Re: [T3] Surging at idle when warm

["Jim Adney" <jadney@vwtype3.org>]

On 26 Oct 99, at 10:33, Nico teWinkel wrote:

> That's exactly the problem I'm having!
> It was worse at first - basically doing this when cold too, so I replaced
> the Voltage Regulator (new Bosch 30amp). That cleared it up somewhat, but it
> is still doing the thing you described - and only when the engine is warm.
> I do have new spark plugs and new distributor points, rotor and cap. A
> mechanically inclined friend of mine helped me set the gaps (which are
> preset on the plugs), and the points gap and dwell (?) or whatever they call
> that thing with the distributor points.

Let me add some background to what I posted earlier.

The FI shuts the fuel off if the throttle valve is closed and the rpms 
are above some trip point. It does this to reduce emissions, 
maximize engine braking, and improve fuel consumption when you 
are slowing down with your foot off the gas. If the idle is set too 
high it can get above the trip point and cause this "hunting". The 
hunting is just the natural result when the engine is running too fast 
and the fuel is cut off, then the rmps drop, the FI starts back up, 
and the rpms rise back above the trip point again.

At the same time that the engine is warming up, the Auxiliary Air 
Regulator should be slowly reducing the amount of air admitted to 
the intake; this is an attempt to keep the idle rpms approximately 
constant. If the AAR is stuck, or not functioning for some other 
reason, the idle speed will increase as the engine warms (because 
of the reduced friction with warmer oil.) In this case, the engine 
may idle just fine when cold, but will start to hunt after it has 
warmed up.

Cars with AT from 70- have an AAR which is electrically heated; 
the power is supplied from the fuel pump relay and the wire exits 
with the oil pressure switch wire. All other cars have an AAR which 
is controlled by a bimetallic spring that sticks down into the engine 
case. In both cases the AAR mounts where the carbed engines 
have a fuel pump, just to the right of the distributor.

To test your AAR you can just pull off the hose that runs between it 
an the air cleaner. With the engine cold this hose should be 
sucking in air and blocking this airflow should cause the idle speed 
to drop. With the engine warm, blocking this path should have little 
effect on the idle speed.

Jim
-
Jim Adney     jadney@vwtype3.org
    Madison WI USA

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