Viscous Fan Coupling Problems

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Mike Fishwick
Joined: Fri 19 Jun, 2009 11:27
Posts: 2009

  Z3 roadster 2.8
Location: Daglan, France

Viscous Fan Coupling Problems

Post by Mike Fishwick » Wed 27 Dec, 2006 11:57

After 85,000 miles I had the viscous coupling of my radiator fan lock up in the ‘fully on’ position, making a noise like a fire siren. This does not seem to be a particularly good record, particularly so when one considers that the air conditioning fan usually carries out engine cooling duties.

Electric fans usually give no trouble at all. Has anyone else had such a premature failure?

As removal on a double-VANOS engine requires removal of the radiator, in order to attach the water pump pulley holding tool, it was a good excuse to change the anti-freeze, and also fit the later water pump, with a metal impeller, not to mention cleaning and painting around the front of the engine etc.

Anyway, the job was easy enough, requiring that the coolant be drained, top hose and radiator removed, and also the air filter to make room for the pulley holding tool.

Although some people will claim that the coupling can be removed (it has a LH thread) by simply hitting the spanner, I have never found this to be the case. To hold the water pump against the nut, it is normallynecessary to unbolt the pulley, and rebolt it via the pulley holding tool. This is a difficult task, due to the very limited space between the front of the pulley and the back of the coupling.

Thise task was made easier by modifying my pulley holding tool so that – instead of being bolted to the pulley - it now slips over the heads of the pulley fixing bolts. It cannot slip off them, as the remaining space is taken up by the necessary 32 mm spanner.

As a bonus, it means that the holding tool can now be used without having to remove anything except the air filter, and de-tension the alternator belt.

This makes the whole job so much easier, as the 15 mm space between the fan coupling and pulley – less the thickness of the tool – makes removal and replacement of the bolts something akin to micro-surgery.

If you do not have a tool, or cannot make one. then try hitting the coupling spanner, but remember that the spring-loaded belt tensioner will simply wind up under the pressure. The belt is not designed to take such tension either, so replace it on assemby.

I had noticed an ‘untracable’ rattle from the front end of the engine during warm-up for the last year or so, deciding it was the hydraulic knock from the lube oil system which some older 2.8 engines exhibit while cold. On starting up it was gone, and must have been an early symptom of failure in the viscous coupling.

On opening the coupling (a hacksaw job) I found that the plates had been touching each other due to the play in the bearing, this causing the rattlling noise, and also wearing away the plastic fluid admission valve, part of it becoming trapped in the bearing and so suddenly locking the coupling.

So – if you have an odd rattle from the front of the engine, and nothing seems loose, don’t forget the fan coupling.

For details on draining the cooling system and radiator removal, see my entry on thermostat replacement.
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Last edited by Anonymous on Thu 28 Dec, 2006 15:33, edited 1 time in total.

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  Z4 M roadster

Re: Viscous Fan Coupling Problems

Post by DavidM » Wed 27 Dec, 2006 13:52

Mike Fishwick wrote:Electric fans usually give no trouble at all. Has anyone else had such a premature failure?
Is this the one adjacent to the radiator - if so - yes - I had one fail at 33,000 miles - and then the fan sheared and went through the radiator - this was in Scotland - literally 15 hours before the start of Highlander II last year - and the BMW assistance chap said that it was surprisingly common - all fixed under warranty though - massive bill of around £4k at the end of the day most of which was the labour - not surprisingly - and the cost of transporting the car back down the length of the UK



Viscous Fan

Post by Guest » Wed 27 Dec, 2006 14:13

David - what a disaster! I checked around the BMWCC forum and found that 50,000 is usually regarded as being an average life for a viscous coupling. It makes one wonder if it would be better to remove it, and rely on an electric fan, or the condensor fan if you have air conditioning.

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Location: Barnsley

Post by AndyBass » Wed 27 Dec, 2006 19:43

I'm coming up to 46,700 miles and recently changed the water pump and drive belt to avoid such disaster as happened to David. As it turned out the pump was OK and so far the viscous coupling seems good too, but I'll be keeping an eye on it. I'd rather spend a bit of money on preventative maintenance than lots on repairing massive damage. Keep this coming, guys, it's good to share information on common problems so we can take action.

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  M roadster S54
Location: Romford Essex

Post by Gazza » Wed 27 Dec, 2006 21:42

I remember Bloke having a similar problem with his 2.0, dont know the mileage figure though.


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