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Old 07-06-2011, 11:36 AM
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4.0 Overheating above 40mph

So I read a couple threads on this but still need to ask the question (gotta love noobies!) I have a 93 YJ, 4.0L and setup is stock gears/axles and 33's. About a month ago, it overheated going over a steep bridge pulling a trailer (small one). Only way to cool it down was to stay under 35/40mph. So I replaced the thermostat and housing. Also took it to get the radiator flushed and filled (I know, garden hose.....what can I say, I was being lazy). So since then I have had it out a few times, no problems hwys speeds and all. Even pulled the trailer again.

Well this past Sat I took it out for a run and after about 20 min at hwy speeds it started climbing over the normal 200/210 range. It was a slow climb but it was going up. I got off the hwy and slowed to 35/40mph and it cooled down.

When going back home, hit the hwy again and tried staying in 4th thinking maybe not enough RPMs in od since it's not geared. Nope, started heating up again so got off the hwy and slowed to the 35/40 range. Again it cooled down. It doesn't seem to have any issues as long as I stay under 40.

Soooooooooo...... what could it be? Tstat is new, housing is new. it was fine for a while but now issue is back (that is the part tripping me up).

I read it could be a fan clutch. I also read it could be a loose intake manifold. Temp sensor too. Checked intake/exhaust and they are tight. No visible leaks of any sort either. I am guessing the temp sensor was right cuase it was pretty hot under the hood. Not sure how to check the fan clutch. When the engine is off, I can rotate the fan and feel some resistance. When running, it is spinning (of course).

So any words of wisdom on this? Any help ya'll can provide would be great.
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Old 07-06-2011, 11:41 AM   #2
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Is it just your gauge saying your over heating? Or is it both gauge and your getting steam/water boiling out?

93 YJ I6 5spd
My YJ Build +1.5 inch SOA ,37 12.50 15R Swamper M16's, 5.22:1/2.97:1 Stak Tcase with 92 Crawl Ratio--------SOLD...
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Old 07-06-2011, 11:48 AM
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Well I haven't seen steam yet but I did look at the overflow and it was full so I am guessing if I let it keep going up I would have seen steam. Good point though. I had the same thought of maybe it is just a bad sending unit or sensor or something. Don't know. Since the overflow was full and looked like some fluid got pushed out of the cap I am guessing it got pretty hot.
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