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Any help to understand what's going on with emissions

P

pooga

Silverstone
Joined
3 Aug 2014
Messages
125
I have replaced all pre and post O2 sensors, MAF and CAT (200 CEL - TopGear)
(had a problem where post sensors were mirroring the pre sensors - was idling rich until the MAF was replaced)

But still failed MOT emissions

:sad:

(2003 - C4S)
 

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Hi deMort - thanks for your response,

i assume you mean exhaust air leak? i.e. exhaust joints?
 
Or do you mean the vacuum line? Is there a vacuum circuit drawing – where do I start?


Could one of the vacuum lines become raptured when my AOS went?
 
Mine failed emissions a couple of times, but sailed through once the manifolds were replaced.
 
Stotty said:
Mine failed emissions a couple of times, but sailed through once the manifolds were replaced.
Click to expand...

thanks - Manifolds + box + cat were replaced 3 years back
 
Your image no. 2 shows pre and post cat lambda sensors all reading low .. this would indicate an air leak on the inlet mainifold .. possible a maf sensor over reading but an air leak basically .

I assume at the end you reved the car hence the higher readings ?

i would start by doing a smoke test on the inlet tract.

Any exhaust leaks can cause similar but i think a blowing exhaust would be obvious and you would hear it .
 
probably unrelated but I had a problem with mine for about 5 years, it would start but not idle, maybe 20% of the time, just die, and needed throttle to keep it going, after a minute it would be ok. Porsche did a check including vacuum leak check on induction,, no problem. I spent load of time running obd trace on flow etc, but no obvious difference between good and bad starts. Then Idling in traffic jam for half an hour it died. I got a fault code on the evap. I took off the evap valve which leaked.
Replaced it and never a problem since. The engine was sucking in air and messing up the maf reading, as it thought all was ok, but it was running really weak as a result. Its weird to have a problem for so long, finally to be solved.
 
I had a similar thing with my 996.1 -from when I bought it it had an erratic idle and would be very tricky to pull away from a standstill without stalling. The seller told me it had a lightweight flywheel, and this was the reason. It turned out that the AOS was leaking air and oil into the inlet tract. When the AOS failed completely (2 years down the line, in a massive cloud of white smoke) and I replaced it, the idle problems went away. So it turns out you can have air leaks from inside your AOS that probably wouldn't be detected by a smoke test, so something to bear in mind.
 
thanks all - the funny thing is that everything is running fine - no issue on idle/driving - just MOT failed!

not sure where to start with smoke test - so booked in week.
 
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