Strange Engine Revving Problem

[AidenM]

Hello all,

Hope this is the right place for this;

I've been having an unusual problem with the engine in my Mercedes CLC 220 CDI, when the temperature is cold out and during the first 10 minutes or so of a drive the engine revs itself when you get in the RPM range of 1200-1500 regardless of a steady foot on the throttle, and effectively the revs just "bounce" up and down between these two figures when you're in this range - it doesn't do this either side of this rev range.

I've only noticed it now during the winter and can't seem to work out the problem from googling, it doesn't do this at all when the temperature is not low and it seems to stop doing it after the first 10 minutes or so of a drive presumably once the engine has warmed up a bit and then continues to drive normally.

Any ideas what may be causing this and whether it's a concern?

Thanks,

Aiden

[GreenV8S]

This is just a guess, but the ECU is likely to be running in closed loop under those conditions in which case you may be feeling the effects of oscillation in the air fuel ratio because the feedback loop isn't working properly. Somebody able to connect a diagnostic scanner and see the lambda PIDs when the problem occurs could tell you whether that's the cause. It's normal for them to cycle every second or so, but not normally by enough to be noticeable.

[stevieturbo]

It's a diesel and cold, so the above is highly unlikely.

Could be any number of things, view sensor data to see if there are any that arent making sense.