[T3] Cold idle Problem

[Jim Adney]

On 21 Jul 2011 at 1:41, SCS Crewcab wrote:

> Thanks for the suggestion, but again, if I had a bad electrical 
> connection, it seems it would run lousy when I revved it up. I've got 
> good compression and the car runs well except at cold start up. 

The bad connection possibility was more of a shot in the dark. My 
only thought was that a bad connection might be more of a problem at 
idle when there is more mechanical motion of the engine, because 
higher rpms usually let the engine smooth out. It's also possible 
that a bad connection would make a larger difference when the 
injection pulse is short.

> Actually, Bobsnotch seems to be on to something. Reading up on the AAR, 
> it supplies extra air to a cold engine to match the extra fuel supplied 
> by the ECU. If the AAR is malfunctioning, there wouldn't be enough air 
> to match the extra fuel. This would create a rich condition until the 
> engine warmed up enough to make the ECU reduce the volume of fuel. 
> Although this seems reasonable, I do not notice black sooty exhaust when 
> the idle is acting up.

The AAR lets in more air. Since that's all it does, it's completely 
equivalent to opening the throttle a bit more. The amount of gas 
injected is calculated from the temp, rpm, and manifold pressure, 
which depends on the rpm and the amount of air being let in. The 
brain does not "expect" some amount of air; it measures it and 
responds to that measurement. Lots of cars run around with stuck AARs 
and no problems other than idling low in cold weather. If your car 
never sees cold weather, you'd never know the AAR was stuck.

And for you guys in SoCal, cold weather means freezing or below.

It's possible that the pressure sensor is a bit off calibration, so 
that its response to air pressure is a bit off at the idle extreme, 
when the pressure is the lowest, but that's unlikely. Was this a '70 
or '71? If so, suck on the pressure sensor to make sure that it 
doesn't have a gross vacuum leak.  

Try unplugging the throttle position switch. If that cures, or 
changes, the problem, the TPS may need to be adjusted.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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