[T3] running rich and mystery material

[Jim Adney]

On 9 Nov 2012 at 21:10, Dave wrote:

> -          Check the cold and warm resistance on your 2 temp. sensors. If
> they are not sending the correct resistance to the brain, then the brain
> will think you are running hotter or colder that it really is..maybe
> creating a too rich mixture for the real operating temp. of the motor.

A bad temp sensor is always a possibllity, but I've never seen a 
problem based on one that had slightly higher or lower resistance. 
There are 2 kinds of problems that they can have: Sometimes they will 
develop an intermittent internal contact, going occasionally to 
infinite resistance, or sometimes they can develop a short to ground.

The only common problem here is with the '68-9 version that mounts 
under the #4 exhaust port: Those get shorts to ground due to the 
sharp bend in the wire. The other problems are rare.

> -          Is certainly sounds like your AAR is not working because you have
> no increased idle speed when cold. It is not supposed  a large increase in
> cold idle speed, but it will be higher than when the motor is warm. The
> brain is supposed to make the mixture a little richer when cold (like a
> choke on a carbed car), and the AAR is supposed to supply extra air into the
> IAR that will provide more air to the cylinders when the motor is cold and
> the mixture is richer. The extra air combined with the richer mixture
> results in a higher than normal idle speed. 

I know this sounds reasonable, but it ignores the fact that the 
pressure sensor compensates for any extra air, regardless of whether 
it comes in via the AAR, the idle adj passage, or thru past the 
throttle butterfly valve.

AAR problems do not cause mixture changes.

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