Diagnose cooling problem

Hello to all.
My son's 325iC began to overheat today. The auxiliary fan does come on. Do we imagine that it is the thermostat, or are there diagnostics we should work on first? Unfortunately, no manuals came with it, so we are operating blind. All help is appreciated.
Tamara

It could be a number of things does it overheat right away or does it happen when you're like stuck in traffic?

bluepony, welcome to the fest!

I sent you a PM which may help with the manuals.

On the SA Models the Auxiliarry fan is triggered by a thermo switch that is screed into the side of the radiator.
Hope that helps to diagnose & repair

Many thanks. Pressure check is scheduled for Friday.

Temp increased after 20 minutes of highway driving. No previous overheating issues. Coolant sytem appears full, no visible leaks or fluid loss.

could be the clutch fan. common problem.

The clutch fan would be the problem if it heats up only while idling, not during highway driving. And if your aux. fan is turned on while your AC is not on then you've reached a cooling temp that is not normal and requires your attention.

When was the cooling system last serviced? If there is no coolant missing or there are no leaks then I'd suspect either a bad t-stat, bad water pump or a clogged line or radiator.

We checked the aux fan by turning on the AC to make sure that it does work.

First suspect would be the thermostat.
On an E30 vintage car, 16-20 minutes at highway speeds is the typical amount of elapsed time for cylinder head temp to cause the water temp to spike in the red.
I would suggest replacing the thermostat, the water pump and the lower front radiator hose. This is based on the vintage of the car, and perhaps because Sonny has run to Mama with his problem. (ie he is not particularly car observant)...No Offense intended.

m

Car is having the coolant system pressure checked today, to be safe. Then we head into the t-stat etc. Son isn't unobservant of the car, but hates computers

How old is the coolant? Mileage on said car? Has the radiator ever been cleaned? Could be just debris in between the fins. A light pressure hosing out of the radiator will help.

Cooling fan clutch
To save energy, the mechanical fan (as opposed to the electric fan) will spin at engine speed only when the engine (and therefore, the fan clutch) gets up to a certain temp.
Symptoms - in hot weather, while idling at a red light, the temp gauge will creep up towards the red zone.
How to check? With the engine hot, using a rolled-up newspaper, GENTLY try to stop the fan by pressing the side of paper roll against the MOVING (watch the fingers!!) fan. If it chews the paper, the clutch is probably ok. Easy fix IF you've got access to a 32mm open end wrench! Part is about $70 from various suppliers.

if its taking that long and you live in colorado, id push alil more towards the water pump. if its the plastic blade, it could be cracked and when cool, hold tightly on the shaft, then when it warms up, it starts slipping and doesnt move the coolant fast enough.

Many thanks for all of the suggestions. The pressure test was great, so no damage to head gasket/head. And....it is the fan clutch. On order, soon to be back on the road. I know, the symptoms weren't quite right, but there you go