1995 840ci Emissions Test - Causes of a Fail?

Took the 8 for the MOT this morning and all going well until the emissions test. Everything else good.

On the 1st Fast Idle Test she failed on CO-%vol = Value 0.498,the max allowed being 0.300 = Fail. 2nd test Value 0.418 =Fail

On the 1st HC - %vol = Value 41, the max allowed being 200 = Pass. 2nd test Value 39 = Pass

On the 1st Lambda Test = Value 1.034, the max allowed 1.030 =Fail. 2nd test Value 1.036 = Fail

Natural Idle Test = CO-%vol Value 0.457, the max allowed 0.500 = Pass

So I have 10 days to source the problem(s) and rectify and return. The mechanic, who is a real gent, said that it's probably un-metered air getting by and throwing things off because he struggled to hold the revs steady when he had to stay in the 2500 revs range. It raised the revs without his input and then came back down as if it was trying to constantly balance things.

So I'm thinking,

1.The PCV has been done (couple of years ago) as have the manifold gaskets, but worth rechecking all bolts and seals.

2. Recheck all of the rubber plugs/caps that tend to deteriorate and fall off.

3. Renew the gasket on the Throttle Housing Assembly

4. New MAF?

5. New Vacuum Hose on Fuel Pressure Regulator (on order)

6. Renew the spacers and washers on the manifold housing

The idle surging has it's causes, but I thought I'd caught that with reattaching the vacuum hose the last time when it had a split. I'm wondering whether a failing MAF will also cause that?

I suppose what I'm trying to achieve is some form of hit list as to what to tackle to rectify this and other common causes of air leaks, surging idles, stalls, and emissions test failures.

My engine is the M60B40 variety, so I'll also look on the other models forums that share the same engine for any answers.

Other relevant details:

Pulls like a train, starts first time every time, no stalls, no warning lights, MPG showing around 20, MAF at least 5 years old, mileage is 136,000. New sports Cats 2 years ago.

Slight up and down in revs when idling in traffic, when decelerating the same thing happens, revs momentarily drop then immediately pick up again. Thought I'd fixed that previously by cleaning out ICV, but that's crept back in, but not as bad. Revving the engine whether hot or cold, is always preceded by a slight hesitant 'suck' of air (I suppose that's a big clue). Not crisp like some that I've heard.

As always, any input gratefully received especially from any 'been there, fixed it with...' type answers.

Mick

Thanks Reinhard, the MAF was on the car when I purchased it, so I don't know how old it is, but it is a Bosch unit with the correct numbers on it. I'm conscious that they don't last forever, so it's a contender. I'm going to get some codes read and do a smoke test as starters.

mick

Thanks Steve, that's an interesting test, it's going on the list

Ok, two minor updates

Can't find a vacuum leak with the spray/WD40 on a cold engine method. No change in idle or revs and I sprayed all joints. I think a smoke test will convince me also.

Whilst it was idling after the spray test I unplugged the MAF and it immediately stalled and I got a Trans Fail Safe message on the OBC. I plugged it back in and started her again, but I couldn't reproduce the cut out, it didn't stall, it stuttered and then continued. So am I right in thinking that the adaptations took over after the first unplug?

Going into an Indy tomorrow to get the fault codes read and see what he can find.

Update on the fault codes = Only code that came up was me unplugging the MAF, other than that, nothing. That's good I suppose, but the hunt is still on. Plugs and all filters next. Then a good tear up the motorway to give the Cats a clean out.

Thanks for your input Steve, I appreciate it

Mick

Ok, some closure to this problem.

All new filters, oil, plugs, engine and injector cleaners and a 'good drive' resulted in a Pass for the emissions test.

The mechanic agrees with the notion that the Cats are possibly causing the issue and said that the after market ones do not last as long as the BMW ones or are as efficient.

Mick