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Failed mot on high lambda reading

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Sat Aug 08, 2015 11:16 pm PostPost subject: Failed mot on high lambda reading
Cupman
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Joined: 22 Jan 2011
Posts: 942
Location: Barrhead

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Looking for some tech help with my Impreza P1 as its failed on high lambda reading on its mot test today.

It's the first mot in 3 years after the engine blew and me fitting a new sti8 short block along with using my P1 heads. I also changed the single cat in the down pipe for a newage Impreza Prodrive sports cat modified to suit the V5 exhaust, a straight through centre section and prodrive backbox. Turbo, injectors, inlet are all the original but I have also fitted a new walbro 255 pump. The car has not been mapped yet for the modifications yet and I'm going to Andy Forrest on Monday for this.

My test results were:

Fast idle test 1 results
CO - 0.171% (<=0.300%) Pass
HC - 62ppm. (<=200ppm) Pass
Lambda 1.163 (0.97-1.03) Fail

Fast idle test 2 results
CO - 0.028% (<=0.300%) Pass
HC - 20ppm. (<=200ppm) Pass
Lambda 1.116 (0.97-1.03) Fail

After doing a bit of reading on this it seems leaks in the exhaust system seem to cause the most issues with high lambda readings and so with my CO and HC being very good and so if there was a leak in the exhaust system before the O2 sensor on the down pipe the ecu would try to correct the AFR and so the CO and HC would increase. This would mean the leak would have to be after the O2 sensor as the tester would check it at the tailpipe. This means it could be at the flexi joint between the cat and centre section, centre section to backbox or the bung I have for the 2nd lambda in the down pipe that not being used. All will be checked tomorrow!

2nd thought is due to the better flow from the PPP sports cat compared to the original P1 cat, it is causing the car to run lean giving this reading with a similar effect as having a decat fitted. This is the desired effect and should be corrected after mapping on Monday but with the chicken and egg thing, the mot came first before the remap and so failed for running lean and should pass once mapped.

Any thoughts on my train of thought or any other pointers on things to look for?

Thanks for any help.
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Subaru Impreza P1 (Fast road and shows)
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Sun Aug 09, 2015 8:51 pm PostPost subject:
Russ@PHQ
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Have you checked that the lambda sensor is working?
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Sun Aug 09, 2015 10:06 pm PostPost subject:
Cupman
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Joined: 22 Jan 2011
Posts: 942
Location: Barrhead

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Russ@PHQ wrote:
Have you checked that the lambda sensor is working?


Hi Russ,

No I haven't, but it's not throwing up an EML light.

Edit - the lambda sensor is at fault. Ordered a new one but got it mapped today and 341/312 was the result which I am very happy with!
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Forged GT2860rs Clio 172 Cup (track)
Subaru Impreza P1 (Fast road and shows)
Audi A7 Black Edition (Daily)
Seat Leon Cupra TFSI (wife's)
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