Hi, guys

I have a 84 GMC 3500, manual. I swapped the engine with a older guessing 70s engine, 350, nothing special about it as far as I can tell. Everything works great, except when traveling on the highway. It gets pretty close to the red line when I'm going 65-70 mph, but if I slow down to 55, or even in stop and go traffic, the temperature stays around 230 degrees, a bit to the right of the middle range of the gauge. I never get any coolant or other weird smells indicating something is going on. I occasionally get belt slipping, but I can't figure out what is causing it, everything works great most of the time.

Before I start yanking water pumps, thermostats and radiators, I was hoping that someone could give me some wisdom. This is the first vehicle I've owned that gets hotter when driving at steady speeds, but cools down in stop and go. Thanks!

 

i had a friend that this happened to, and it was a thermostat, it should be cheap fix, good luck

 

Sounds like you have a bad thermostat. as stated in the other post by mrfisher86 its a cheap fix