I flushed my coolant and put a new thermostat in my truck. When I'm on the highway and I drive 75+mph my temp gauge rises to past 3/4. But if I drop back to 60 mph,my gauge will go back to normal. Why is this? The water pump seems to be working fine why is it only overheating when I drive at highway speeds?

 

Bump

 

truck have a/c?

 

Fan clutch?? Low of antifreeze?? Anytime you drain your cooling system you need to have the heater on to fill the heater core cavity.

 

Check your lower radiator hose, theres a good chance that the hose is collapsing. Some hoses have a spring inside others have the spring molded in the hose. As the hose gets older it gets soft and the spring can collapse.
regards
rikard

 

Check those hoses and double check that thermostat. Sometime even new ones can be bad.

 

I agree its possible the thermostat could be bad even though its new. I had this very problem on a ford ranger, replaced the thermostat and never had that issue again. Good luck

 

Thanks for the suggestions. Will double check the thermostat and all of the hoses again.