NCT emissions - high lambda

BrianD3

I have a petrol car here about to do its NCT, it passed emissions last time but the lambda reading then was 1.027 which was only barely within the 1.03 limit. The HC and CO emissions were low. I have a feeling that it could fail on the lambda this time around

Am I right in saying that a low emissions + high lambda reading from an exhaust gas analyzer probably means that the exhaust gases are being diluted by fresh air AFTER they pass the lambda sensor. Eg a hole in the exhaust back box causing fresh air to be drawn in.

If there was an air leak around the intake or exhaust manifold or anywhere before the sensor then would the sensor detect the leanness and compensate by making the engine run richer (resulting in low lambda reading at the tailpipe and increased emissions)

Is this correct?

Gerry

Yep brian, I think that is correct. Though I think an intake leak need not result in a rich mixture, it depends on how well the ecu compensates for it.

You can check for the exhaust leak by blocking the tailpipe with a rag, the idle should dip down and the car should start to stall. If the engine note doesn't change at all, get someone to hold the rag there while you inspect the system, you should be able to hear the leak.

Cosmo K

Its definitely a small leak somewhere in the exhaust system. Even a small one is enough.

crosstownk

Basically your right, Brian. Normally a leak is audible. If you can get it up on a ramp and listen and feel for the leak as the engine idles then all the better.

I know this is unlikely, but do a check on another car to be sure the equipment is functioning OK.

netwhizkid

I put my car a BMW 316 through the NCT last Thursday after driving it around for the last 7 months without the NCT. However the English MOT (imported car) which ran was good until January ran out and even though the Gardai are not enforcing it, Stopped several times (randomly breath-tested once) and they never commented on the NCT bald tyres and Insurance was what they seemed to be looking for.

I put mine in for it despite my dislike of the NCT. Anyway it passed easily but I got the impression that alot of it has to do with the mechanic testing the car. The fella who tested my car gave it a good revving while while he had the wand up the exhaust.

Then a different fella hardly revved up a Nissan Primera 02 and it failed the NCT on the emissions - It had me totally puzzled. I don't know it is the same everywhere else but where I had mine done you see out the windows of the waiting room.

My overall image of the NCT was good because if your car is in good enough shape it will pass it, however you hear people complaining, these are the same people who would have cardboard on the floor and Baling twine holding the door shut. If your car fails the NCT you either have a total banger or else you obviously don't treat the car right.

I know one person (English) who has the same car now for thirty years he bought it new, he keeps it in a garge and regularly puts up 10-15k miles a year on it. If you get a good car and treat it with respect and look after it then you'll be fine, This man is English where they know how to treat motors right, I have seen several 07 cars here already with huge scratches and dents already :rolleyes: