MOT Emissions Fail

[Airbusguy]

Hi, I'm hoping someone had help me with this. My car is an MGF SE 1.8 VVC, delivered 9/2000. It has been dealer serviced annually from new, has 27,300 miles to date, is kept in my garage and in VGC! The problem is that 2 weeks ago it failed the MOT on emissions, see attached FAIL certificate. The station suggested I take it for a high RPM run, put a petrol additive in the tank, and bring it back for retest. I took it for a 40 minute run having first filled up with BP Ultimate 97 RON but did not use an additive.
Now at this stage I should mention that of late I’d been having an erratic engine RPM problem. Quite often the RPM wouldn’t return to its normat 950 value but would stick at anything up to 2200 rpm. I’d checked that the throttle cable was fully returning. Also I’d repeatedly been doing the IACV resetting procedure which always worked for a while, but then the problem would re-occur. Anyway, back at the MOT station I did it once more and they rechecked the emissions – in fact they were happy for me to watch the technician.

He was an older guy who clearly knew his job, and he tried for 40 minutes to get some good readings. Finally, he did manage to get the Fast Idle Test results within the acceptable limits – I’ve no idea what he did – and gave me a PASS. The certificate shows it failed on the 1st Fast Idle Test but passed on the 2nd, hmmm. I spoke to him later; he suggested I had an after market catalytic convertor (I’m sure I haven’t), he also said he didn’t think the lambda sensor was the problem.

So, I have an MOT pass for this year, but the RPM problem persists, which I assume is linked to the emissions difficulty.

From the above can anyone suggest where the problem lies?

[Chris Tideswell]

What exhaust back box do you have fitted?

[Notanumber]

If you haven't done so already search for IACV (Idle Air Control Valve) on this and the other MGF forums. Much is written about it's symptoms and cure.

[Rob Bell]

Looking at the data from the emissions test, and that it the lambda eventually comes into range, I think that this is your catalytic converter on its last legs and is now failing.

The carbon monoxide is high, that can point to an inefficient burn, but the Lambda and hydrocarbons are normal, so this is unlikely.

The other reason for a high CO is that the CO (and NOx, which is not measured in an MoT test) is not being oxidised - and hence the likely explanation that the catalyst has failed (or a previous owner has removed the catalytic converter - but that would probably only apply if the car were new to you!)

The catalysts on MGFs are generally extremely reliable in my experience. The one on my car (owned since new), has over 120k miles and 27 years of use, and still works fine. But the internals can shatter and disintegrate.

I'd aim to find a decent quality replacement if you can - some aftermarket catalysts are not hugely well designed and can lead to reduced power due to poor gas flow.

Above is a screenshot of a spreadsheet I created some while back - one day, I shall make a script and put it on my website, but unfortunately, I currently have no editing access to the server since Steffan's server crashed, and the version that is currently online is an older version that is now 10 years old

[Airbusguy]

Rob, thanks for your reply, but would or could a failing catalyst also cause the erratic idle rpm problem I have? As I said I can get this high idle speed back to normal for a while with the IACV resetting procedure, before it returns after a few days.

Chris, In answer to your question about the back box, I don't know but I assume with 27K miles it is the original factory fitted item. It doesn't leak to my knowledge.

[Rob Bell]

No, the erratic idle probably is not related to the emissions problems that have been documented. Ockham's razor suggests that the two should be linked, but a car can have as many problems as it likes...