Re: <E30> Thunking Noise form Heater Area

[Norman Boehl <nboehl@xxxxxxxxxxxxxxxx>]

Is there a possibility that there might be some air trapped in your
cooling system?  I'm guessing an air pocket might be forming/trapped
in your heater core, and the water pump might be causing a "resonance"
between the water and air.  You might check/bleed your cooling
system (its possible on a 6-cylinder, anyway).

Just a wild guess/speculation :^)

Good luck,
Norm


Stroes, Gustave wrote:

> Digesters,
>
> I recently replaced the heater core and heater valve on my 88 M3.
>
> The other day I turned my thermostat about 30 degrees clockwise form the off
> position.  Just far enough so that the heater valve solenoid should be open.
> I was driving in town, with many stop and go's.  I noticed a "thumping"
> sound coming from behind the center console.
>
> After some experimenting I have discovered that the frequency of the thumps
> is related to engine RPM.  The thumping starts at about 1000 RPM's and
> increases in frequency until about 2800 RPM's at which point it ceases.  So
> driving on the freeway with the heater on there is no thumping.  If I slowly
> decrease the RMP's through the range from 2800 down to 1000 there is no
> thumping either.  But if I come down to idle and then slowly bring the RPM's
> back up again, then the thumping manifests itself again.  The relationship
> between the frequency of the thumps and RPM is very repeatable.
>
> It sounds like some kind of valve opening and closing (the heater valve
> maybe?).  It is very peculiar and I am not sure where to begin looking.
>
> Does anyone have anything to suggest?
>
> Thank you,
>
> Gustave Stroes
> 88 M3
> Redondo Bch, CA

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