AJM Automatic: Intermittent Idle Surge Issues

Passat 3B5, year 2000, engine code AJM, automatic transmission, 85,000 km.

Error description:

"When the gear is engaged and the vehicle is stationary, the idle speed occasionally spikes very briefly (approximately 0.5 seconds) to around 1000-1100 RPM, then returns to the normal range at intervals of about 10-20 seconds. It feels like the engine speed drops slightly (barely noticeable) before it spikes, similar to a brief misfire."

All error memory locations are clear. I suspect the temperature sensor may be the issue. Unfortunately, the vehicle was already at 65°C during the diagnosis. The corresponding measured values were plausible. After a test drive and reaching operating temperature, the error could no longer be reproduced.

Does anyone have any ideas?

Hmm, could it be that, especially if the engine speed momentarily drops, something is drawing power, and that the increased load on the alternator then causes the idle speed to increase?

Quote:

Hmm, could it be that, especially if the engine speed momentarily drops, something is drawing power, and that a heavier load on the alternator then causes the idle speed to increase??

Possible, but what is it? I've turned off everything I could. The voltage is a constant 14V. I'll check it again, though.

Quote:

Plausible, but what?

Solution: LMM from Herzog It's unbelievable the kind of errors these things can produce.

Why I overlooked this here: https://community.dieselschrauber.org/en/viewtopic.php?t=12257&start=0&postdays=0&postorder=asc&highlight=herzog is beyond my understanding.

"Ergo - if there's nothing sensible written on it, there's nothing sensible inside."

Unfortunately, I couldn't get a Bosch mass airflow sensor (MAF sensor) at that time (Saturday afternoon), which is probably why this happened.

So, stay away from that stuff!