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Old 09-29-2010, 12:39 AM   #1
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4.2 Running Hot

My 89 YJ is running hot at highway speeds 60+ and does great at lower speeds on and off road, anybody come across this problem. I changed the thermostat and water pump and has a larger capacity radiator but still gets hot while going down the highway for lenghty periods of time. Any helpful suggestions would be greatly appreciated.

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Old 09-29-2010, 01:08 AM   #2
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My 89 YJ is running hot at highway speeds 60+ and does great at lower speeds on and off road, anybody come across this problem. I changed the thermostat and water pump and has a larger capacity radiator but still gets hot while going down the highway for lenghty periods of time. Any helpful suggestions would be greatly appreciated.
Flow. Usually a flow problem. The engine creates more heat at higher rpms but this is usually countered by incrased coolant flow and higher air flow across the radiator. First with the vehicle in park rev the engine to highway rpms and check the bottom hose to make sure it is not collapsing and if it is replace it with one that has a spring in it. Check and make sure nothing is blocking the grill or radiator i.e. mud. If that is okay some possible causes of low flow are bad water pump, corrosing restricting flow in engine or radiator , heater core plugged which you can bypass and check easily.

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Old 09-29-2010, 11:30 AM   #3
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Other than what Fweaky recommended are you at least running a 50/50 mix in the radiator. Even though the radiator has a larger capacity how long has it been in there, maybe it's clogged, getting back to the flow situation. Did you put in a 195 deg. thermostat when you changed it.
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Old 09-29-2010, 11:48 AM   #4
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also, you have to absolutely make suer that you burp the system after you top it off. i had coolant in my system with bubbles and all it did was heat up at highway speeds.

if you've got all that taken care of, make sure you have no flat spots in your fan clutch.
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Old 09-29-2010, 07:10 PM   #5
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Wait wait.............. Your YJ does 60+ MPH ?????
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Old 09-29-2010, 10:11 PM   #6
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Wait wait.............. Your YJ does 60+ MPH ?????
When I bought my YJ it had 200,000 miles on it and the engine was slam wore out. There was a small hill, and I mean small, that I had to go up but it was on Fort Lee and you dont speed there so I couldn't get a running start. I swear I would start up the hill at 50 mph and when I reached the top I was going 15 and smoking like hell. It looked like one of those Castrol GT commercials with the car dragging the boat anchor! It was embarrassing.
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Old 09-30-2010, 11:10 AM   #7
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I want to thank all of you for your suggestions and I guess I should be happy that it does go that fast but at a cost. I will see about looking at the flow more closely and possibly go with high flow pump. Thanks again.
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Old 10-01-2010, 10:29 PM   #8
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One thing I wanted to add but didn't think about at the time. Check your engine timing. Also check your vacuum advance, if it is not working that can also cause heat issues.
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Old 10-02-2010, 12:52 PM   #9
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Do you buy a new radiator or a used one? With a bigger radiator, it doesn't make sense to have it over heat at 60 MPH. What RPM are you turning at 60 MPH? If you got a used radiator, I would put it, take it to a radiator shop ane get it cleaned out.

Is the motor stock? It would make sense if you were pulling a trail, up hill, in a head wind, in 90 degree weather, I'd say try a lower gear. But just by itself, there is something not right.
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Old 10-02-2010, 06:42 PM   #10
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tbois, I had my radiator redone at radiator shop keeping top and bottom replacing original cores with 3, seem to work for couple of years. This was done after engine having been rebuilt. It's running at about 2600 rpm at 60MPH and temp climbs noticably, when I hit 3000 65-70 MPH steady causing me to back off to avoid overheating.
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Old 10-04-2010, 10:52 PM   #11
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Glad to hear you had the radiator done. Between what has already been suggested and the things you did when you put the new motor in, it doesn't make since.

There must have been something that has changed if it was fine for a few years and now it's a problem. Could it be that it's not over heating, but the temp sensor is leading you a stray? Does it over heat to the point that it's blowing fuid out the overflow?

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