Beiträge von E31

Just a post about my experiences with my viscous fan and overheating; two weeks after car purchase!

My car is a low mileage example and hunted out for that reason. However, countryside blasts instead of battling with the daily commute may be a contributory factor to the following. Low mileage cars can cause problems too!

Problem
The car had come out of storage (previous owner) and it was my first time in stop start traffic. It was a very very hot day, unlike my test drive!
When stationary at traffic lights, the temperature gauge would climb and climb. As soon as the vehicle started moving, the temperature would fall.
This was obviously a relationship controlled by the amount of air passing through the radiator. At worst, the temperature gauge just kissed the red before I turned her off. I managed to turn her off before any warning on the MID, although I imagine that warning wasn't too far away.

Tests
With the coolant level fine and no leaks apparent, I was immediately suspicious of the viscous fan. With the engine very hot again, needle climbing, I tried to stop the fan blades with a rolled up newspaper. I could stop the blades dead, with very little resistance.

Viscous Fan Operation
We all know why a viscous fan exists; when cold, it free-wheels for quicker engine warm up times, greater engine efficiency and reduced fan noise.
However, its operation is often misunderstood. One of those misunderstandings (by me initially) is in the word 'viscous' and the belief that the change in contained oil viscosity (due to heat) is the basis of control. In fact, this is the complete opposite; the oil in a viscous fan hub is chosen for its constant oil viscosity across a broad temperature range.

The operation is based around centrifugal force and where the oil is contained within the fan hub. Quite simply, there are two chambers within the hub, a 'reservoir chamber' and 'drive chamber'. The division between these two chambers is controlled by a pin valve, operated by a bi-metallic spring on the external face of the hub. This bi-metallic spring is mounted directly in the airflow of the hot air after the radiator; its behaviour is therefore related to engine temperature. When hot, the bi-metallic spring bends, allowing the pin valve to move, allowing the oil to flow (due to centrifugal force) into the drive chamber. The drive chamber pushes the oil to the outer diameter, giving greater centrifugal force and also acts against an internal drive disc, much like a viscous plate in an LSD. This locks the hub and drives the fan.

The problem is the pin. It is exposed at the front of the hub, and if it doesn't have regular hot/cold cycles, it never moves and seizes.

Repair
I removed the fan (left hand thread!)



I then removed the bi-metallic spring and freed off the pin with WD40 and a bit of brute force. I never removed the pin entirely. I greased the pin (as WD40 goes funny after time and can be more of a hindrance) and refitted the bi-metallic spring.

Photo below = Dissasembly & Pin Seized



Photo below = Pin free



The car has been good as gold since. The temperature needle sits bang in the middle and when hot, I get that audible 'roar' from the fan which proves operation.

A new viscous fan hub is about £150 from BMW (not fitted) so I am quite happy. I think you can get pattern items, but I am not a 'fan' of pattern parts!

Thanks for the warm welcome guys. I managed to get two months enjoyment from the now finished summer but she is SORN’d now until March. Diamond cut wheels and salt do not mix.

The 840 has been awesome so far. A compelling blend of looks, luxury, power, refinement. I am a massive fan of all pillar-less coupes; and yes, that even includes the Jag XJ6C!

If I had to criticise, and this is really scraping the barrel, I have two observations:

• For a car of such huge external dimensions, interior space does not correspond!

• She is a very big car to ‘hussle’ on a country B road. It just needs some getting used to I guess. Scrubbing off speed into a tight hairpin and her weight and external dimensions are very very apparent!

Very happy with this website too – found some wonderful info; especially the repair manuals. Have found the wiring diagrams too thanks to this site and am a lot happier understanding the double batteries. You hear so many technical horror stories with this car (most unfair, I’m sure) so I naturally assumed the twin batteries were regulated by some over-complicated split-charging system. But no, they appear just linked in parrarel!

With such a free resource, my forthcoming membership to BMW Car Club GB seems in doubt really.

Anyway, thanks again, roll on the summer!

Hi!

First post, so seems fitting to have some pictures of my 1995 840Ci in Orient Blue. There are only a handful of BMW’s that I’m mad for, the 8-series being one of them.

It was a three way fight. Porsche 928, Mercedes SL (R129) or an E31. Bumped into this car and bit his arm off!

All original, with owners books, tools, torch, FSH and original bill of sale. 2 owners from new and currently on 72k miles. She is a 4.0 (M60) automatic with Silver Grey leather.

I have just defaulted from Porsche Club GB to ‘tick the 840 box’ and have absolutely no regrets. A truly awesome and wholly underrated car. I will keep her for many years.

I will be doing all my own work, so hope to share some of that with you guys soon. Plus meet you at a show and shine. Best Regards, James.