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Yes, I noted your comment above about unmetered air entering after the MAF but this is entering before the MAF just after the throttle body. I had checked this breather before but did not remove it. What caught my eye was a fine mist of deposits on the elbow. I guess it explains the smell I noticed earlier. Not sure how the MAF is not detecting this extra air and the mixture being adjusted accordingly.

Trying to book in for an MOT this week.

Failed again on emissions - carbon monoxide after 2nd fast idle, at idle, and lambda reading after 2nd fast idle outside specified limits.

It seems that there is more of a smell from the exhaust than before. Crankcase breather pipe still attached after I replaced the seals.

Sounds like the cat is failing?

Unless I'm completely wrong, that engine does not have a MAF away far out the inlet tract, it has a MAP sensor on the inlet manifold, so the only area that unmetered air would get in from is due to a leaking or cracked inlet manifold.

I can't answer your final question though, is the cat "lighting up", you can check using a temperature probe I'd think, though better to know what its temperature used to get up to when it was initially working though, a garage would know what temperatures to expect form a working cat.

Correct, there is a MAP sensor on the o/s of the inlet manifold. I did not remove the inlet manifold but inspected what I could in situ. It was refitted with new gaskets and correctly torqued when the engine was rebuilt. I'll have another look and maybe try a cold start spray test.

On the previous MOT where it failed on emissions it did not fail as badly and I'd given the car a good thrash beforehand (as well as having put a bottle of Cataclean though it). This time the engine was warm but I didn't have time for a thrash. Spoke with the tester afterwards and he said he wasn't able to get it through.

I forget what temps the cat should be but when I last connected the diagnostics it was up to temp and lambda was still out (before and after changing the front O2 sensor). Based on the aforementioned, even well up to temps after a good thrash and cataclean wasn't enough.

I'm just waiting on the recent set of readings which should help with diagnostics.

Natural Idle
CO 1.01 (0.50)

Fast idle
CO 1.08 (0.50)
HC 64 (200)
Lambda 0.969 (0.970-1.030)

2nd fast idle
CO 0.95 (0.30)
HC 110 (200)
Lambda 0.969 (0.970-1.030)

It’s still overfuelling then. This makes little sense

Have you got a faulty injector I wonder thanks flooding the engine with fuel? You said the exhaust fumes smell noticeably fuel ly?

Yes, it smells fuelly but not eggy.

I did clean the injectors at the rebuild in Jan 2017. I’ll pull the plugs tomorrow and check the coloring. Could it be the MAP or the Hella CTS (replaced as above)? Temp gauge reads OK, engine warms up OK, hot air on the blower. Drives OK. Last time I checked there were no codes. Changed the fuel filter recently (3bar regulator).

Edit. Last time I checked on diagnostics no misfires detected.

Found a broken pipe on the breather on top of the camcover that goes into the air box. Taped it up. That might explain some of the smell.

Lambda value is still floating around 0.968 with the engine warm

Also noted the intake vacuum at idle with the engine up to temp is about 322 millibars Or 9.5” hg). Is this normal?

As I set out to have a look at the injectors this morning out of curiosity I pulled of the top belt cover. It seems there is too much play in the main belt as it can be deflected a cm or so just under the pulley using finger pressure. It's possible that the cam belt tensioner has failed or come undone. Would this be a plausible explanation for overfuelling? If its jumped a tooth would valves be damaged?

Seems like a fair bit. Cam tensioner may not be working correctly. Yes if the timing was a tooth out, that could cause an overfuel scenario

Rear O2 sensor replaced. Emissions good enough to pass MOT but creeping up due to a degrading cat.

Very strange. If you replace it, go genuine only. Aftermarket cats are just rubbish. One from a lower mileage used car would be better

Noted. Who are the OEM suppliers?