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XJ overheating at idle but everything is new?

707 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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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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