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Re: car comes close to overheating while doing highway...
Sounds like either the thermostat or water pump. Either way, there appears to be a lack of circulation in the cooling system. Check the overflow reservoir. If it's backing up or not taking coolant, it's likely the thermostat, if it's working correctly, I'd lean toward the water pump. All the best. Greg
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May be a bad fan or thermal switch not working. The a/c condenser sits in front of the radiator. It may be that the load on the engine going up the mountain is too much for just the air blowing across it to keep it cool, and the car shuts the a/c off to try and keep engine temp down. Cruising along on level ground is enough to keep things cool. Going up the mountain means more engine heat and probably reduced speed, which reduces air flow, especially if a fan isn't working.
Let car cool off, start engine and let it warm up without driving. Monitor temp and fans. When temp gets about normal, look for fan to kick on. Then when turning on a/c, look for the other fan to kick on. Don't let engine overheat. If one or both fans don't come on, start there. Check fan motor not locked up, relay working, fuse/wiring good.
if it has a viscous fan hub ( fan clutch) then the fan will hook up when the radiator temp reaches around 95 degrees C
this coupling increases the fan speed to close to engine rpms and there will be fan noise as it sucks air through the radiator
it prevents overheating by moving extra air through the radiator when there is insufficient road speed to do the job
it is most noticeable when in slow traffic , sitting at lights or with engine rpms high and under load but not noticeable at highway speeds
When driving down highway only, it overheats? When running steady, I take it you mean, sitting idling? If it's only overheating at highway speeds, and all else has been checked out & is o.k., I'd take a close look at rad hoses for soft spots (easy to squeeze) that may be causing hoses to collapse at highway speeds & restricting flow of cooling system. Another thing to try is, turn your heater on high fan & high temperature when it starts to heat up & see if temperature gauge goes down (cooler) at all. If it does, then rad not doing it's job properly.
Well, it could be a clogged radiator, stuck thermostat, bogus fan clutchbut it is disconcerting that it heats up at highway speeds and cools down when slowing down. This really sounds like a blown head gasket to me. Combustion gas is seeping into the coolant and overheating it. I would have a reliable shop check your coolant for the prescence of carbon monoxide. They can check it w/ a simple test kit. There should be no CO in the coolant.