strange idle after revving

[76Passat]

This might be a dumb question, in the 125kw TDi engine, I revved it gently while it was idling out of gear (just for fun, to see what it sounded like...) and when it returns to idle it kind of "bounces" up and down to find the right idle speed. Sounds wierd but not serious - just wondering if that's normal.

[gregozedobe]

Sounds like it might be cutting the fuel completely as it is slowing down, then re-applying fuel as the revs get close to idle (so the engine doesn't stall/die ). Do you get a similar sensation if you back off completely at say 15Kmh in 1st gear ?

[76Passat]

I'd say that's what's happening, it sounds like some kind of control doing its job, rather than something that's not right. I'd just never heard an engine do that. I had another listen to it, you can hear it blip the throttle a few times after you lift off the pedal, until it's found the right idle speed.

[gregozedobe]

Sounds weird but not serious - just wondering if that's normal.

Perfectly normal, it is meant to improve your fuel economy by not using fuel when you decelerate (check your instantaneous fuel consumption on your MFD).

It does take a bit of getting used to, particularly with a manual transmission in stop/start traffic. If you try to keep moving at low revs in 1st gear like I do (letting a gap appear, then slowly driving up to the car in front to save a stop or two), it decelerates strongly until it slows down to near idle speed, then the engine gives itself a bit of accelerator so it doesn't stall.