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I was recently towing our toyhauler for President's Day weekend and ran into a strange intermittent overheating issue. For the first hour I could pull sustained grades (and the larger grades) and never exceed 220 degrees. After about an hour of operation, when pulling grades where the RPM was sustained around 2,500-3,000 RPM, the engine coolant would slowly creep to 240+ degreesand throw a check gauge light. Upon cresting the grade or stopping, the engine coolant would immediately (within 5-10 minutes) go back down to 180 degrees. This occurred on the way out and the way back.

A couple of things to note. I installed a 7.3 fan clutch last September, although I didn't hear the fan as much as normal, although I'm chasing a recent loud drone that might be a cracked y-pipe, so perhaps I just couldn't hear it over that. And lastly, I have never replaced the thermostat, and as far as I know its the original thermostat with 116k miles on it.

On the surface, it seems like this is likely a fan clutch issue, but its a BPD fan clutch that's less than a year old. But the intermittent issue is odd. If it was the clutch I would think that it would happen the whole time. What is the easiest way to test a mechanical clutch?

Lastly, I plan on replacing the thermostat either way though since its only $20 and it'll provide piece of mind. So with that, does this sound like a bad fan clutch, or something else? Thank you for the help!
 

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When the engine is "cold" do the coolant and oil temp sensors read the same? Those sensors are the same, and can be swapped to see if the problem is different

Too bad the clutch was replaced, that was a superior system

You can use a hand held laser tach to measure pulley RPM and fan RPM -- cold and when it is 'overheated' -- if the clutch is working, it should idle the fan when cold and almost match pulley speed when hot

You should hear the fan roar when hot, the 6.0 fan is pretty aggressive - unless there is other noise

The 7.3 clutch relies on air passing thru the center of the cooling stack to activate the bi-metallic spring -- radiators clean and no fin damage?
 

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Discussion Starter #3
When the engine is "cold" do the coolant and oil temp sensors read the same? Those sensors are the same, and can be swapped to see if the problem is different

Too bad the clutch was replaced, that was a superior system

You can use a hand held laser tach to measure pulley RPM and fan RPM -- cold and when it is 'overheated' -- if the clutch is working, it should idle the fan when cold and almost match pulley speed when hot

You should hear the fan roar when hot, the 6.0 fan is pretty aggressive - unless there is other noise

The 7.3 clutch relies on air passing thru the center of the cooling stack to activate the bi-metallic spring -- radiators clean and no fin damage?
Under cold conditions the ECT and EOT are the same, the sensors seem good.

I know you are one of the proponents of the stock fan clutch, lol. I never liked how warm it let the truck get prior to engaging, and since I really only tow with the truck it seemed like the mechanical clutch would be better, although it might be proving me wrong right now. I agree on the fan roaring, after I installed it I was wondering if I had made a mistake with how loud it was when cold until the fluid warmed up a little. I will check the radiator, I did notice an oily buildup on the fan blades that I couldn't track down so maybe something got on the radiator and is plugging it.
 

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I don't think the clutch is the issue. I think you need a pressure gauge on the degas bottle.
 

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I am. Hard to say for sure, but sometimes a head gasket leak will cause a drone AND can restrict coolant flow.
 

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Discussion Starter #7
I am. Hard to say for sure, but sometimes a head gasket leak will cause a drone AND can restrict coolant flow.
Ugh- I have been trying to avoid blowing head gaskets (babying the truck and no tuners). If it was head gaskets I would see coolant coming out of the degas cap right? I will rig up a pressure gauge on the degas system. What should be the max pressure I see?
 

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You wouldn't necessarily see coolant spewing out of the degas bottle. The degas bottle cap is a relief valve. If the head gasket leak were minor, the cap can relieve a small volume continuously and make the drone sound. The small release may not be enough for liquid to spew out. It is possible for it to relieve only the gas.
 

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...it is possible for it to relieve only the gas.
Ayup, that's why it is called a ...degas bottle

About to roll out of my chair

You may get up to the relief pressure of the cap(16psi) -- what you are looking for is a sharp rise in pressure that coincides with load on the engine

Soo … a short WOT run that makes 16psi -- sometimes just starting the engine will do
- I have had engines project coolant out of the filler port … like 3 feet high
- Fill the degas bottle all the way to the top and watch the bubbles continue to rise ...kinda like Lawrence Welk Show
 

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Ayup, that's why it is called a ...degas bottle
Doh- *facepalm*

If I get a pressure gauge in line with the degas bottle vent going to the radiator, and do a short WOT run, will the pressure (sub-16 psi since the excess pressure will vent) remain in the system so I can see the pressure on the gauge under the hood?

Thanks for the suggestions gents.
 

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Doh- *facepalm*

If I get a pressure gauge in line with the degas bottle vent going to the radiator, and do a short WOT run, will the pressure (sub-16 psi since the excess pressure will vent) remain in the system so I can see the pressure on the gauge under the hood?

Thanks for the suggestions gents.
get a longer hose on the gauge and zip tie to the wiper arm -- you need to be able to see the gauge

We gotta have some humor here, else life gets boring
 

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I finally got around to rigging up the gauge on the coolant system. Here is a video of a WOT run from a dead stop to 65 MPH up a slight grade. The gauge barely moves but does move up slightly.


I also did a Napa leak test to confirm the presence of exhaust in the coolant. The test came back negative both before the engine was warm and after it was warm. We confirmed that the test fluid was good by putting it near the exhaust outlet and it turned yellow immediately.

https://www.napaonline.com/en/p/BK_7001006

Since neither test showed an issue is it safe to say the head gaskets are good?
 

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When the coolant temp went up to 240, what did the oil temp do?

Even when they read the same at the low end, they can still be off at the high end.
 

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Time to install a new thermostat I guess. Glad the degas bottle pressure test turned out good!
 

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Discussion Starter #16
Not gonna lie, I was stoked when the gauge didn’t move. I also have a laser tac to confirm fan clutch operation. But I think it’s getting a new thermostat either way. Any recommendation on whether to go with a stock thermostat or the 200 degree thermostat?
 

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I know some people like a hotter engine. IMO things will last longer if it stays around 190 coolant temp.

Don't ask me what the most reliable thermostat is. I haven't found any that I really like. I am currently running a stock thermostat that I bought for BIG bucks from International.
 

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Thermostat seems like a good 25 dollar next step.

Not sure the test fluid is reliable for diesels, but the gauge seemed good.

For my 7.3 fan, I notice at idle i can feel the Breeze. Snd the stock fan clutch had no bteeze. Only way i know how to check it.

Coolant into the radiator should be20 degrees warmer than out, but don't know how to measure.
 

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Strap a digital temp probe to the lower radiator hose -- look at your Barbecue supply -- Bluetooth would work well
 
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