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My daughters 2004 Grand Cherokee (4.0) a week ago Friday threw a code - said Thermostat, so I replaced the thermostat, OEM 195, easy job. Drives for a week all is fine. Last Friday it overheats - she has it towed to my house. No leaks, system is full, pressurized, all looks fine. When she bought the car 3 yrs ago it was loaded with stop leak, which I told her was a big concern but it was too late. But that was 3 yrs ago and it's been fine. My first thought is that when I changed the t-stat and replaced a lot of coolant that it stirred stuff up inside and maybe the t-stat was stuck. Took it out, looked fine, put it in water, heated, it opens fine, etc, cabin is smokin hot, etc. Reinstalled t-stat, new gasket. On to plan B - radiator, was original, and with all the stop leak thinking ok that has to be clogged and the problem. Replaced it yesterday, refilled the system again, still overheating. Water pump is only item left , showing no signs of being bad, not weeping, pulley is true, no noise, but replaced it anyway. Refill again, STILL overheating. UUUUGGHHHH......

I do not think it's a head gasket as shows none of the expected signs such as exhaust vapor, no coolant in the oil (its clean), no oil found leaking, idles fine, starts fine. Not likely a temp sensor as that would throw a code and there are no codes. Now - the only thing I've noticed is that the electric fan doesn't seem to be turning on until it's pretty hot past the 195 -210 operating range, and I don't know if the fan is functioning at full capacity as it's not my car, but the fan is essential to cool if just sitting idling but would that alone cause an overheat? There's nothing left as there are not that many moving parts to a cooling system. Again for the past 3 yrs the car has been fine - no issues, and even a week after t stat change.

Would REALYY APPRECIATE and insight here as I'm at wits end on this one. The only good news is it's easy to work on, but I'm tired of working on it.

THANKS!
 

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2015 WK2 Overland
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It should have a mechanical fan, yes? When spun is there a good amount of resistance? I'm wondering if the fan clutch is not so great.

210 would be a normal temp for the 4.0 so if it does have some type of electric fan turning on above that temp would be an effort to further cool the engine. That's how my XJ worked. The mechanical fan would spin all the time, the secondary electric fan would kick on around the 210 mark to keep things under control.

If no prior overheating issues before the thermostat replacement I'd lean towards air trapped in the system. I've always let my older cars idle on a hillside with the radiator cap as the highest point to allow all the air to work its way out.
 

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It should have a mechanical fan, yes? When spun is there a good amount of resistance? I'm wondering if the fan clutch is not so great.

210 would be a normal temp for the 4.0 so if it does have some type of electric fan turning on above that temp would be an effort to further cool the engine. That's how my XJ worked. The mechanical fan would spin all the time, the secondary electric fan would kick on around the 210 mark to keep things under control.

If no prior overheating issues before the thermostat replacement I'd lean towards air trapped in the system. I've always let my older cars idle on a hillside with the radiator cap as the highest point to allow all the air to work its way out.
the 2004 didn't have mechanical fan only electric . If I let it idle without the cap on coolant starts to come out of the filler neck after about a minute - is that normal?
 

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Is that with the engine hot or cold?

Sorry about the fan, the parts site I was using must have their stuff wrong. Switched and I see the e fan now.
 
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