So I acquired a 1991 xj for $300, replaced the fuel pump and filter, changed the firing order (perviously had the coil going to a spark plug output) and replaced the plugs. With a cough and a bang it started right up and after a minute of finding a sweet spot, it idles better than my daily driver. smoke goes away, running like a top, great oil pressure, and it overheats six minutes later. My question is has anyone had symptoms of it heating up that quick? And if so, what fixed it? I have a flush and new fluids in the agenda already. Thanks in advance.

 

First off 4.0's are designed to reach operating temp quickly. What are the symptoms of overheating?

 

Boiling out of the cap and temp getting close to the red.

 

And I forgot to mention that it had been sitting under a tree for 8 years not running.

 

Does it look like there could be coolant in the oil or oil in the coolant?

 

not at all

 

Are all the components of the cooling system there? There could still be an oil/coolant mix happening even though it's not visible so I wouldn't run it.

 

They are all there. Ill change the oil to have a better look at it but everything looked really good for 160xxx miles and sitting under a tree. In fact, the coolant still had its neon tint to it when It flowed out of the cap. No trace of anything that shouldn't be there. I will say that the cap that came with it was worn out though. When it overflowed out of the old cap, the first time, it was at 210. I put my new cap on from my dd and it held its pressure even when the temp went to the red zone (Which happened I think from the engine not having enough cooling from the first overflow).

 

It could be something as simple as a bad cap but not knowing the condition of all the components is an issue. I guess I'd start with a good flush and see what comes out of the system.

 

I have had thermostats stick closed from sitting so long like that. And 6 minutes is about right with a stuck thermostat.

 

x2 on the thermostat. Very easy for them to get stuck shut when not moving for so long.

 

Wow, well in that case ill do a stat, a cap, and a flush. That would make sense because my cooling fan didn't come on when it reached 210, and Im pretty sure the stat opening engages that.

 

Sounds like a plan. The auxiliary electric fan comes on at ~218* or when the a/c cycles. Did yours not come on at all?

 

It did not

 

Fan is activated by coolant sensor. Sitting 8 years flush, new upper and lower hoses and thermostat replacing at the minimum should be done.