CO Emissions at Natural Idle vs Fast Idle

[944fan]

My car has just had its MOT, it passed so no worries there but looking at the printout from the emissions test it only just scraped through on the natural idle test. The values are:

Fast idle
CO 0.005% PASS
HC 13ppm PASS
Lamda 0.999 PASS

Natural Idle I got:
CO 0.298% PASS - The limit for my age car is 0.3

What would cause them to be so low at Fast idle and yet high at natural idle?

It is my understanding that a high CO reading means incomplete combustion which I would guess is likely caused either by incorrect fueling or a missfire. I would have expected any issue like this to be worse whent he engine is under load not at idle.

Any ideas?

[anonymous-user]

if i had to guess, i would say that the cat had "gone out" during the natural idle test, when they reved it for the fast idle test, the cat lit off, hence the low Co number for that test.

it is certainly not unusual to have a low idle co result much worse than a fast idle one, even when looking at pure "engine out" emissions (i.e. pre cat), as engines have a very poor burn rate when idling slowly, espec modern 4valve ones etc.


Most MOT emissions tests are not rigerous enough to really get a proper emissions value, other than to show an engine that is grossely F**ked......