K series MOT fail - emissions

[Ivaan]

I would also check the cam timing. Do you have verniers?

If you have SS cams and the timing is off a fraction, you will have overlap between the inlet valve opening and the exhaust valve closing. Then you'll get some fuel going direct into the exhaust. I used to adjust on the verniers whilst being tested during the MOT.

[mudguard]

Yes, but the major "misfire" lasted one day 6 weeks ago and vanished immediately on cleaning the HT springs. The symptoms were two things it has never done before or since:

On idle, run very rough, hunt and never settle.

On acceleration, stutter and then recover.

The other "misfire", the more long-running one, whose onset was immediately after the HG work, seems strictly idle-related, and this was improved significantly by cleaning the IACV. The symptom of this is occasional stumbling and sometimes stalling when slowing down, and then (if not stalling) recovery to a fast idle, and then settling down. Absolutely no stutter under acceleration, and no idle hunting. 

The reason why I think these are not the root cause of the cat failure is that they both began long after the original cat was in significant distress. 

[mudguard]

No verniers, stock pulleys, stock cams, stock everything really.

[Ivaan]

I see your's is VVC. I know nothing about these, but could it be a fault in the mechanism of how it's been set up since the HG replacement?

[mudguard]

Could be, but my feeling is this is less likely. 

The reason why I think this is that the original cat was already destroyed before the HG work, by (presumably) the same problem which has now, 8 months later, apparently destroyed the second cat. This problem appears to be overfuelling, root cause unknown, so it seems reasonable to think it's been the same problem all along, and not detected and corrected during the HG work. 

 

[Geoff Brown]

I would hazard a guess & put it down to the VVC 'clock work' & the set up.  Correct me if wrong but the VVC system alters the valve timing with increase in RPM so also altering valve opening & closing - overlap?. So could the VVC timing be out causing the timing to allow fuel to enter the exhaust system resulting in the cat damage & confusing the Lambda sensor creating a vicious unending loop?

[aerobod - near CYYC]

I'd start the tracing for over fueling / over oiling at the plugs. After a long run are they all even and tan coloured or lighter? If yes, then it is likely incorrect mixture at low loads such as due to an air leak, valve timing. TPS position or IACV issue. If there is variation between the plugs, then fuel injector, ignition or cylinder specific wear (especially if oil on the plug). If all plugs are dark after a good run, then fuel pressure regulation, valve timing, water temperature sensor, incorrect TPS position or lambda sensor failure could be the best candidates.

[revilla]

You wouldn't disassemble the VVC mechs to the point where their internal timing could be affected changing a head gasket, you would need to split the ladder off the head to do that. All you could get wrong would be to get one (or two ...) of the pulleys out by a tooth. Which comes back to a quick check of the cam timing (by which I mean just check all the marks align simultaneously, not a full DTI gauge type of check).

[mudguard]

Thanks for all the suggestions on this thread, and indeed during my four years of ownership. Not many get to have the privilege of owning such a machine as their first car. 

I've sold the Seven - with, of course, full disclosure of its difficulties. I'm sure it'll be sorted out and be back on the road making its next owner very happy. 

For me - not quite sure yet what's next. Maybe another Seven, or maybe I'll try something else. Nothing will ever live up to it, I'm sure. 

Many fond memories of chip runs, weekend blats with the Kent and Sussex groups, Dunsfold, the evening meets at Toys Hill and Penshurst, all with great camaraderie from you. And outside of the club, most corners of England and Wales, and a bit of France, 28,000 miles.

Thanks - it's been a blast!

[SM25T]

That's not the best way to fix an emissions MOT fail.