! Great job. Coach