[T3] Engine "stumble' on Start

Jim Adney jadney at vwtype3.org
Thu May 25 06:54:52 PDT 2023

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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