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Old 11-20-2009, 09:14 AM   #1 (permalink)
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Need help with 4.0 overcooling!

I have an 88 cherokee XJ with the non-HO 4.0 renix motor. It is supposed run at around 210 in order to produce max power and low emissions. my jeep barely gets halfway to 210 (about 160 degrees or so), and the only time I ever see 210 is in 95+degree weather when im stopped in drive idling for 10 or more minutes. and when i'm at 210 and start driving away I can tell the engine has WAY more power/response and is way smoother, but after the first minute or two of moving the temp drops back down and the jeep is running weak/rough again.

I've changed over to the open cooling system, ever since then I have been over cooling. Ive re-checked the way I ran the hoses to the new open cooling system heater control valve and thats all good.

The thermostat housing was kinda corroded and pitted between the heater bypass and the thermostat so I thought that was the problem (cooland leaking past the closed thermostat) so I replaced the 195deg thermostat and thermostat housing with brand new dealer pieces, also replaced the radiator and 16lb rad cap with new parts, and both the fan and guage temp sensors, as well as my them guage itself, the O2 sensor, and the fan clutch.

It still dosen't get to running temp, horrible mileage, less power, bad emissions, and the heater is barely warm. Its been 30-40 degree outside when I leave in the morning and it takes about 10 minutes to even get half way to 210, then just kinda fluctuates around there.

I'm running out of ideas and I'm about to just remove the fan and install another electric fan with a on-off-auto 3way switch so I can control the cooling a little more. what else should I try? I know a couple other guys with similar cherokees and they all run around 195-210 no matter how cold or hot it is out.

Last edited by Brandon_6667; 11-20-2009 at 09:15 AM.
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Old 11-20-2009, 09:53 AM   #2 (permalink)
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are you using a stock water pump?
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Old 11-20-2009, 10:03 AM   #3 (permalink)
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I had this problem once, strangely it happened all of a sudden one day on an engine that had been running at the right temp for years. It turns out some piece of metal crud had dislodged itself from somewhere inside the engine and jammed the thermostat open. Must have been somewhere inside the water passages from the original block casting process, and it never made itself known until it came loose and jammed the thermostat.

Sorry for the long story, but make sure your thermostat isn't stuck open by some piece of crud that got dislodged when you changed over your cooling system.

Here's a photo of the offending piece of metal, it's about 1/2" across.

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Old 11-20-2009, 10:24 AM   #4 (permalink)
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Sounds like your thermostat is stuck open to me.
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Old 11-20-2009, 10:49 AM   #5 (permalink)
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The waterpump was replaced right before I bought the jeep, looks like a stock pump though, not a high flow or anything.

my thermostat isnt stuck open, its brand new and the old one wasnt stuck either (I tested them both) I thought coolant was leaking past the thermostat because the housing was so pitted around the heater bypass so I replaced it and that helped a little bit but not much, temp just goes up a little quicker, still stops warming up at the same place.

Last edited by Brandon_6667; 11-20-2009 at 11:48 AM.
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Old 11-20-2009, 01:30 PM   #6 (permalink)
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Have you tried a higher temp thermostat? Maybe go with 205.
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Old 11-20-2009, 02:47 PM   #7 (permalink)
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Have you tried a higher temp thermostat? Maybe go with 205.
yes, try that unless you find out that the thermostat was stuck open
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Old 11-21-2009, 03:07 PM   #8 (permalink)
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How did you test the thermostat? did you boil it. I had a 195 that opened at about 150, I put mine on the stove in a pot of water, put a thermonitor in with it. checked the temp when it opened ( it was brand new as well. )
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Old 11-21-2009, 06:09 PM   #9 (permalink)
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I got rid of the mechanical fan on mine and replaced it with an electric fan because it was running hot and I was missing the shroud. Worked good. But now I have the same problem you have. Sometimes I don't even turn the fans on and it runs about half way between 210 and the 0 unless it is really hot out. Mine has a 210 thermostat, stock pump, and dual electric fans that are both controllable from in the vehicle (not the A/c). Thought I would share this with you since you thought going with an electric fan would help you control the heat better. You're not the only one, mine runs cold all the time now. I don't worry about mine because I remember those days when I would have to kill the A/C or even worse shut down because it ran so hot. And by the way it seemed to go from running hot to cold overnight. Hope this helps.
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Old 11-23-2009, 09:36 AM   #10 (permalink)
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How did you test the thermostat? did you boil it. I had a 195 that opened at about 150, I put mine on the stove in a pot of water, put a thermonitor in with it. checked the temp when it opened ( it was brand new as well. )
hmm, good point, I did boil it but I didnt have a thermometer to check exactly when it opened, but I do know it got pretty damn hot (about boiling) before it started opening.

So what did you do about it? did you replace it with a better one you found to open closer to 195?
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Old 11-23-2009, 01:24 PM   #11 (permalink)
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hmm, good point, I did boil it but I didnt have a thermometer to check exactly when it opened, but I do know it got pretty damn hot (about boiling) before it started opening.

So what did you do about it? did you replace it with a better one you found to open closer to 195?
yes i took it back and got another it open at about 196 much better.
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Old 11-23-2009, 04:04 PM   #12 (permalink)
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Try using an actual temperature gauge and not the stock gauge in the dash like it sound's like your doing. " half way to 210 " well what is that really ? Those gauges suck so get one that actually read's a temp. a little better. I think your gauge is reading wrong and it's screwing with your head and your thermostat is working properly.

On top of that you need to flush out your heater core which is most likely the reason your freezing your ass off.

running like crap and poor MPG is just normal for a cherokee.
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