overheating

my 1990 735IL is overheating. i replaced the radiator and thermostat about 6 months ago. it over heats while idling. the gage goes red. when driving down the road, the temp drops back down. the fan is running.. what could be the problem? please help. thanks.

If she overheats at idle but not at speed, I would suspect a bad viscous fan clutch. This is a very common problem and replacing the fan clutch is rleatively easy. The clutch is that big aluminum looking casting on the primary fan (not the elcectric fan). You will need to take the fan off and that means unscrewing that 32 MM nut (left hand thread) that holds the fan. The hard part is to keep the fan from spinning while you loosen the big nut (there is a tool for this). Once fan is off you unscrew the cluth and replace with new clutch and screw the fan back on (watch that left hand threaded nut again). I had this happen to me in my 1988 750 while on vacation is SW Florida and there are not many foreign parts places in that area. I found a parts shop bought and replaced the clutch and she runs just fine again.

thanks for the response. brainstorming here with some mechanic friends and they figured fan clutch too. one question though, how do you tell if the clutch is not working properly?
thanks.

Generally the fan will spin too freely. With the engine at normal temp, attempt to stop the fan using a a rolled up newspaper, If you can stop the blade, replace the clutch. This is not fail proof but is a good start. You can check the auxilary fan 9electric behind radiator) by starting car and turning on AC, if this fan runs the relay is good.

Bear your good... You took the words right out of my mouth. The fan clutch is bad. Incidently my 87 750 had the same problem as yours. I at first thought it was the thermostate but the new fan clutch did the trick.

I have found that with the different engines that all fan clutches are not created equal. With the 735's you can stop the fan blades while the clutch is engaged. There is a little resistance however. The BMW cooling system is actually pretty simple and the fan clutch is a good start. Also the water pump could be an issue here in this case. The main thing to remember is not to let it overheat to bad or too much, because it will blow a head gasket or worse crack the head.