Case Study - E38 - Overheating

Date of manufacture   March 1997
Mileage   102,000
Model 740i
Engine M62B44


The temperature gauge sat right in the middle most of the time, occasionally heading towards the red but as soon as it hit 3/4 of the way the temperature started dropping again. It would only do this if the car was stationary or travelling slowly. With the bonnet open the viscous fan could be seen to be rotating reasonably fast, as the needle started rising towards the red the auxiliary (electric) fan started operating and the temperature dropped again.

The fix

This is a common problem with the E38 and the E32. It could be the thermostat, but, in that case the A/C fan would not help a great deal as the viscous fan should be doing most of the work. The thermostat is temperature-mapped and under control of the ECU to a certain extent. Diagnostics will show if it has failed as it will produce an error code. More blurb on the thermostat:

E38 Temperature-Mapped Thermostat

The usual culprit is the viscous fan which has a temperature-controlled fluid-clutch. When the engine is cold it ambles around producing little puff. The hotter the engine gets the more the clutch grips and it should start puffing out masses of air. The usual symptoms of a failed viscous fan is that the car starts to overheat when the car is stationary or moving slowly and the temperature is reduced by the A/C fan only (it is designed to provide additional cooling when the temperature exceeds safe limits). More blurb on Auxilliary fan:

E38 Auxiliary Fan

Testing is usually done by rolling up a newspaper. When the newspaper is poked into the fan blades the fan should stop rotating when the engine is cold and should shred the paper when the engine is hot! In this case the viscous clutch was definitely the problem, even with the needle heading for the red the fan could be stopped with the rolled-up newspaper. Replacement is pretty simple and can be done with minimal tools although a 32mm open-ended spanner is needed. These spanners are similar to those used on propane cylinders and are available from calor gas stockists.
Here is the method for replacing the viscous clutch. It is best to get the clutch from BMW as the after-market versions can be a false economy.

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