My Audi A8 is overheating. But the weird thing is that it only overheats sometimes. I checked fluids and it does have enough in there. Could it be the thermostat?? but if it really was the thermostat would it not heat up all the time?? I guess it could also be the sensor but unlikely because it does not shoot up the needle. it is a gradual increase and a gradual decrease. also the other day, it just went a couple of bars over what it usually has. but it just stayed there. Meaning that the temperature was constant, but hotter than usual. The small fan is new and when it has overheated i checked it and it was working but the big fan looks weak. it spins but i can stop it with my hand. could that be a possible problem?? thank you.

 

Need to make note of when it's running hot (if it's not the sensor)...cooler while moving, hotter when sitting still? ...could be fan problem.

Sensors can do weird and inconsistent stuff, from past experience. Mine erred mostly to the hot side but not a real pattern...it started malfunctioning by rising slowly to a hot reading.

A thermostat stuck closed follows a more consistent pattern. My failed ones would always heat up normally to the exact perfect spot and stay there for a while, then it would start rising relatively fast several minutes later to high readings. Purposely running at high rpms (lower gear) would cool it slightly, by maybe forcing a little cool water through the thermostat (maybe just the bleeder hole). Running the heater full blast at high fan setting will also cool it down some, if it's a stuck thermostat.

It's not completely impossible for a thermostat to stick, then free up more than once...but not common.

Sensors are the preferred problem (obviously) and don't cost too much for a mechanic to check and/or replace.

 

i heard it could also be the radiator, can it be a probable cause?

 

You "heard" it could be the radiator? Crap man just verify its actually overheating, get it really hot then see if the coolant is boiling in the resivior by popping it loose. If it comes flying out like Mount St. Helens its actually overheating. Now you check your radiator hoses, if its hot going into the radiator and cold coming out, the radiator is plugged. That almost never happens on these cars so i would say its either your water pump or thermostat. Either way you have to take apart the timing belt to get at it so you might as well buy an entire repair kit from www.blauparts.com and get it all done.
However if its not overheating just buy the coolant temp sensor.