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XJ overheating at idle but everything is new?

715 Views 8 Replies 9 Participants Last post by  edrrt
I know I know... There are a million "XJ overheating" threads but I'm really at a loss here even after reading every forum I could find...

I have a 1991 4.0, AX-15, XJ... I overheat while idling, but here's the catch... Everything is new on that car! Motor has 3k miles on it, entire cooling system is new, has new fan clutch out of a ZJ, runs fantastic! I burped the system several times, no bubbles... But what doesn't make sense is it doesn't overheat every time I idle, I went a week with no overheat, then today it overheated once when I got home... If I open the vents it cools down basically instantly... I was an hour in traffic is 100+ degrees with AC on last week and didn't overheat... It's totally random

Any ideas?
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Is the electric fan working correctly? Is the fan shroud installed ? Is the engine running rich ?

Is the radiator cap rated at 16 lbs and a Stant or genuine Jeep part ?
Also might want to change the temp sender on the thermostat housing. If your electric fan isn’t coming on at temp without the AC on then it might be reading incorrectly.

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What brand radiator did you put in?

Ensure thermostat and rad cap are quality brand.
Simple and cheap enough to try a new thermostat and see if that is the problem. Any chance you got air trapped in the system? (I had a similar problem with a non jeep vehicle once and it was air in the system)
Aftermarket caps and thermostats are sadly crap. for thermostat I would only trust factory or Robert Shaw. Caps only OEM
When you burped the system did you have the heat on and full? Some times you can get air in the heater core.
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Are you sure it 's the right pump? they have a reverse rotation one for postal vehicles. You could have easily ordered the wrong one on rock aut or whatever. I believe they look the same except the impeller.
Sounds to me like your electric fan isn't kicking on when the AC is off but engine is hot. You need to verify the temp sending unit is working and the electric fan.

2 reasons.

1) It overheated when you got home. This is usually because when running the clutch fan and driving are enough to cool the radiator. When you shut off it relies of the electric fan for a bit to stay cool after the clutch fan stops. My radiator literally exploded on my XJ after I got home and turned it off because I forgot to plug in my efan after adjusting my stearing box. Bad radiator caps cause boil over on shutoff commonly. If that sounds like you replace the cap.

2) It doesn't overheat sitting in traffic with the AC on. Turning on the AC forces the efan on OR your efan may not be working but the clutch fan is enough.

I highly recommend you pull the thermostat and verify it opens fully and reliably in boiling water at the specified temp. They fail a lot.
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