[mdr0303]

My BMW has been revving by itself when cold. (n47)
I recently scanned for codes and found only A6CF (AUC sensor malfunction), which controls the recirculation flap when detecting air pollution.

Could this be related to the revving issue ?

[DomMuc]

when cold to get warm faster idle is about 1100 then lowers to normal idle revvs

[mdr0303]

Originally Posted by DomMuc

when cold to get warm faster idle is about 1100 then lowers to normal idle revvs

It is not about higher revs, it revs up and down, more so when I turn the Heat on, up and down up and down

[DrVenture]

So," surging" at idle. I'd assume the AUC sensor is faulty and disrupting the air/fuel mixture. Or there is an air leak into your intake (unmetered air). The AUC sensor is usually mounted somewhere near/under the air box.

[RockCrusher]

What DrVenture said.

I'll add FWIW (and probably not much) is I had a car (Boxster) out of the blue manifest a varying cold idle condition. Idle RPMs would go up then down. Not by much but clearly not idling as it had before.

Long story short the cause proved to be an air/oil separator that had failed (at around 80K miles).

Which reminds me this same car at around 250K miles after a normal 30 mile drive to work when I pulled in to park the engine idle (hot idle this time) started going up and down. By a considerable amount. The RPMs would go low enough I felt the engine was about to die.

I used a handy OBD2 code reader and found a pending code pointing to the variable valve timing system. In the shop the dealer tech diagnosed the problem as a failed variable camshaft solenoid/actuator. After their replacement engine was fine. I drove it another 60K+ miles before I sold the car.