Idle speed dropping on ALH engine

Hello everyone!

I have observed the phenomenon of the idle speed dropping without any load twice so far. It always happened under the exact same conditions: the engine was idling, the vehicle was rolling very slowly, and the estimated coolant temperature was around 50-60°C.
Normally, the idle speed on an ALH engine is exactly 903 RPM (according to VAG-COM). When the aforementioned phenomenon occurs, it slightly drops to an estimated 850 RPM and remains there. After a few gas pedal presses, the engine speed may return to 900 RPM, or it may settle at a speed between 850 and 900 RPM.
After a restart, the engine runs normally again with an idle speed of 900 RPM.
The error memory is, of course, empty, and otherwise, no anomalies can be detected on the vehicle.

Does anyone have any ideas about what might be causing the LL speed to drop?

Regards,
Alex.

donalexo wrote:

Does anyone have any ideas about what the cause of the LL speed dropping might be?

Hi Alex,

Perhaps it's a piece of software?

At the AFN (presumably a specific vehicle or system), I remember very well that when coasting without a gear engaged (at about 40 km/h approaching an intersection, coming out of 5th gear and too lazy to shift to lower gears), the LL (likely referring to a pressure or level reading) would also drop significantly. However, it would then rise again to the normal value when the vehicle came to a stop.

Hi Alex,

The software allows for different idle speeds to be set for a moving and a stationary vehicle, but this is not implemented in the Golf with an ALH engine and manual transmission. Both target values are set to 904 in this case. There are still several corrections that can be made, but they all seem to increase idle speed.
There is still a switch that changes the idle speed from the cold engine setting to the warm engine setting after 270 seconds. Pay attention to whether this phenomenon occurs exactly 270 seconds after the engine starts.

I'm familiar with this phenomenon, even when approaching a traffic light, and it always happens when I'm braking and have disengaged the clutch. When the vehicle comes to a stop, the engine speed increases again after about 1 second.
I particularly notice this after heavy braking, but it seems less dependent on the temperature.

I also know this problem from the Bora with the AHF engine , although here it is more sporadic and independent of the temperature.

I've also been paying closer attention to it lately, but I haven't noticed any drop in idle speed on my ALH engine.
Regards,
Michael.