BMW E30 (M20) Problem - Overheating and Overcooling!

[JWatkins]

Hi All,

A bit of a weird one this... In traffic, the temp gauge slowly creeps up past 3/4 but in free traffic at around 50mph +, the needle drops to around 1/4 (i.e. well below half temp). It has just started doing this in the last couple of days.

It seems that in traffic it overheats but on an open road it over cools. My guess is that the thermostat is stuck partially open, (enough to overcool at speed but not enough to cope with traffic).

Has anybody else had experience of this on a 320i (1988) with the M20 engine?

By the way, the viscous fan seems OK and the coolant and oil are not contaminated. It's not the solder connections on the back of the gauge either - I know about that one!

Cheers,

Jason.

[scjimbo]

Definitely sounds like a stuck open thermostat mate. You can test the theory by going down hill in a low gear. That will keep the coolant circulating quickly with no combustion. The temperature needle should nose dive.

[JWatkins]

Yes, I tried exactly that and it behaved as you described.

Thanks,

Jason.

[jonny323i]

mine doesnt go over half way in traffic but runs at about 1/4 when driving might be due to the very large aircon radiator i have fitted

[JWatkins]

Changed the thermostat but no difference. Also, if I turn the heater on in traffic / at idle, the temp comes back down to 'normalish'. Beginning to wonder if it is the water pump or viscous fan now.... hopefully it isn't the head gasket!

The bit that really puzzles me is the over cooling bit going down hill with no load on the engine.

Confused!

Jason.

[lamby]

mine is an 88 320 and it stays at quarter most of the time,sounds like the fan going

[jmc330i]

Changed the thermostat but no difference. Also, if I turn the heater on in traffic / at idle, the temp comes back down to 'normalish'.
The bit that really puzzles me is the over cooling bit going down hill with no load on the engine.

Confused!

The temp will drop when you put the heater on as it takes some heat from the water.
The temp will also drop if your moving with no load on the engine as the airflow cools and the engine isn't producing any more heat - the same as at idle which the viscous should be coping with.

My 325i temp gauge sat at 1/4 when on the move and between 1/2 and 3/4 while sat in traffic, only heading over 3/4 in very hot weather - it would drop as soon as we started moving. I would occasionally use the heater on trick just to lower the temp to under 3/4 if it was bad traffic (windows open obviously!). Thinking about it, it was pretty much the same with the S50 fitted.

Once over 30mph+ the airflow should be enough to bring the temp down to normal.
If its running a good temp on the move, but running high whilst stopped, I'd be looking at the viscous. Not difficult to change and remember its a left hand thread.

[JWatkins]

Quick update for everyone kind enough to offer suggestions and for those who need future guidance..... all now fixed, it was the viscous fan coupling. The fan was turning but not fast enough, (evidenced by the load fan 'roar' the new coupling makes when the engine is hot, and the lack of any 'roar' with the old coupling). Temperature stays rock steady on 1/2 which is exactly where it always used to be.

Cheers,

Jason.

[bmwbmwbmw]

Try the viscous