Hey all. Hopefully this is a simple one - I'm just looking for any insight on my thought process and a few questions answered. Sorry if I run a little long winded, but I'm trying to lay out as much info as I can. Disclaimer before I continue though: I didn't think to bring a temp gun along with me (my bad) so I'm relying on just what I can tell by the idiot gauge. I've got one in the truck now and if the problem comes back I'll update this thread with some empirical evidence. Now, on to the backstory.

My '96 F-SuperDuty has a problem with getting hot, but only when I'm pulling something behind me and mostly only on warm days. It's happened to me a few times but yesterday was the worst. Here's what happened: I was towing a dump trailer loaded with maybe 4000 lbs of roofing material (so I think somewhere between 8000-8500 lbs total weight). I got caught up in northern VA rush hour traffic on an unseasonably warm October afternoon and the truck started to get hot. Typically it'll run in between the "N" and "O" on NORMAL unloaded and "O" and "R" while towing. This time, however, I chose to pull off when it got past the "L". When I found a parking spot and could take a look, the truck was indeed puking coolant out of the reservoir, so I'm sure it was getting a little cranky with me. At no point did the truck start to run poorly or anything. I should mention, it will only go thermonuclear like this if I'm stuck in traffic on a warm day - if I can get some decent speed the truck will cool back down fairly quickly. Furthermore, the truck will not ever get hot when unloaded, whether sitting in traffic, cruising, or idling in the driveway. This issue only occurs when I'm towing with it.

Here's what I'm going to do in an attempt to get this problem resolved:
- Check base timing to ensure it's not too advanced. I believe 6-10 degrees btc is spec, right?
- Replace thermostat with 195 degree Motorcraft
- Flush the block, radiator, and heater core with a garden hose and replace with fresh coolant. I'll flush the normal flow way and reverse flow as well.
- Replace the fan clutch. I assume this needs to be done as the truck heats up when idling in traffic but cools down once I can get moving.
- Check for adequate water pump flow. I assume the thermostat has to be open to do so, right?

Does this seem like a good way to proceed? Anything else I'm missing, or any other insights from those who've experienced this issue before? As I previously mentioned, I don't mind putting a little money into the truck to make it right - it's my camper hauler and I also don't want to melt down my 90K miles 460!!

 

I say start by testing the fan clutch before replacing anything. Sounds more like an airflow issue than a coolant flow issue, as once at highway speed, it's fine.

 

Check or replace your fan clutch. Make sure your radiator, etc fins are clear.

I would also check and service if necessary the air filter and your temp sensors.

You may need a more aggressive fan clutch to keep up.

 

Thanks for the advice Junktownbill! I definitely think that's likely going to be the main issue as well. I'll probably take the truck for a long drive this evening to warm it up, then check how stiff the fan clutch is. My guess is it's going to need replaced.

On the other points, thermostats are cheap enough that I wouldn't feel bad about replacing whatever's in the truck now. As for the coolant, I've topped it up with water a few times so I need to get all that out and replaced with coolant again before we have too many cold days.

 

Definitely going to be at the top of my list to pull everything away from the rad to make sure there's no debris plugging it off. I'll be checking the fan clutch as well to see if it gets stiffer once the engine is hot.

Air filter is new - I did a tune up on the truck earlier this year. I also checked the air box and all the air piping and they're clear of obstructions. On the temp sensor, how do I check it? And what am I checking for? I don't think its faulty, per se, as it was telling me the truck was hot and coolant was boiling over the coolant overflow and puking out.

Where does one get a more aggressive fan clutch? Hell I'd go with a direct drive fan if I knew it would help - not like I'm worried about losing fuel economy

 

Hayden 2799 for a fan clutch. I will look for a Ford part later but most likely it is not available.

The computer temp sensors (intake air and coolant) provide variable resistance based on temperature. You can test them by getting a resistance reading and checking if it is in spec. The air intake temp sensor sometimes is just dirty. These are separate from the gauge senders.

These being in good working really help your computer adjust fuel trims properly.

 

I recently installed an additional transmission cooler and with this update, I spent a fair amount of time sealing all the places where the air could go around the radiator. I could highly recommend to invest a fair amount of time to make panels and use foam tape to force any air that goes past the grill to only be able to go through the radiator.

I never had an overheating problem, but I now know that all the air has to go through all the coolers/condenser/radiator.

 

When the truck gets warm the fan clutch should lock up. You'll know based on sound alone. Mine roars when the clutch comes on. If it doesn't make much noise you can pretty much be sure it's not coming on correctly. It should be pretty loud when you first start the truck too, and loosen up after a minute or two.

My last fan clutch failed in the other way, it was locked up and so the fan ran full speed no matter what.

 

I will also back up the fan clutch theory.

When my turns on it sounds like a plane is taking off in my engine bay, it is very audible.

Picked up a couple of fan clutches and its been a hit or miss with them, have had some that never turn on and some that are always on lol. A way I test them, again not a professional, I used a blowtorch and some ice to see if the bi metal coil would expand and contract and then I would rotate them by hand to ensure the fluid was actually being released.

 

Thanks for the info! I'll check both those sensors and if they aren't reading in spec, I'll replace 'em!

Should I determine I need a fan clutch, I'll check out the Hayden. I appreciate the suggestion!

 

That's a very interesting idea, and it makes sense!! I'll look into it while I'm inspecting the radiator area for debris and other ish.

 

I think you both might be on to something here. I've heard others say that you can't mistake or not hear the sound of a clutch fan at full lockup. Unfortunately, I don't think I've ever heard that loud, roaring sound coming from this truck. So it's possible mine isn't locking up all the way. Certainly feels that way with the symptoms the truck's presenting.

So, with testing the clutch fan with a blowtorch, do I just put some heat to it and determine how stiff the fan then becomes to turn? Should it turn at all when I heat it up?

Furthermore, is there any real issue with a fan clutch failing at full lockup all the time??

 

Have you replaced your radiator cap?
If its weak, it'll go into the reservoir then you'll be low on coolant, then overheat.

 

We've got a 6.9 IDI with a seized fan clutch. I recently had to remove the hood insulation, so it definitely sounds like a jet engine now. I noticed some of the steel blades are chipped.

Fully seized, it can overcool the engine and soak up unnecessary power.

 

I think your radiator is getting plugged up