Yesterday I took my boat to the lake and my truck got hot. My boat weighs about 8000 lbs on the trailer. My truck is a 2004 3500 HO turbo diesel with 91k miles, the water pump was replace about 5000 miles ago.

It was hot yesterday, got up to around 90 degrees. I first noticed my truck was getting hot about 20 miles into the trip, usually my temp needle is rock steady just a bit left of the center tick mark, but as I was approaching a traffic light I saw the needle moving to the right of the center tick mark. As I sat at the light it kept getting warmer and warmer. As I got moving again, it initially moved up, but after a couple of miles the temp got back down to normal. During the next 30 miles I kept seeing the same thing, I'd come to a stop and the truck would get hotter and it would cool down as I got moving again. The last 6 miles were the worst, I hit a light and the engine got warmer, then I had to accelerate up a hill which brought the needled to the top edge of normal. I had to shut down the AC and turn the heat on full blast to bleed off the heat so it wouldn't go over. It responded pretty quickly and the engine was just above the center mark as I pulled into the marina. From there it never got extreme again.

The ride home everything was fine, I had the AC on and it never moved past the center mark. I did notice though that I could see the thermostat open and close, the needle would move from just under the center mark to just over the center mark and go back and forth. I don't recall ever seeing that before, usually it'll warm up, I'll see the thermostat open and from there on it stays steady.

My first thought is I was low on coolant, the truck did this once last year and I was low so I thought that was the cause last time and would be again this year, but nope, I had plenty of coolant. Yesterday I pulled the radiator cap and watched the water looking for bubbles JIC my head gasket might be bad, but after letting it run for about 5 minutes I didn't see anything so I think that might be OK. I also checked the oil for signs of emulsion but didn't see any.

My best guess now is either the fan clutch or thermostat. I can see that the engine would heat up under load, but at an idle it should be fine. I might expect an initial jump in heat when coming to a stop, but after a few seconds it should drop. So, I'm thinking the thermostat is not fully opening or I'm not getting enough air flow.

Am I thinking right or am I way off? Thoughts?

-Bob

 

IMO I would replace the fan clutch. T stat. Depending on where you live may want to run a 5-10 dog cooler t stat. You can also try a water wetting agent. FYI. I run royal purple's purple ice.

 

What is a water wetting agent?

 

Auto trans? What's your trans temp doing?

I would suspect either the fan isn't moving air like it should, or the radiator isn't flowing coolant like it should. Or you put cardboard in front of it this winter and forgot about it.

I'd check for cold spots on the radiator.

 

No cardboard, I don't do that. I have a nice warm down jacket that will keep me warm while the truck heats up.

How do I find my trans temp?

How would I check colds spots? I have an AC cooler and a trans cooler in front of it.

Thanks
-Bob

 

Both radiator hoses hot? Feel the inside of the radiator for cold spots. Water wetters will allow a 10-20 deg drop in coolant temp.

 

Water wetter is a bandaid at best.

Cold spots - go drive the truck, get it up to operating temp, shut it off and feel the surface. It should be fairly even in temp all the way across, with about a 10 degree difference between inlet and outlet temps.


If you have a laser temp gun, that'll work great.

 

I'll buy one and give it a shot.

-Bob

 

I bought a Ryobi infrared temp gun and after taking my truck on an 80 mile ride, with temps at times near 90 but closer to 75 at home, I used the gun on the various parts of the truck.

All readings are in Fahrenheit

The radiator was 185 at the top and 135 at the bottom, this seemed pretty consistent across the radiator, I'm not sure if this is normal or not. The AC cooler and trans cooler were both pretty cool, around 90. The top of the engine was 185 like the top of the radiator and the transmission was around 160.

None of these temps seemed alarming to me.

One thing I did notice is that my AC isn't working as well as it should. The air it puts out isn't very cold unless the truck is moving, this suggests to me the coolant is low and needs to be topped off, I can recall when the truck was new it would freeze you out at an idle. I'm wondering if this could be part of the problem, I don't know much about AC systems, does the AC cooler produce more heat when coolant is low? It would seem to me that with low coolant the efficiency isn't as good as it should be.

-Bob