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99 XJ Overheating


NAXJA Forum User
I have recently purchased a '99 XJ. I found it behind a building and the business owner sold it to me for a song.

Since I purchased it (not running) I installed a new belt, a new battery, a new fuel pump and a new thermostat with hoses.

The thing keeps overheating. It doesn't do it right away. It takes some time at idle. Today I had it steady at 230 or so for a while.

There is a squeeling when you start it up and sometimes when you first accellerate. A friend said it might be a bad fuel pump but I get pressure in all hoses.

How much coolant should be in the system? Any input would be appreciated.
Squeeling at startup could easily be a belt that is not tight. The XJ belts need to be super tight compared to other vehicles I have owned. Tighten the belt and see if that cures the squeeling.

Overheating could be a bad water pump, clogged rad, etc. First thing I would do is do a flush and see how much crap comes out. It could easily be the original radiator. At 20 yrs old - it probably has tons of corrosion.
sounds like diagnosis and then maybe a cooling system rebuild (water pump, thermo,radiator, hoses,fanclutch). is the e fan coming on?
Thanks guys! Yeah. the Efan is coming on.

What is the fan clutch? I know a lot of guys on here have talked about them as an issue.

If the radiator was clogged, why would I get pressure at all the hoses and fluid running in to the thermostat housing?

How much fluid should be in the system? I noticed some backing up in to the bottle and it overflowed last night due to boiling.


The fan clutch is attached to the mechanical fan. It allows the fan to "freewheel" when the Jeep is in motion. If the clutch is not engaging the engine will run hotter when the Jeep is not in motion or moving at slow speed. A good rule of thumb for coolant capacity is around 2 gallons. When refilling your coolant it is a good idea to refill from the upper radiator hose first. This lessens the chance of air being trapped in the system. Once it is full there, attach the upper hose and fill the overflow bottle to the top of the inner stem. Idle the engine with the heater set to high and the overflow bottle cap off until the thermostat opens. Let the engine cool and recheck the coolant level. Add as necessary. Does your coolant bottle constantly vent while hot and running? Constant overpressure could be a sign of a blown head gasket. If you tighten the serp. belt and still hear a squeal your water pump may be bad. Is coolant weaping from under the water pump?
It takes some time at idle.
But does not overhead at any real speed right? Bad clutch fan .
If it overheats at speed. 45 MPH with light loads. Bad rad, Bad T-stat (new or not) or T-stat in backwards. clogged rad inside and/or out. AT very last water pump.
I have fixed two overheating problems this year do to T-stat in backwards.
I believe I put the Tstat in correctly.

Thanks for the input guys.

I will have to try running it with the radiator cap off until the Tstat opens. I'm thinkin I may not have enough coolant in the system and too much air.

The fan spins when the engine is running. Wouldn't that mean the clutch is working?

I don't have any coolant weeping from the pump.
Wouldn't that mean the clutch is working?
Fan clutch are a hard call sometimes. They generally go bad slowly. Keep slipping more and more until they just don't move the amount of air needed to keep the engine cool under a given load. The E fan will kick in sooner and more often but because the breakdown of the C fan can be so slow you may not notice the e fan working harder.
Key off engine cold. Spin the C fan by hand. there should be very little drag to it. There should also be VERY little wobble to. Then warm engine to full operating temp. Then key off and very quickly try to spin the fan. There should be a much larger amount of drag. If not replace C fan. You should know what the drag on a good warm c fan feels like before trying this test but your just going to do your best.
If the stuff looks old, I would consider an entire cooling system rebuild as mentioned.

Parts aren't expensive if you can do the job yourself.

Radiator, water pump, fan clutch, radiator cap, (you replaced stat and hoses already) and you should be back to good temps.

BTW, the squealing is almost assuredly the serpentine belt. As already said, they need to be awfully darn tight on the Jeep 4.0.

Good luck!
2000 XJ Sport 4.0 4WD 147k miles

Is a radiator flush using a store-bought kit (like Peak Radiator Flush and Fill Kit - ~$4) sufficient? Or, is it better to got to a shop and have the system flushed (~$80)?
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Make sure there is enough coolant in the system, if that doesnt work sounds like a clutch fan if its overheating at idle. If you are going to replace the fan clutch upgrade to the HD zj fan clutch. the part number is NAPA 272310. $44. Do not get the max cool as it will not fit. its bit louder than the stock one but moves a lot more air. well worth the extra $5 from the stock one. its a very easy swap, and well worth it. try that and see if that works..
For the belt squeal, you'll NEED a Krikit II belt tension gauge from NAPA; 10 - 15 bux. Belt tension should be ~185 to 190 lbs. for a new belt, and ~170 for a used (not new) belt.
A "new" belt is one with less than 15 minutes of engine run time on it iirc.

The 1/2" belt deflection, or 90* twist tests are no good on the 4.0.