Fiesta MOT emision failure

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Thu 20 Oct 2005 17:53
Fiesta MOT emision failure - far0n
My trusty old 1.3 Fiesta has racked up 97K, which isn't so bad for those old push-rod units. Anyway, it failed it's mot on it's emmisions. Had it tested twice. Here's the results....

failed on CO the first time 3000rpm 1.11 % CO

after having it serviced the CO was fine (0.00%) but it failed it's lambda test giving a value of 1.22

So couple of questions, what is the lambda figure a measure of ? How come the lambda figure passed first time but then failed the second time after a service ?

Cheers


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Thu 20 Oct 2005 23:01
Fiesta MOT emision failure - AlastairW
How old is your Fiesta? Its age governs the emissions it is tested on.
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Fri 21 Oct 2005 01:10
Fiesta MOT emision failure - far0n
it's a 1995 N reg
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Fri 21 Oct 2005 08:35
Fiesta MOT emision failure - Number_Cruncher
To me, it sounds like the garage may have left you with an air leak into the manifold somehwere, giving a weak mixture. Hence, the excess air ratio, lambda, is quite a bit greater than one.

CO drops to near enough zero once the mixture becomes weak (the spare oxygen enhances the conversion from carbon monoxide to carbon dioxide)

If the mixture becomes weak enough to make it difficult to burn, you may also see a rise in the HC emissions, although these may be masked by the catalytic converter combining them with the excess oxygen in the exhaust stream.

Number_Cruncher



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Wed 26 Oct 2005 20:37
Fiesta MOT emision failure - far0n
Just an update, I've taken off the cover to the air filter and it's vanished !! No air filter, strangely it was in there when I took it in. I can only assume that they took it out to get the CO figures down when it failed the first time. Now, without the air filter the lambda probe is picking up a weak mixture.

My mechanic had better have a good excuse. :o(
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Fri 21 Oct 2005 13:47
Fiesta MOT emision failure - John S
Lambda is a measure of how close the mixture is to the theoretical optimum - ie just enough air to fully combust all the fuel. 'Correct' lambda is 1.00, and MOT requires it to be between 0.97 and 1.03.

I think NC is right. If CO is zero it's probably running fuel weak. Normally the ECU adjusts the mixture based upon input from the lambda sensor so your car must be running beyond the limits the ECU can adjust for.

Did it fail only on high CO and not lambda the first time, because that seems a little strange. High CO could be caused by a blocked air filter, especially at the 3000 rev/min when air flow is high, and an air filter is certainly a standard service item. Did they do anything other than a service? I'm struggling to think of a service operation that would give a serious air leak, because the car seems to have gone from one extreme to the other. I wonder if the lambda sensor is faulty or coated from the rich mixture. Maybe some injector cleaner and an Italian tune up would be a first step.

JS
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Fri 21 Oct 2005 15:35
Fiesta MOT emision failure - Cliff Pope
I'm struggling to think of a service
operation that would give a serious air leak, because the car
seems to have gone from one extreme to the other.



Air intake jubilee clip loose or not located properly? Air filter box not snapped closed completely? Tired/fragile air hose cracked during removal? Vacuum pipe, eg for crankcase ventilation, left off by mistake?
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Fri 21 Oct 2005 15:45
Fiesta MOT emision failure - John S
Cliff

With respect, few of those. Got to be downstream of the throttle butterfly to have any effect on mixture, which severely limits the options. That leaves the main vacuum hose to the brake servo, and a large leak there makes the car undriveable. Crankcase vent pipe leaks can upset idling, so are equally noticeable. Wonder if the vacuum hose to the ECU has come off?

JS
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Sat 22 Oct 2005 21:08
Fiesta MOT emision failure - Cliff Pope
OK, I bow to your better knowledge JS. I was just remembering the time I didn't put the air duct back properly on the AMM, and the air drawn in by-passing the AMM caused bad running until I realised. But may be as you imply, that was some a different effect not weak mixture.
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