Help with overheating 4.0

xtultegra

xtultegra

New Member
Joined
Jul 28, 2020
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2
Location
Redding California
Fresh 4.0 build. Will run all day in stop and go traffic or idling up some knarly trail in 110 degree weather and never go over 195. Pull a hill on the highway or travel down the highway at 3000 rpm and it will over heat in about 5 minutes. Temp comes down if I slow down.

What I have done: new flow cooler water pump, high flow 194 thermostat, new double row aluminum radiator, new CPS, flushed the holy hell out of the system with Red Devil Flush (3 times), added wetting agent, running 60/40 water to coolant ratio.

Engine sounds great. Pulls like a beast. No stutters, backfires or misses. However, it idles past 12 degrees advanced, more like 20 degrees.
 
This sounds exactly like what happens (from what I've read) to the 4.0 when there is a big air bubble somewhere in the cooling system along the head. Look into properly "Burping" the air out of the system possibly by raising the front end.

About 20 years ago the place I worked had an old XJ Cherokee 4.0 that overheated on the highway and seized the engine. The dealership explained they had a problem with the air getting trapped. They mentioned some method or mod they did to prevent it.

I filled the radiator up on my YJ 2.5L then drove up on ramps and ran the engine with the radiator cap off until it warm up and thermostat opened up. Lots of air came out an I ended up adding another quart of coolant.
 
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I ran it with the radiator cap off and added coolant until it overflowed. Then I took the thermostat put and did the same thing. No dice. I did not jack the front end up. So I will try that next.
 
Ok the T-stat out and still overheats still leads me to believe it's something else. Thermostats are used to control the minimum normal operating temperature not the maximum temp. The radiator should be designed to cool the high temp limits.

One additional wild guess since you mentioned the new flow cooler water pump, and I just came across this, is there's two different pump impellors for the same pump housing. A standard direction and a reverse direction impellor depending on which way the pump rotates. I'm not saying you have the wrong one but shipping errors do get made. I don't know about the high flow pumps or even if they make the two different impellors. Just a wild guess trying to help here.
 
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