Question about 1990 Jeep Wrangler

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1990 4.2 Jeep Wrangler that is overheating

I have a 1990 4.2 Jeep Wrangler that is overheating. I've replaced the thermostat and radiator. The waterpump is not leaking or making any noise. I believe there is an air bubble because the top hose from the radiator is soft when the car is running. I've tried to get the air bubble out buy running the engine with the radiator cap off but the engine overheats within minutes. Any suggestions?

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  • Darren North Sep 28, 2009

    I'm pretty sure the thermostat was installed right but I guess I could take it out and try again. There hasn't ever felt like there's been pressure there

  • Samuel Charles Spriggs May 11, 2010

    The soft top hose sounds like the thermostat is not opening or not opening all the way. Could the new thermostat be installed upside down? It would take a major air bubble to keep the top hose from pressurizing -- does it ever feel like there is pressure on it? Charlie

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  • Jeep Master
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Still sounds like a bad radiator...new or from scrapyard? If you can't add coolant faster than a really slow dribble that's a big clue.
You can run the engine with the heater on high and the engine should not air-lock. I'd also remove the thermostat just to make sure it is not defective. Run the engine without it and see if it still overheats. Last, it is possible that your head gasket has failed, and engine compression is causing the overheat. I tend to think not only because the hose stays soft.
While it's running, also make sure that the lower hose is not "sucking in" if it is, replace it. On 4.0 engines you can also get the serpentine belt on wrong and it will drive the pump backwards. if you replaced the belt check that too.

Posted on Sep 28, 2009

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  • Jeep Master
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Is there any chance the thermostat got installed upside down? Flat end down into the engine is right. How hot does it get? Did you turn the heater on to circulate through it and get the air out of the heater core?
Thermostat usually open at 190 degrees, you may need a "candy thermometer" to double check the water temp in the rad. Get it in housewares. It's possible the thermostat is defective, I've seen that before. Hope this helps.

Posted on Sep 28, 2009

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