350 runs hot on the highway - maybe?
Ok, this is actually in an '81 GM 3/4t van but please help me!
Recently the gauge started going up to and usually beyond 260 degrees on the highway. The funny thing is that as soon as I hit 45-50 mph it climbs up there rapidly, and once I pull over and let it idle it drops very quickly.
I have replaced the thermostat and sending unit for the gauge. When I ground the wire the gauge pegs. The wire is not touching anything like spark plug wires. Coolant is fresh and full. No coolant in oil. Fan clutch feels good.
It just started this out of the blue - one day running great, next day thinks it's overheating at hiway speeds. It doesn't boil over (haven't tested rad cap yet tho). The radiator *could* be plugged internally I suppose, but the problem happened overnight like I said.
I'm still not convinced it's running as hot as it says, and the fact that it's SOOO speed dependent makes me suspicious. Any thoughts?