Case Study - E38 - Overheating
| Date of manufacture | March 1997 |
| Mileage | 102,000 |
| Model | 740i |
| Engine | M62B44 |
Symptoms
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.
