Hello all Im new to the forum I just bought an 83 F250 4x4 with a 351w. It was a California emission model I say was because it didnt come with a smog pump and i have removed all of the emission lines. The engine did not overheat when first purchased but has since begun to do so. I have replaced the thermostat, radiator cap and all hoses, flushed the cooling system / radiator and the cooling system appears to flow fine. I have run a pressure test on the cooling system it has a very slow leak which is not visible I see no visible coolant weeping. Under stop and go traffic it takes about 45min to start to over heat however at highway speeds it overheats rather quickly it does not smoke out of the exhaust but it does however develop a lot of pressure under the drivers side valve cover and eventually starts to smoke through the valve cover vent even spraying oil out at high rpms Im not sure if this is a separate issue or the cause of the overheating issue. I plan on doing a compression check on the engine But am wondering if anyone else may have run across these issues. Any insight would be appreciated Thank you.

 

I would do a leak down test (google it for a good description)
might be a leaking head gasket.

 

Overheating at highway speeds is a sign of a coolant flow problem. I would check and make sure the lower radiator hose has a spring inside it. I would also pull the sparkplugs and look at their condition, and see if any of them have green coolant on them.