[T3] Engine "stumble' on Start

[Gary Forsmo]

Jim ...

Thanks, for ...

1st ... the explanation.
2nd ... for a thorough & comprehensive answer to my simplified  observation.
3rd ... your understanding of how those German engineers designed,
configuered & executed the building of our FI engine.
AND
4th ... & not withstanding your knowledge plus 62+(?) years of "messing
around" with our Type 3 cars.

It helps me & possibly many of us on The List.

Thanks.

On Thu, May 25, 2023, 9:31 AM Jim Adney <jadney at vwtype3.org> wrote:

> On 24 May 2023 at 21:19, Gary Forsmo wrote:
>
> > After the engine ran at idle  for a short time, it started to "lope".
> > High idle followed by extremely low idle.  Wash, rinse, repeat..... for a
> > few minutes.  Then without touching the throttle pedal, the engine
> settled
> > down to what I assumed was steady idle.  About 750(?) RPM.
> >
> > I think that's normal.  But what engine component is responsible for
> this?
>
> It usually means that the idle speed is set slightly high.
>
> It's caused by the interaction between the AAR (Auxiliary Air Regulator)
> and
> the RPM shutdown threshold in the brain. When the engine is hot, the AAR
> is wide open, causing a high idle. As the engine warms up, the AAR slowly
> closes, pulling the idle down to normal.
>
> The brain has a feature that shuts off the FI if you take your foot off
> the
> throttle at high speed. This gives max engine braking, min fuel
> consumption,
> and min pollution during coastdown. The FI comes back on at some lower
> RPM. If the cold idle speed exceeds the high threshold, the FI goes off,
> letting the speed drop until it gets below the low threshold, when the FI
> comes back on. Thus you get the loping.
>
> Note that this ONLY happens when the throttle is completely closed, so it
> does not interfere with normal driving.
>
> If everything is correctly adjusted, the cold idle speed should not go
> above
> the high threshold, but if the idle is adjusted just a bit too high, this
> can
> happen. It does not cause any harm, but you might want to check to make
> sure no hose has fallen off, creating an air leak into the intake, that
> has
> accidentally increased the idle speed.
>
> If everything seems fine, just adjust the warm idle speed down a bit. This
> usually solves it.
>
>
> If the high idle
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
>
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