Overheating only at highway speed

[JohnGrawcock]

I've been fighting an over heating problem on my 79 Chevy pickup. It can sit parked and idle all day without getting hot but after 4 or 5 miles at 60mph the temp gauge is pegged. It just doesn't act like the typical cooling system problem. Just to see what it would do I flushed and refilled the system and then took it out on the road with no thermostat. It took longer but it still overheated.

Could it be that I'm running to lean at speed and need to work on the carb? I'm going to pull a plug and check as soon as it cools. I'll also double check the belt tension. Does anyone else have suggestions?

 

[JohnZ]

First, put a 180* thermostat back in it. Is your vacuum advance working? Probably is, as you don't have an idle cooling problem, but I'd check it anyway; retarded timing or inoperative vacuum advance is a frequent cause of heating.

Generally, OK cooling at idle/in-town and heating only at highway speed says the radiator has finally accumulated enough scale in the tubes that its heat transfer capability has deteriorated to the point where it can no longer cope with cooling the engine under load. If it's the original radiator or is more than ten years old, it probably needs to be re-cored or replaced.

 

[JohnGrawcock]

:duh Doh!! I should have known that. Been away from the old high mileage stuff for too long. Thanks JohnZ.