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Unread 11-03-2013, 10:20 PM   #1
jared-f
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Overheating at highway speeds

So my 89 4.2 only starts overheating on the highway at RPMs over 1600. Was running 1700-1900 the other day and it overheated in about 10 miles. I drive in the city everyday at around 1500 RPMs and no problems. Just did a tune up, new distributor, radiator, new carb(mc2150). Any ideas that might solve my problem?

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Unread 11-03-2013, 10:26 PM   #2
Xpress
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My initial assumptions are the waterpump.... Does it overheat at idle? Putting around on the trails?
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Unread 11-03-2013, 10:30 PM   #3
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Quote:
Originally Posted by jared-f
So my 89 4.2 only starts overheating on the highway at RPMs over 1600. Was running 1700-1900 the other day and it overheated in about 10 miles. I drive in the city everyday at around 1500 RPMs and no problems. Just did a tune up, new distributor, radiator, new carb(mc2150). Any ideas that might solve my problem?
Did you replace the radiator cap ? When you replaced the radiator. How about the Thermostat ? At hiway speeds you should have enough air keeping it cool. Mine runs at 175 on the hiway. Did you get all of the air out of the system ? Any seepage around the waterpump?
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Unread 11-03-2013, 10:33 PM   #4
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Also does your thermostat have a tiny weep hole in it? The weep hole is essential to allowing air to properly bleed out of the system, otherwise air will be trapped behind the thermostat, which will prevent it from opening.
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Unread 11-03-2013, 10:47 PM   #5
Chrisnvegas
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Cheap t stats often do not open enough to cool at highway speeds. Is it possible during gasket scraping some debris could have gotten into the block? This could also restrict flow through the t stat.

And as mentioned above, a weak radiator cap spring can allow the coolant to turn into steam.

I know people will disagree, but a properly functioning fan is essential. Especially during highway driving where the engine is creating much heat trying to move all the air.

When you refill the cooling system, remove the heater hose from the t-stat. It will fill nicely and get most of the air out. And if you switched to an aluminum radiator from the original brass and it has plastic tanks, you just reduced the cooling by as much as 30%. The brass tanks in the original really add to the cooling. Plastic tanks do not transfer much heat.
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Unread 11-04-2013, 06:16 AM   #6
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besides making sure there is no air in your cooling system, with a new distributor and carb installed I'd pull the spark plugs and see what they are telling you too..

you may be leaning out and running hotter at higher rpm's
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Unread 11-04-2013, 09:04 AM   #7
jared-f
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Quote:
Originally Posted by Xpress
My initial assumptions are the waterpump.... Does it overheat at idle? Putting around on the trails?
That's my thought too, so I'll be taking a look at that this weekend. No overheating at idle or on the trails. Just when its consistently running over 1600 RPMs
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Unread 11-04-2013, 09:32 AM   #8
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come to think of it, didn't the 4.2's come with either v-belts or serpentine belts and have different rotations on the pump depending on the belt system used?

make sure you have the correct pump..I think the serpentine belt set up used a reverse rotation pump
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Unread 11-04-2013, 10:26 AM   #9
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Quote:
Originally Posted by crackedcornish View Post
come to think of it, didn't the 4.2's come with either v-belts or serpentine belts and have different rotations on the pump depending on the belt system used?

make sure you have the correct pump..I think the serpentine belt set up used a reverse rotation pump
They both spin the same direction with Serp and V configuration
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Unread 11-04-2013, 01:19 PM   #10
crackedcornish
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Originally Posted by Tom95YJ View Post
They both spin the same direction with Serp and V configuration
is it the 4.0L or the 2.5L that have reverse direction then?...I've heard there are Jeeps that use a reverse direction pump on them, and that they have an "R" cast into them to signify that they are in fact a reverse direction pump.
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Unread 11-04-2013, 01:28 PM   #11
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4.0 with serpentine is reverse rotation. It's stamped "R" on the impeller or body.
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Unread 11-04-2013, 02:21 PM   #12
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Quote:
Originally Posted by crackedcornish
is it the 4.0L or the 2.5L that have reverse direction then?...I've heard there are Jeeps that use a reverse direction pump on them, and that they have an "R" cast into them to signify that they are in fact a reverse direction pump.
Op would have the 4.2
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Unread 11-04-2013, 03:11 PM   #13
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Originally Posted by Tom95YJ View Post
They both spin the same direction with Serp and V configuration
no, the v belt is standard rotation and the serpentine is reverse.
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Unread 11-04-2013, 03:23 PM   #14
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Originally Posted by roadyrob
no, the v belt is standard rotation and the serpentine is reverse.
The belts are driven off the crank. I am pretty sure the direction of the crankshaft rotation has never Reversed.
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Unread 11-04-2013, 03:46 PM   #15
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your of course correct, the engines all rotate in the same direction but the belt configuration is different. I just changed my waterpump about 3 weeks ago, they show 2 different pumps for the 4.2. standard rotation for v belts, reverse for serpentine.
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