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From digest.v6.n663 Wed May 14 19:50:55 1997
From: "Brett Anderson" <>
Date: Wed, 14 May 1997 18:01:57 -0400
Subject: Re:525i E28 get warm by idling.

> From: tore at (Tore Vulpius)
> Date: Tue, 13 May 97 20:01:10 GMT
> Subject: 525i E28 get warm by idling.
> Hello wizards!
> My 525i gets pretty warm after say 5-10 minutes of idling (after normal
> driving, temperature around 20 degrees centigrade). Now, the first thing
> to check for is the thermostat: it was changed with no effect. Secondly,

> the visco coupling for the fan (no - no electric fan on the E28) might be
> suspected to be faulty. However, when the engine idles it appears to me,
> that the fan will follow the fanbelt revs even when the engine is cold:
> the visco coupling es effectively only forcing the fan to rev up with
> the engine (this is a question)? Consequently, while idling the engine
> temperature should NOT be sensitive to the presence of a defect visco
> coupling (this is another question)? I HAVE ordered a new visco coupling
> (around 200 $ in Denmark), but I am really not sure about this being the
> right solution. Anyone out there with similar problems and experiences?
> Best regards
> - ----
> Tore Vulpius
> Denmark
> tore at

Before you go replacing an expensive fan clutch, you may want to check your radiator.

The fan clutch can be stopped by hand when the car is cold, even above idle. I don't suggest actually using your fingers though.

As the car heats up to the temperature of the thermostat, the thermostat opens, allowing the still cold fluid from the radiator to enter the engine and forcing the hot fluid into the radiator. Up until this point your fan clutch should be freewheeling. Once the hot fluid is in the radiator, air movement , created by vehicle motion, blows hot air from the heat transfer fins of the radiator, into the engine bay. This hot air is what heats up and locks the fan clutch.
At a stationary idle, the freewheeling fan still creates enough air movement to draw hot air through the radiator onto the clutch. If your radiator's fins are full of dirt and bugs, which I can guarantee on an E28, there may not be enough air being pulled through the radiator by the freewheeling fan, but plenty of air still gets through while driving, so the temp only goes up at stationary idle. You may well find success by removing the radiator and pressure washing the fins, this must be done out of the car and carefully, the high pressure water jet must pass perfectly through the fins at 90 degrees to the radiator or it will bend the fins, closing up the radiator and compounding the problem.
As a test, clean half the radiator, shake out the water from between the fins and then hold the radiator up to the light, you will see how much more light is visible throught the clean side as opposed to the untouched side.

This is a very common cause for erroneous replacement of a fan clutch.

Brett Anderson

BMW and ASE Master Technician

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