86 GMC truck overheating

BOBZAM216

MEMBER
1986 GMC

I have and 86gmc 3/4t p/u with a 454. It has a new radiator, waterpump, thermostat, fan clutch and hoses. The engine has been rebuilt. The problem is the truck runs cool as long as the speed is 55 or below. It will also run cool at idle for long periods in extreme heat. As soon as the speed is increased the emp seems to climb with the speedometer. Oh the truck also came with an aux electric cooling fan I also added a second fan and put both on a switch so I could control them manually. The cooling system has been pressure tested and is ok. Any help will be appreciated as I am at a loss

1 Reply

MIKEYBDMAN

MECHANIC
623 POSTS

It almost sounds like something went wrong with the engine rebuild. We tend to think that since it is an overheating problem, it has to be a coolant issue. Maybe the engine is producing too much heat for other reasons?
Was anything special added to the engine when it was rebuilt? Was the compression ratio changed, cam shaft style replaced, or any other variation made away from the factory specs?
I know the head gaskets have orifices to limit the water flow thru the engine to control cylinder head temp, but these are set to control it with the operating parameters set at the factory standards.
Have you tried to run the truck without a thermostat? See if it continues to climb that way, and you may end up taking the heads off and taking a look at your internal water ways. Maybe some sort of debris has clogged part of the water passage way and limiting the flow of coolant.
These are just thoughts, and I hope you can find the source, but it sounds like the heat the engine is producing is more than your cooling system can handle.
Good luck.