Hello, I have a 92 735i. 2 months ago I had an overheating problem and it was the waterpump, also noticed the fan clutch was slow so I replaced both and also changed the thermostat and all hoses and belts... Put in BMW coolant and bled the system of air several times and it held temp great for 2 months... All of a sudden while sitting still the temp creaped up... didn't make it to the red... When I moved the temp dropped a bit... I checked the coolant and all was fine... no leaks in the system... Coolant bubbles out of the return hose into the resivor... Top radiator hose is hot so the thermostat is opening and coolant in going through the radiator... Usually I see the temp rise while sitting still and if I put the car in nutral and rev it up the temp drops... Fan clutch seams like it is working great and blows hard... Sometimes while driving the temp will creap up a small bit past center and then back down a bit below center and it keeps doing this... Usually at 60mph the temp stays dead center but normally before the problem the temp would be a tad less then center... Sometimes nothing happens and the temp stays perfect, even while sitting parked but other times the temp jumps up... It's strange cause all parts are new and were working fine for 2 months...

What I was thinking is the thermostat is slow or going bad... normally I would think the fan clutch but it seams to be working fine and strong and stops quickly after shut off...

Does the temp sensor only do my gauge and if so could this be a possible problem and in reality the car isn't overheating and only my gauge is going because of the sensor...

Seams like I have covered all bases except checking the flow through the radiator but I do know the top hose is hot... If this is a possible problem how do I check it and is the check reliable...

I have another thermostat so I may pop it in tomorrow... I have read about drilling a small hole in it so air can be bled easier... is it worth doing and if so where to drill it... any pics would be nice...

Thanks everyone for your help...