Odd emissions problem

[handpaper]

The Touring (320i) has just failed it's MoT on the emissions test.
It's failed on HC (hydrocarbon) emissions, with 3551 ppm (limit is 1200), which suggests poor combustion, but CO is well within at 0.141% (limit 3.5%).
Its idle is a bit rough and lumpy, but I can't find any air leaks, the ICV is clean and the spark plugs are recent and clean.
Any ideas?

[VitesseEFI]

High HC but low CO suggests a complete misfire on one or more cylinders to me.

This could be one cylinder persistently misfiring or a number of them taking it in turns. Multitude of possible causes.

If a single cylinder misfire then plug leads, dizzy cap, partly blocked injector or low compression on that cylinder are all suspects.

If random misfire on multiple cylinders then all the above except low compression still apply and you can add in excessively weak idle mixture. A non-standard cam with aggressive timing can have the same effect.

Very heavy oil burning can also bump the HC up but at that level you'd surely be aware already!

Food for thought....

Nick

[driftwood]

High hc can be caused by a weak mixture as well as a rich one. Try 0.5% co if you definitely have no air leaks at the manifold or hoses. You could try clamping the servo hose too. If it's lumpy at idle you might just have tight tappets. A compression test might give you a clue. I presume you've tried injector cleaner and given it a good thrashing. What colour are the plugs and are they all the same colour? Does it run ok at other engine speeds?

[handpaper]

Air leaks now found, I'll investigate further once the inlet manifold is properly sealed

[driftwood]

I bet it'll be fine once you've cured them.

[HairyScreech]

Just about to say, sounds like a lean misfire caused by an air leak to me.
Get that sorted and it should be ok.