Overheating wtf...

1990 xj, 4l.
5 inch lift on 31s.



My jeep started overheating recently, this was after only being driven once or twice a month through the spring.

It first happened climbing a mountain here in BC. Then after it overheated there i noticed it would hit the red when on the highway at around 100 kmh. When driving around down or sitting at a light it doesnt seem to go over normal temp (~100 c)

I have..

replaced the thermostat
checked all hoses
burped twice
flushed till crystal clear, heater core too
added electric aux fan from newer jeep and put it on toggle, didnt do much.
radiators fins are clean and clear
new pressure bottle and cap.
Did a compression test to make sure no blown head gasket, everything between 130-150 psi,
no coolant in oil
no oil in coolant

im about to replace fan clutch but i figure highway speeds should work just as good so it "shouldnt" be the fan clutch.

One thing i did notice though, when driving, if i stomp on it, once the rpms get to around 3k it seems to have a serge of power! not sure if its related or if thats a whole other issue.


I need ideas... Thanks in advance!

My vote is your headgasket.

Headgaskets are tricky beasts, they can manifest themselves in many different ways. I've never had low compression due to a headgasket, and you dont always get a mixing of coolant and oil.

Do you notice the coolant level dropping?

In fact, I had one on my 4.0 and it had no symptoms other than what you describe. Overheating under load is the tell tale sign.

Time to yank that heavy piece of iron off and change the gasket.

the coolant hasnt changed at all.

should i do a pressure test? not sure how to do it on a renix

Compression test won't necessarily show a head gasket problem. Need to do a leak-down test. Also, check the cap on the pressure tank...if it won't hold pressure, you'll overheat.

go to get a block tester. it checks for exhaust gasses in the coolant

I would verify the accuracy of the temp gauge by either sticking a good ol' thermometer
into the rad filler neck - or use one of those fancy-schmanzy infrared thermometers
and take a reading from the engine side of the radiator in order to not confuse it by
inadvertently reading the AC exchanger (front side of radiator).

Find someone with a block tester and check it for combustion gasses

Replace the clutch first, with a ZJ clutch. Get an IR temp sensor and see what the radiator cold and hot side temps are and report back here.

Is the Renix E-Fan coming on, if yes, at what temperature, on and off?

Is the shroud still on the mech fan?

What radiator does it have, specs, and age????

Check for an exhaust leak blowing on the block!!!! And check the EGR valve!!!!!!

If it is a stock black Renix bottle cap (that comes with the bottle), toss it and get a stock Volvo stock bottle cap, the green ones.

Note from DJ

Here is the information on the cap Ecomike was referring to.
Fits the following models:Volvo 740 1986-1992 and a bunch of other Volvos
Go to NAPA and buy this cap: 7031396
Product Line: Balkamp</SPAN>
Dimension: 2 63/64" W x 1 3/32" H x 1 13/16" W Base
Manufacture: Stant Manufacturing P#10244
Pressure Rating: 22 lbs.
Note: Sometimes you have to push the cap down with a fair amount of force till it catches the threads. Do to the fact that the ID of the bottles very slightly.

I thought it was a 16 lb cap, 22 lb really?

Try this, get some CLR, take all the hoses off the radiator, plug the bottom hole and fill it with CLR. let it sit a bit and flush it out.
might want to see how much water flows thru it before and after the CLR treatment.