I hope someone can help me out because I'm stumped. I have a 1985 E350 box truck w/ a 351W, 185 deg high flow thermostat, four row radiator, C4 trans, 4.10 rear end. When running around town the temp gauge stays right in the middle of Normal, when I get on the Turnpike running 75-80 it runs very hot, this is with or without a load. I have to constantly watch the temp gauge on the highway and when it gets too hot I back down to 60mph and it cools right off. It has 4.10 rear end so the engine isn't screaming at 75-80mph. I'm running timing at 10 BTC. Air/Fuel ratio seems to be ok. Checked for exhaust gasses in the cooling system w/ chemical, but none. No strange bubbles in cooling system and I've made sure all of the air is bled out of the cooling system. I've even tried running without a thermostat which helps but it still gets too hot, just takes longer. There is a slight difference between winter and summer, just takes longer in the winter.
The engine was built 3 years ago but has less than 25,000 miles and it has been like this the whole time. I had a mid range high torque cam put in it with Edlebrock intake and 4 bbl carb. I've tried all of the usual, fan- ok, shroud-ok, water pump-correct rotation, timing-ok. I'm wondering if the air flow into and through the radiator might be getting blocked by a pressure build up under the dog house at the 75-80mph range. Any ideas would be appreciated. TIA

 

Has anybody checked out the catalytic converter for partial blockage?
Also, is the spring in the lower radiator hose to prevent it from collapsing at hi RPM when the water pump pulls the most coolant from the radiator?

 

Can't be cat since no cat. I just checked the Lower Rad hose and no spring, I hope that is the problem. A collapsed lower Rad hose never occurred to me. I will add a spring and test drive it this afternoon. Suppose to be near 100s so it will be a good day to test. Hanky, thanks for the quick reply. I’ll let you know what I find out.