Author Topic: Jeep overheating at highway speeds  (Read 458 times)

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Offline jfrabat

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Jeep overheating at highway speeds
« on: February 14, 2020, 02:40:13 PM »
So this happened some months ago, before the Jeep was taken out of commission, and I thought it was just the thermostat gone haywire.  So I replaced the thermostat, but, surprise surprise, I still got the problem.  The Jeep keeps cool with AC on and everything at city speed limits, but once I hit the highway, if I keep the AC on, it starts overheating.

Now, for those new to my Jeep, my jeep has an electric fan.  It originally was a Lincoln Mk VIII fan, but at some point, the mechanic I used to use replaced the fan motor for a new one.  And the new one apparently was the wrong polarity, so he had to reverse it.  So the new fan motor is nowhere near as powerful as the original.  And not only that, but now I can only use it with a relay, as because he reversed polarity, the 2 positives are now 2 negatives, and I cannot use the 2 drive options like I had it originally wired (I used one positive - low speed - for override on, and the other for the DC Control automatic controller, which was the one I used by default), and I could also turn it off to fording rivers.  I also have the winch up front, and I have AC and a winch up front.  Oh, and also a power steering cooler in front of both the AC and radiator.

Now, I have a new water pump (a Bosh one I bought years ago) that I can install (technically, it is higher flow than the original at idle, and although idle is not my problem right now, I have it, and it is not that difficult to change out the water pump anyway), and I do have another fan I bought that says it is from a Ford Taurus, but I have my doubts.  But at least it is a 2 speed one, which I could technically change and try it (it has its own fan shroud, but it is smaller than the one I have in the Jeep, so I would probably just swap out the motors).

So what do you guys recommend I do with this issue?
'94 YJ 2.5L with 4" RE lift, Superwinch EPi9.0, FoMoCo e-Fan, SD30 and SD35 w/ARB-5.13, 165A alt., 33" BFG KM2 on 15" AR wheels, Sony sound system, Pavement Ends Hardtop, Hydroboost

Offline Jeffy

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Re: Jeep overheating at highway speeds
« Reply #1 on: February 17, 2020, 10:53:09 PM »
That's weird.  If anything it should be able to cool itself without the fan while on the highway.  Have you taken the radiator cap off and watched for the coolant to flow?  If you take teh cap off you should be able to watch the level rise till the thermostat opens.  Then the level should drop and you should be able to see the coolant moving.

With the fan, as long as your moving the Jeep shouldn't overheat even if the fan is off.  If the fan is off and the AC is on, you may notice the air warming up till you start moving...
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Offline jfrabat

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Re: Jeep overheating at highway speeds
« Reply #2 on: February 18, 2020, 06:23:32 AM »
Well, turns out the fan motor was full of cooked mud, so replacing the fan fixed the issue.  My guess is that the mud was limiting how fast the fan cluld turn, and the fan was actually blocking air at highway speeds.  There was also some mud in the lower part of the radiator that I cleaned up.
'94 YJ 2.5L with 4" RE lift, Superwinch EPi9.0, FoMoCo e-Fan, SD30 and SD35 w/ARB-5.13, 165A alt., 33" BFG KM2 on 15" AR wheels, Sony sound system, Pavement Ends Hardtop, Hydroboost