Engine Revs High After Warm Start (Solution)

I've been experiencing a sporadic problem with my 2000 Passat, which has an AJM engine, specifically when starting it when the engine is warm.
Sometimes, immediately after starting, the engine revs up to approximately 2000 RPM, and the speed fluctuates around that value with a frequency of about 4 Hz.
Approximately +/- 100 RPM (it sounds like a nervous scooter driver). This lasts for about 2 seconds, and then the speed returns to the normal value. This effect occurs more frequently when the outside temperature is above 20 degrees Celsius and when the engine has been idle for 5-10 minutes.

Regards,

floppine

Hi,

I used to experience that occasionally with my Golf 1Z as well.
Also, this typically happens in warm temperatures, and especially after driving for a long time at a fast pace.
I always believed (or still believe) that this is intentionally done by the engine control unit.
Perhaps to clear the engine of any obstructions, or to quickly restore oil pressure.
It sounds like there's a 'ghost foot' effect, where the accelerator is briefly pressed multiple times immediately after starting the engine, right?

I cannot give you a specific answer as to why this issue has only recently started occurring in your car.

Markus H. wrote:

I always believed (or still believe) that this is intentionally done by the engine control unit. Perhaps to clear the engine of any obstructions, or to quickly restore oil pressure. It sounds like there's a "ghost foot" effect, where the accelerator is briefly pressed multiple times immediately after starting the engine, right?

Hi everyone,

If that were intentional, it would probably happen with every car. But it's not, which suggests there's likely something wrong with your cars.

Maybe you'll find something in the error database.
I vaguely remember having read something similar before, but I no longer have any idea about the cause.

Hi,

My Golf has been doing that occasionally for the last 150,000 kilometers.
If it's lazy, it can't be a drama (at least, not for me).
Of course, I don't know how often this happens to you - and I don't want to downplay it at all!!

Hello everyone,
It happens to me about twice a month. I also experienced it the day before yesterday, even with low outside temperatures. Could it possibly be air bubbles in the fuel supply? The PD engine has a complex and therefore potentially problematic fuel cooling system.

Regards,

floppine

Hello floppine,

The threads don't quite fit, but maybe this will help you:
/viewtopic.php?t=919&highlight=rauf
/viewtopic.php?t=1181&highlight=revolutions

Have you ever checked the engine control unit's fault memory immediately after the error occurred?

Best regards, Rainer.

Hello again.

I just found what I vaguely remembered.

/viewtopic.php?t=15

I, of course, don't know if that applies to PDs as well...

I found the problem: the coolant temperature sensor is defective. It wasn't a complete failure, but it only malfunctions at certain temperatures (around 50 degrees Celsius), causing the reading to jump and the engine to register a temperature below zero.
To diagnose the issue, I used the Climatronic function diagnostic tool. Channel 45 displays the coolant temperature, which allowed me to observe the behavior clearly.
Regards,
floppine

Hi Rainer,

If there were a category in the error database for "exotic, but I've seen it before," I would move the article there...

What do you think about that?

Yes Thank you very much for the feedback, floppine!