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Old 15-05-2018   #1
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MOT failed on high lamda reading

Got a fail on my mk2b 1.2 8V on exhaust emissions, high idle test1 and test2 generated a lamda 1.07 and 1.06 respectively. It should not be more then 1.03.
I replaced flexible pipe and rear box prior the MOT test as the flexible part was rotten inside and the exhaust made more noise then normal. After the replacement everything was OK, apart of small leak between cat and flexible pipe joint.

Apart of high lamda reading, CO and HCC levels are well below the acceptable reading.
I am going to have another go to try to fix the leak. I am trying to use a ring gasket rather then a flange as the exhaust I fitted appears to need a ring gasket. I have two questions.


1. Will complete seal of the leak fix the exhaust lamda emissions readings?
2. Any more suggestions to seal that joint?
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Old 16-05-2018   #2
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Re: MOT failed on high lamda reading

Can't say for certain it will sort the lambda issue but I wouldn't bother with trying to fix emissions if there was a leak anywhere on the exhaust especially at the front end.
Exhaust putty is an option but with the proper gasket in there, there shouldn't be any need for it. It's more for pinhole leaks in boxes and sleeve joints in pipes.
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Old 17-05-2018   #3
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Re: MOT failed on high lamda reading

The more I think about this the more I believe the leak is the issue. So the leak is definitely up front at the joint between the cat and rest of exhaust. I can feel it pulsating air when idle when I put my hand near the joint. As the air is pulsating I am hoping that the exhaust gas flow withing exhaust is not always under pressure (MOT test fast idle, no load, hence little combustion) and between the exhaust cycles there is some vacuum created within the exhaust. So when the vacuum areas are moving over the leaky joint is possible air from outside is drown into the exhaust. As the air contains oxygen, it will foul the reader causing it to show elevated lamda (lean) readings. So hopefully I can seal the joint better and see if this theory is valid.
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Old 17-05-2018   #4
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Re: MOT failed on high lamda reading

I think you're on the right track. Air can get sucked into gaps. Let us know how you get on and have a close look at the flange at the bottom of the cat while you are there. Some cars develop cracks - ours still sound at 135k.
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Old 17-05-2018   #5
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Re: MOT failed on high lamda reading

I had an MOT failure on my daughter's 1.2 8V. I'd also replaced the exhaust but I hadn't sealed it properly at the backbox joint, once that was sealed it was fine.
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Old 18-05-2018   #6
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Re: MOT failed on high lamda reading

Used this evening the ring gasket and seems to be tight. Hope to last until the retest. The engine does sound a lot quieter, so finger crossed it will do the trick.
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Old 18-05-2018   #7
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Re: MOT failed on high lamda reading

Quote Originally Posted by dumbledore View Post
Used this evening the ring gasket and seems to be tight. Hope to last until the retest. The engine does sound a lot quieter, so finger crossed it will do the trick.
If you hold your hand over a cold exhaust (just started up) bl9cking the exit
Does it build pressure up and blow your hand off?? Or does it keep going with a hiss?

Ziggy
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Old 18-05-2018   #8
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Re: MOT failed on high lamda reading

Quote Originally Posted by Ziggy122 View Post
If you hold your hand over a cold exhaust (just started up) bl9cking the exit
Does it build pressure up and blow your hand off?? Or does it keep going with a hiss?

Ziggy
I tried this yesterday and it seem to be building up pressure more then before I replaced the exhaust. Did not insist more as I did not want to get disappointed. I am not recalling me making this test after replacing exhaust prior the MOT, which had a leak in the joint between cat and rest of exhaust. The good news today is that the car passed the retest. The lamda value was 1.03 which is at the limit. So not sure now if the seal did the trick or the tester let me off the hook, hopefully the earlier. So I can't say for sure fixing the leak sorted the MOT, but as Brendan suggested, don't try to look for other causes of failure before a leak is rectified should be the rule.
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Old 18-05-2018   #9
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Re: MOT failed on high lamda reading

The NCT testers round here do the old fashioned rag on the tailpipe test first and if it doesn't build the right kind of pressure they don't even bother sticking the probe in - automatic fail.
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