There seem to have been a few threads on this topic over the years but this has been dogging our Twinair Panda ever since its first MOT three years ago.
The lambda figure in the MOT has always been borderline on the high side of the limits and sometimes the tester has had to work really hard to acheive a pass. I've had the same guy's signature on my MOT test certificates now for well over 20 years so I know he's doing a trustworthy job. A few months ago I had cause to replace the pre cat Lambda sensor due to a misfire when cold so I hoped that the problem at MOT would now be solved.
..unless there's a specual nuance with the Twinair engine?
This year it's failed on the same reading! with as usual the HC's and CO indicating that actual emissions wise, the engine is running perfectly clean ...so no need to try the garage's £20 fuel treatment then thankyou! ..Or taking it out for a thrash wouldn't really improve matters would it?
Here's the readout..
Now, the MOT's Lambda figure is derived internally in the testing machine from measuring the ratio of oxygen in the exhaust gases. It's nothing to do with a figure from the Lambda sensors.
The high figure indicates an excess of Oxygen. This could possibly suggest a misfire of some sort but that would be noticeable and associated with a high HC figure which there is not. (spark plugs have also been replaced with Iridium types in the last 3 months too)
A very highly possible cause reported on other threads and boards is that there is a leak in the exhaust pipe downstream of the catalytic converter that's letting in air and hence oxygen. I can find no such leak. However, there are two drain holes, one in each end of the silencer that could be the only possible sources.
How could I best temporarily block up these holes for the re-test on Monday and is it indeed worth doing? Is there anything else I could look for? Multiecuscan shows no faults.
The lambda figure in the MOT has always been borderline on the high side of the limits and sometimes the tester has had to work really hard to acheive a pass. I've had the same guy's signature on my MOT test certificates now for well over 20 years so I know he's doing a trustworthy job. A few months ago I had cause to replace the pre cat Lambda sensor due to a misfire when cold so I hoped that the problem at MOT would now be solved.
..unless there's a specual nuance with the Twinair engine?
This year it's failed on the same reading! with as usual the HC's and CO indicating that actual emissions wise, the engine is running perfectly clean ...so no need to try the garage's £20 fuel treatment then thankyou! ..Or taking it out for a thrash wouldn't really improve matters would it?
Here's the readout..
Now, the MOT's Lambda figure is derived internally in the testing machine from measuring the ratio of oxygen in the exhaust gases. It's nothing to do with a figure from the Lambda sensors.
The high figure indicates an excess of Oxygen. This could possibly suggest a misfire of some sort but that would be noticeable and associated with a high HC figure which there is not. (spark plugs have also been replaced with Iridium types in the last 3 months too)
A very highly possible cause reported on other threads and boards is that there is a leak in the exhaust pipe downstream of the catalytic converter that's letting in air and hence oxygen. I can find no such leak. However, there are two drain holes, one in each end of the silencer that could be the only possible sources.
How could I best temporarily block up these holes for the re-test on Monday and is it indeed worth doing? Is there anything else I could look for? Multiecuscan shows no faults.