Weird Overheating Issue

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Aug 28, 2021 | 05:32 PM

Oliver Roe

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Weird Overheating Issue

1993 F250 5.8L 4x4 with the ZF manual transmission. About 131K on the clock, but I doubt it's accurate.

New coolant hoses, radiator#1, coolant temperature sensor + housing unit (had cracked) in Jan 2019
Radiator#2 (rock put a hole in it), Motorcraft water pump, Motorcraft thermostat back in February of this year. ***NOTE: I broke a water pump stud doing this service. It's not leaking - coolant level is normal - but I'm short one stud on that water pump - has been running fine since, and I don't have the $$ for engine-out repair to get that dang stud out.***

Truck started overheating only at LOW speeds this weekend, so I figured the fan clutch wasn't engaging (was perfect at 60+mph). Replaced fan clutch this morning and now at idle + low speed it's great, but once I get up to 30-40 it starts to overheat, then instantly drops back down when I slow down. So now it's overheating the *opposite* way (higher speed).

Driving me crazy!

Heater core works, but it's the only part I haven't replaced of the whole cooling system (well, that and the reservoir, but that's just a plastic box).

Is the truck possessed? What could be going on?

Aug 28, 2021 | 06:52 PM

torq'ta 5 8

Lead Driver

Joined: Jan 2019

Posts: 5,081

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From: N.W. Alabama

Quote:

Originally Posted by Oliver Roe

Is the truck possessed? What could be going on?

Yes...they all are, they give you the correct reverse rotation pump, did you test that new thermostat, opened before install. What temp did you get?

Aug 28, 2021 | 11:07 PM

Oliver Roe

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Yes it's a reverse rotation fan clutch. The thermostat and water pump were installed back in February and I've driven the truck around town all summer with no issues; this just suddenly cropped up. Aggravating! Thanks for confirming it's possessed; I'll have to find a priest.

Aug 29, 2021 | 11:52 AM

FuzzFace2

FTE Legend

Joined: Nov 2015

Posts: 30,935

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From: Angier, NC

So this started after the fan clutch change?
I would look into the fan was it put on right first?
Do you have a spring in the lower radiator hose?
If not as the RPM goes up like driving down the road the lower hose gets sucked closed and the water cant flow.
Dave ----

Aug 29, 2021 | 04:08 PM

Oliver Roe

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It started overheating at low speeds before the fan clutch.

I swapped the fan clutch to try to fix the problem.

Now it's overheating still, but slower, and only at higher speeds.

The fan clutch swap seems to have either reversed the problem or solved one problem while exposing another.

Aug 29, 2021 | 05:45 PM

aaa2231

5th Wheeling

Joined: Aug 2021

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From: Shawnee National Forest

New fan clutch defective?

Aug 29, 2021 | 06:01 PM

timbersteel

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Joined: Mar 2005

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From: Mexico, Missouri

When the engine is speeding down the highway, the engine should not overheat. You wouldn't even need a fan at highway speed for the engine to stay at the correct operating temp if the correct t-stat is installed.

As someone mentioned, is the lower radiator hose collapsing when rpms are increased? Is the t-stat correct? And when I ask that, was it verified in a pot of boiling water and a thermometer?
Does the radiator cap hold psi?

The suggestion about a collapsed lower radiator hose sounds like a good path to investigate.
When dealing with cooling issues, I recommend Motorcraft t-stats, and Gates hoses or the closest thing to Motorcraft. I've seen many issues with subpar t-stats, radiator caps, and hoses.

If the fan clutch isn't working correctly, then at I could see an overheat condition at slower speeds and higher rpms.