Any ideas what could be causing this?
It is overheating at idle. No coolant loss. When COLD with the radiator cap off, with the car started, coolant will overflow out of the radiator reservoir.

The car has sat for 4 years prior to this. Arizona summer temperature ambient of 100F today.

92 XJ 4.0
Replaced thermostat already.

Any help would be great. Kinda stuck here...

 

Sounds like the thermostat needs replaced

After sitting that long I'd probably give it a flush and new coolant as well.

 

original post states thermostat replaced already.

 

Did you flush the system? And I'm confused about where coolant is coming out of, the radiator or the overflow bottle?

 

I'd start with a full flush. Sounds like something is blocking flow.

 

sorry, thought the stuff down there was a signature and didn't read

 

the reservoir...overflow bottle...Same place to fill coolant up.

The car is running at idle 240F. Is that dangerously hot? On freeway its running 210F.

 

Yes, it should never see anything above 210.

You SHOULD have both the res and a rad cap, both can shoot coolant out.

Sounds like it needs a water pump or radiator. More likely a radiator if it keeps cool on the freeway.


Wait...what fans do you have? Mechanical? They working? If mech, the clutch good?

 

What I am saying is -- with the car cold, there should be no reason that coolant should be overflowing out of the reservoir RIGHT? I know when the coolant boils it will, but I am talking strictly cold it does it.

This is my brothers car, I will ask him about the fans. He says the clutch fan works, not sure about the electrical one...

I am assuming the 2nd fan (electrical) is given power both via condenser temperature/pressure AND coolant temperature, right? If so, what is the temperature that it should turn on at?

 

I'm still confused. How in the hell does water come out the overflow bottle if the radiator cap is off?

 

Yeah, I'm not sure whats going on anymore either.

 

Sounds to me like a blown head gasket or a cracked head. Exhaust gases are probably somehow getting into a coolant passage and causing pressure to build up in the cooling system.

So why was it parked 4 years ago? Engine problems maybe?

 

That's exactly what I was wondering. Who parks a good-running vehicle for 4 years?

The only way a cold cooling system gets pressurized is exactly what you have described as causes.

Isn't it funny how everyone blames the lowly thermostat first and changes it--to no avail?

 

That'd be my guess too. If you idle the engine with the radiator cap off, do you get bubbles, or coolant spewing out? There's pretty much only one way that can happen with cold coolant.

 

Compression test the engine that will tell if its got problems deep down.