My truck is a 5.9 diesel with a 4.10 rear gear. It cools fine below 75 mph. As soon as I reach 80 mph or so, the truck temperature increases dramatically. I didn't let the truck overheat, but it came close.

My truck has less than 99000 miles. It has never been abused and has hauled a trailer on a handful of times.

The shop says new radiator. Any other ideas?

Thank you.

 

Low on coolant??

If not that, get the truck up to temp, and use one of the IR temp guns to check temps on a bunch of different places on the radiator. Temps SHOULD decrease gradually as you move down, and to the passenger side. If they drop suddenly, radiator likely is clogged there.

 

Do you know the last time the thermostat was changed?

 

The thermostat has never been changed. I think I will start there.

Thank you for your reply.

 

The coolant level is fine.

My concern is that the shop is guessing that it is the radiator without doing a proper diagnosis, as you explain in your post.

The shop says that the thermostat is working properly. How is a thermostat checked?

Thank you for your reply.

 

If the stat opens at the proper temp, then it's fine. Generally, they will fail open, or closed, and that's it. They don't 'partially' fail. (if you have one of the safety stats, they fail all to often, but, they fail open...... so, overheating would be the issue there.) If it hasn't been changed in a while, it sure won't hurt anything to swap it out. (use a stant super-stat. No safety stats......) Don't expect it to solve your problem though.