RE: <E30> Stalling

["Fadeev, Alex" <alex.fadeev@xxxxxxxxxxx>]

David A. Polasik [mailto:dpolasik@domain.elided] wrote:
> 
> I'm seeking some collective wisdom regarding a stalling 
> problem that is becoming more pronounced on my '86 325.
> 
> Last October, I finally had the original ICV and O2 sensor 
> replaced to correct a surging idle that seemed to occur only 
> during cold months (i.e., temperature below 40 degrees 
> Fahrenheit).  Up until these replacements, the RPM's would 
> rapidly cycle between 700 and 1,600.  Now, the idle is 
> significantly improved, but not perfectly steady: it 
> fluctuates between 700 and 800 RPMs regardless of outside 
> temperature and maybe between 700 and 900 RPMs when the A/C 
> is engaged.

David,
Check your temperature sensors (front of the manifold, right behind the
thermostat housing). You will find two of them there. If I remember
correctly, one has a brown connector (instrument cluster temp gauge), the
other is blue (DME temp gauge). Compare DME temp sensor resistance to the
Bentley values. DME is using that sensor's input to decide if the engine is
running in cold start mode (more gas for richer mixture).
Also check for vacuum leaks. 

> I can live with this.  However, my main problem 
> is that the engine now stalls in very humid weather when the 
> A/C is on.  Specifically, if I'm driving along with the A/C 
> on and the RPMs up around normal cruising range (approx. 
> 2,000) and then I disengage the clutch to come to a stop, the 
> engine will quite often stall.  On rare occasions, it will 
> drop to around 400 RPMs and then recover to a normal idle.

2000 RPMs is WAY too high. With AC on, the RPMs should be in the 900-1100
RPM neighborhood (I don't remember exactly, but definitely not 2K). It is
DME's job to bump the RPMs a bit when the AC is running. If everythin else
checks out, perhaps it's time to suspect the DME...

> Is there an adjustment that needs to be made to the ICV?

No, but you can never rule a bad ICV out (even if you had recently replaced
it) unless you swap it with a known good unit.

> Is the O2 sensor bad?

02 sensor would not be playing this much mayhem with your idle.

> Please note also that the original ICM is still installed.

Aha!
That could very well be your problem (I forgot that pre '89 M20's have a
separate ICM outside of the DME). I would almost be willing to bet that your
ICM is kaput.

> The mechanic who replaced the ICV and O2 sensor for me is
> very competent.  His original advice was to delay replacing
> the ICM until we could be sure that it is contributing to
> this situation since this is a rather expensive and non-
> returnable component.  I'm attempting to diagnose the most
> likely cause of my stalling before I commit to a new ICM.

I think your mechanic is right. 

good luck,
alex f

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