[Engine] Annoying hanging revs, pulsating idle in 1.6hdi

[Chips'n'gravy]

Good evening all.

I recently replaced all 3 engine mounts and blanked the EGR valve. The engine runs so much smoother especially at lower rev range, definitely more happy to pull away. Engine judder while pulling away still there but I'm suspecting clutch.

The only annoying problem still there is hanging revs. As soon as the engine reaches 80 degrees the idle drops from 950rpm to 900rpm and pulsates and the revs hang every time I shift through the gears. The harder I drive and the higher the rpm the revs hang for longer. I disconnected the maf sensor which didn't cure fluctuation at idle. Didn't get a chance if it's affected hanging revs yet- hoping to do it tomorrow on the way back from work.

Am I right suspecting the air doser? It's covered in thick oily residue and cleaning it is on my list once the weather gets warmer. Any suggestion will be much appreciated.

[Sol]

It's possibly the doser but that is meant to bypass the turbo intercooler when the engine is cold, that's all. I have long suspected it for issues on the old vans. They did away with it on the later B9s.

The only time I had the hanging issue was in the last M59 and it was the electronic pedal at fault. But someone else had this and never got to the bottom of it.

[Chips'n'gravy]

Thank you Zion. I am familiar with the posts you mentioned plus many more regarding the problem. The hanging issue only appears when the engine is hot which excludes the throttle pedal. Up to 80c the engine runs  lovely, then the idle drops slightly and hanging revs appear. I will inspect and clean the doser and intercooler but if that doesn't help I will be clueless. Probably will have to ask someone with Lexia for help.

[Sol]

You could be right as I've always thought the doser causes weird issues. On paper it shouldn't, but I'm not convinced. There's a later version which is just a plastic model of the original. Maybe you could replace one with the other and unplug the motors for the butterflies. I'm not convinced it knows the position or status of those, there seems to be no feedback so I'd say it can be swapped with no error codes.

[Chips'n'gravy]

Hmmm, that's interesting indeed. Thanks Zion

[Chips'n'gravy]

Bit of an update. I performed a BSI reset and hanging revs issue disappeared. Great improvement in regards to pulsating idle too, still there but less noticable and hardly ever occurs.

[Sol]

Weird. The BSI has no engine functions, but maybe the reset cleared something in the engine ECU too. Glad to hear it's better though.

[Chips'n'gravy]

Interesting but must have done something

[geoff]

I've had " hanging " and a BSI reset made a difference for me - went away for a while but came back.

I'm living with it - sad emoji

[smutts]

For we ignorant, what is the "doser"?