M54B25 idle misfire

[Krikkit]

Hi all,

Been having some bother with my M54-engined compact since I bought it with idle misfires - it's mostly been very tolerable when very hot or left idling stone cold (and never stalls etc), so frankly I've ignored it.

However, things have kicked off this week as I replaced the CCV to try and remedy excessive oil consumption, now the misfire is much worse (paradoxically).

Now I know that the common cause of this is a vacuum leak, however I've been through a lot of items and checked them, even had the inlet manifold off checking connections etc for the common points.

Broad strokes of things checked/replaced:
1) New intake boots

2) MAF is reading OK (steady 16.x kg/hour at 700RPM idle)

3) DISA is OK (good seal, flap actuates as it should, no leakage from the vacuum valve)

4) ICV has been swapped for a replacement, no change

5) Coil packs have been swapped with another good used set, no change - if I get it to full misfire (rather than just a bit rough) on say, cylinders 4 and 6, the misfire doesn't move with moving the coils between banks.

6) Inlet manifold gaskets look fine, as do the little bungs on the firewall-side of the manifold

7) CCV and hoses are all brand new Febi items

8) The VANOS angles all look fine, the camshaft sensors are giving smooth, consistent readings

Symptoms:

Intermittent misfire - reading with INPA the short-term trims move between 0 adjustment (normal mix) and +28 (full short-term enrichment) seemingly without any rhyme or reason. Give the car a blip of throttle the adjustments disappear, then come back in over about a 30s period as the DME adjusts the mix.

0 adjustment:


Then back to +28 adjust:


And an example of the "full" misfire where it's running rough on 4 cylinders and two are practically out completely


At this point I'm at a loss of what to do with it, it seems odd that it's an intermittent problem where the DME will back off the adjustments, it gradually begins misfiring, then it adjusts them back in. I'd have thought with a continuous vacuum leak it wouldn't pull the trims... Even then I'm not sure where it can be given how much of the system I've checked... Worth a smoke test?

Edited by Krikkit on Tuesday 14th September 12:07

[JakeT]

Checked the plugs? I’d also smoke test it.

[Krikkit]

Plugs look good for all 6, it runs perfectly off-idle so I'd imagine any rough running symptoms would be masked once you're moving.

[buggalugs]

Short term fuel trims will be due to the misfire sending fresh air down the exhaust.