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MoT emissions fail - diagnosing?

747 views 3 replies 1 participant last post by  JeremyG 
#1 ·
My 146 is now clear on its Mot apart from the emissions, as follows:

Fast idle:
CO = 0.56% [should be =< 0.3%] = FAIL
HC = 78ppm [should be =< 200ppm] = PASS
lambda = 1.011 [should be between 0.97 and 1.03] = PASS

Second Fast idle:
CO = 0.44% [should be =< 0.3%] = FAIL
HC = 98ppm [should be =< 200ppm] = PASS
lambda = 1.011 [should be between 0.97 and 1.03] = PASS

Natural idle test:
CO = 0.50% [should be =< 0.5%] = PASS

MoT test centre say it's probably the cat; reading this forum I'm not so sure... what should I try first to troubleshoot this?

Car is a 1.8 with 101k on the clock. MAF is genuine Bosch. I do not know when the MAF or lambda sensor were changed.

I have MultiECUScan and will be connecting this tomorrow - any suggestions on what to check?

Cheers,

Jeremy
 
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#2 · (Edited)
Ran the car up and monitored lambda values on MultiECUScan this morning.

Lambda voltage constantly cycles between 70 and 800mV, as per the graph below, with the "Lambda sensor status" constantly cycling through all the available values... Can anyone tell me if this is normal behaviour?

cheers!
 

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#3 ·
... and here are some traces for the MAF.

With the engine warm, I raised engine revs from tickover to approx. 3000 rpm. Air quantity and airflow meter signal both rose proportionally.

However - as revs increase, the actual air quantity is much higher than the desired air quantity.

As revs return back to tickover, actual and desired air quantity equalise.

Is this normal behaviour for the MAF?

cheers,
 

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