My early 99 7.3 is overheating while towing my boat. This started a while back and I changed my fan clutch because I could not hear it kicking in. I also changed the thermostat for good measure. I thought it was cured but I had the truck on the highway yesterday and as soon as I got up to 55 MPH, the temp screamed back up into the red. I got off on surface roads and kept speed at 45 and the temp held just under the red on the gauge. With no trailer, the temp runs just below halfway up the scale. Thought I might have a bad sender but when I pulled over and popped the hood it was scary hot. I'm not sure where to go next. The coolant looks great so I have a hard time believing that the radiator is plugged. You guys have always had good advice so thanks in advance for any input.

 

Even though you changed the thermostat a while back, doesn't mean its working right now. It wouldn't hurt to take it out and check it. I've checked mine in the past before by putting it in a pan of water on the stove and checked to see if it opened as the water heats to a boil.

 

I would have to say your t-stat failed. When was the last time you did a drain and flush on your coolant system?

 

I changed my t-stat 2 times in my old F150 before i got a good one. And when i changed one in my jeep last fall, it failed three months later..

You can shoot the radiator hose with a laser temp gauge, To see what the temp is roughly.

 

Well I agree with everyone on that it has to be the t-stat. Also when it comes to the t-stat they make a short one and a longer version. And our trucks take the longer version. So make sure you have the right one. And the fan wouldn't come into play at 55mph.
And I hate to be the one to bring up the opinion of what could be wrong and that would be a blown head gasket. But that isn't likely since these 7.3's are very good about head gaskets. But check to see if you are losing coolant or over pressurized degasser tank.
By the way Welcome to FTE !!!

 

To test the head gasket idea, Could you remove the t-stat all together and see if the temp still sky rockets? just thinking.

 

Seems to be a consensus on thermostat. I did not know about short vs long either. It won't cost much to give that a try. What do you guys think about danskool's suggestion of temporarily removing it to see what happens?

 

I would take a look between the radiator and intercooler for trash.build up...not letting the radiator breath properly

 

I've only did that once in a gasser as a temp fix. I'm not sure about our engines. I wouldnt go through the trouble of sealing the housing up without a tstat just to see if it still over heats. I would just buy another tstat and put it in boiling water to make sure it works first then put it in. You will save on coolant also. Which one did you put in there the 195* or the 203* tstat?

 

The last post was from my phone so now i will add my 02 a little better..
check for trash in the front of the radiator..The reason I dont feel like its the t-stat is it only does it towing,if the t-stat is sticking it would do it all the time, i guess it could be partally open..When mind did the same thing
(overheated while towing)I followed the same path,new t-stat,new water pump,pulled the radiator to replace and found the front of radiator full of dirt,feathers/straw.a good cleaning and i was good to go... BTW any body need a radiator??

 

Removing the T-Stat wouldn't work as good as having a correctly working T-Stat in place. As you can see in the picture, when the T-Stat opens, it closed off the bypass port below it. Just removing it would also leave the bypass port open so you won't get the full flow through the radiator.

 

I couldn't see this at all until after I removed the rad..

 

I think I see some eggs in that nest!

 

There's been more then one guy on here that did everything and it ended up being the radiator. Had a guy coming from RRE one year keep heating up, replace water pump and all. Got home replaced radiator problem solved. Just a thought,

Chet

 

I'm gonna put a new tstat in it tomorrow and if that doesn't fix it I'll look into a new radiator. I did check the fins for trash and it appears to be clean. Thanks for the inputs.