Is the fan turning off on the highway? It should not be trying to actively run on the highway, it ought to be freewheeling since you don't need it with the air moving through the radiator. If the fan relay is not turning off, then the fan is fighting the air moving through the radiator, which could be the problem.ca_surveyor said:
Yeah, getting that temp set just right is tricky. Need to set the turn on temp just higher than the thermostat wants to keep the coolant temp so that the fan kicks on just higher than the natural temp of the coolant in the radiator. The radiator should be sufficient to keep the engine cool moving on the highway with no fan or anything. If that is not the case, then the radiator is too small. If the fans themselves are running while on the highway (and really anytime you're moving at a decent speed) they could be keeping enough air from flowing rather than helping. I have a 3 row core radiator in my truck, which when I'm going downhill on a long pass will actually cool the engine to the point that the thermostat will close. I didn't expect that problem, but I did intentionally size the cooling system to be much more than I need. I ended up finding a Toyota 2-stage thermostat (it actually has two springs and valves) that keeps the engine temp from crashing when the engine isn't working on descents.ca_surveyor said: