A dip below idle speed after acceleration and neutral gear engagement.

[Alex75]

Hello, please tell me what could be the reason? If you accelerate and put the car into neutral, the revs drop to 400-500, and sometimes the car stalls.

[Alex75]

Audi s4 AAN

[Ian Simmonds]

Lightweight flywheel?

[Alex75]

There were no dips in speed before, the flywheel was not changed.

[Tractor Dave]

Try giving the idle valve a good clean. Failing that, vacuum leak maybe.

[Alex75]

Thank you, I cleaned the idle valve, checked it on the table with a device, there are no jams, the shutter moves smoothly. Can the speed sensor affect the operation of the idle valve?

[newsh]

Idle stabilisation is triggered by the idle switch on the throttle body, perhaps it's not closing or the connector needs a clean.

[Alex75]

I checked the idle switch and throttle position sensor circuits and the wires going to them. In all modes, the idle speed works normally, only when switching to neutral gear there is a dip in revs and sometimes the car stalls.

[Tractor Dave]

Auto gearbox?

[Alex75]

No, manual transmission.

[Tractor Dave]

Ok, I would check for vac leaks then try another ISV if you have a spare. After that a few other things can cause low revs and stalling like a faulty coil pack, spark plug, low compression, head gasket failure, timing slightly out etc. Just a case of working through things methodically.

[Alex75]

Thank you, I will check everything.

[-ice-]

vac leaks and also the AAN has a damper at the throttle plate to make it close slower.

[Crasher]

I am struggling with this on a 3A which uses the same ISV 034 133 455 B and I bought a new one from you know how in Berlin, made by Essen, it made a big difference to the stalling but not perfect. Does it do this when driving and coming to a stop? With the 3A I found it was the throttle closed switch not being triggered fast enough ahead of the damper due to corrosion in the spindle but the AAN uses a potentiometer like the ABY/ADU, part number 078 133 154 and this can cause the same fault, you need to check its position in measuring blocks using VCDS and the 2x2 adapter.