My 2003 Jeep Wrangler (6 cylinder) overheats at at highway speeds. I've replace the fan clutch, the water pump, the serpentine belt, the thermostat, and most of the hoses. I've flushed the system and back flushed the system. Still, it overheats. My radiator doesn't leak.
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no car make or model mentioned so getting an answer is hit and miss
however what you describe indicates a fan drive problem so if it is electric fan cooled-- check the fans for full blade speed ( weak motor operation) or if viscous fan hub ( fan clutch) replace the viscous hub
it is all about moving air through the radiator when the vehicle speed is slow and the engine rpms are high
if the fan is not doing the job of pulling air through the radiator , the car will overheat
at highway speeds the ram action of the air flow virtually makes a fan useless as there is more than enough air passing through the radiator to cool the water
Im guessing here but I suspect that you have a viscous fan hub
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
Well you could have a bad water pump that is not circulating the water right while you driving you car on high speed but if you wan to try if is the thermostat take off and drive the car like that if the problem goes away just put a new thermostat
When does it overheat? At idle, or at highway speed? Is the coolant level where it should be? Overheating at idle or highway speed may be a faulty thermostat. If it overheats only at idle then it is likely a cooling fan fault. To determine if the fan is working start the engine and switch the A/C on, the fan should come on, if it does not, then you need to have the computer "scanned" to see if it can detect the reason for the fan not coming on. If the fan is switching on, then you need to have the water pump checked to see if in fact it is pumping coolant (this could also be the cause of overheating at highway speed) If you are overheating only at highway speed you may want to have the enigine checked for a blown head-gasket, or faulty radiator.
If this vehicle has a clutch type fan it is probably the problem, good at idle but once the rpm's speed up it does not keep up with engine speed, thats why it heats up at highway speed. Difficult to diagnose because you cant always tell if it is spinning as fast as it is supposed to be.
I had the same overheating problem in my jeep ! My problem was finaly found out to be the clutch in my fan its a different kind of clutch system it work as this. When your jeep gets warmer the tighter your fan clutch gets .While cool your fan moves slowly. now to test your clutch just warm up your jeep and rev it up a bit if your fan dose not pick up speed then it is sliping.What will come out of this is that you fan will the seez and run full all the time. if you jeep dose not overheat wile moving then it is not your thermo stat period.
Check your rad hoses. They will tend to collapse (**** in) & cut down on flow at highway speeds, more than city, due to increased demand & flow on cooling system. If they don't feel relatively stiff, I'd change them. You might even see them **** in while sitting and revving the engine a bit.