Creeping Temp at Idle, Aux fan wont turn on

[brutus87]

Hey guys,
Trying to figure this out before I get tired of throwing money at it.

Car: 1988 M20 325i 5 speed.

Problem: If car sits at idle on a "warmish 80 degrees and up" day for longer than 5 mins, the temp needle will creep up to the 3/4 mark. If I turn on the heater, it goes down, or if I take it for a spin.

I have already replaced with new parts:
Thermostat
Fan clutch
temp sender
water temp sensor
Aux fan switch


I noticed that the aux fan will only turn on if the AC switch is turned on, will never turn on if the temp creeps up.


I think I need to replace the resistor or relay, but im not sure, and im getting tired of throwing money at the problem. Any help/tips are greatly appreciated!


Thanks R3v.

[JCarp90]

Mine had the same problem. I built a new motor and it fixed it.

Seriously, though - thermostat. I've gotten countless new thermostats that turned out to be bad.

Oh and I never did get my aux fan fixed after replacing everything but the fan.

[jeffnhiscars]

Check for a bad "new" tstat for sure if its overheating, but if your aux fan is not kicking in when the temperature rises past the dual stage temp switch set point then either the switch is bad or there is a short.

With the AC off the aux fan is not a substitute for a properly functioning cooling system.

[jlevie]

Unless you just got unlucky and got a bad fan clutch and/or thermostat, both of which can be tested, my guess is a partially plugged up radiator. Since the aux fan spins if you engage A/C, the fan resistor and low speed relay are good. You can test the high speed relay with a jumper at the thermo switch connector. If both speeds work, the thermo switch is bad or the thermo switch isn't covered with coolant (air in the radiator).

It is likely that the radiator is original, in which case it is way overdue for replacement. Along with all hoses, the expansion tank, and cap. The cooling system could be gunked up also. Unless I can see that it is less than 10 years old, the first thing I do with and M20 is to replace the entire cooling system and then flush the system. If I see lime or rust in the system when changing the water pump and thermostat I'll do one detergent flush and follow that with an oxalic acid flush.

Oh yeah, the fan shroud is important. If it is missing the engine will run hotter than it should at idle.

If everything is working properly, the aux fan will only come on when A/C is in use. It isn't otherwise needed to cool the engine.