Need Help Diagnosing Over-Heating

[stevefreides]

1991 318is, 180xxx miles. 

It doesn't overheat driving moderately in ~80 Farenheit weather yesterday

It does overheat if idling but not moving - was able to nurse it home by shutting it off at traffic lights, and pulling over to let it cool a couple of times.

Radiiator fan spins turns like nothing is wrong.

Has fluid in plastic "overflow" tank that's on driver's side of radiator, right up to the line that's about 2/3 of the way up.

I'd appreciate suggestions as to what part I might try replacing - need to fix asap, if I can't fix it within a few days, it'll have to go into the shop because I need it for next weekend.

Many, many thanks in advance for advice.

-S-

[DesktopDave]

I'd test the auxiliary cooling fan. Does it ever turn on? It's in the front of the radiator. You should be able to manually power it up when the car is running by switching on the A/C. The fan will run on low speed regardless of whether the rest of the A/C system works. It's a good 'emergency' backup for a failing fanstat.

If the fan isn't working, you can test the switch, on the passenger side of the radiator. There is a small brass thermostatic switch with three wires (sometimes called a fanstat). Pull the connector and make yourself a wiring jumper with a short piece of thick wire. With the ignition on, shorting the green/black wire to the black wire should turn the aux fan on low speed. Shorting the green/black to the black/brown wire should turn it on at high speed.

If neither speed works, check fuse 19. If A/C won't power up the fan, check fuse 20. If high speed works, but low speed doesn't, check fuse 3. There is a large resistor on the fan that can also fail, for a temporary fix you can cut off the wires on either side of the resistor and splice them together. The resistors are still available new, but they're not much fun to install.

[stevefreides]

Fan turns whenever engine is ON, stops easily with rolled up newspaper, fan clutch seems the likely culprit.

-S-

[DesktopDave]

That engine-driven fan is a likely culprit too. If the fan clutch (metal part in the center of the fan) is good and hot and the fan won't shred a newspaper, it has seen better days. It should turn all the time though - and it should be easy to stop with a rolled up paper when the fan clutch is cold.

But I was talking about the  auxiliary electric fan, in front of the radiator.

[Tgoode318]

Maybe its just the wording of your post but "Radiator fan spins turns like nothing is wrong" IF by that you Mean its Spins as in spins Freely With NO resistance then your clutch is bad. The fan, engine off, should be able to be turned, but not spin freely. It should be tight and smooth. Also You said "It does overheat if idling but not moving " This is The typical symptom for failed clutch the car heats in stop-n-go traffic, but cools again when the traffic starts moving. This is because the airflow through the radiator is lost when the viscous couple fails to lock, then as the car speeds up the airflow increases with the speed.