MOT Emmisions Fail Help

[N1xsta]

Hello

Took my car for MOT today and it failed on the emissions test

VW PASSAT 2l petrol 2004

Fast idle test
Lambda 1.530 fail

Second fast idle test
Lambda 1.496 fail

Can anyone give advice - I dropped the car off hot and waited for it. I noticed when idle the car revs up and down a bit and the car will stall now and again for no reason just dropping revs

The garage said about taking something off and cleaning it in the morning to see if that helps

And advice will be greatly helpful

Thanks

[E-bmw]

Why don't you just take something off & clean it in the morning then?

[N1xsta]

Sorry haha couldn't remember what they said they was going to do

They said about them removing throttle body and cleaning it - they are unsure what is causing the high lambda reading. Just wanted to get some more advice

[E-bmw]

I am sure there are others out there in PH land that know in more detail than me, but here is my understanding of that.

There are 2 sensors in your exhaust, 1 measures how much oxygen there is in the exhaust gases coming right out of the manifold to see how well the engine is burning its fuel and it uses this data to tweak the fuel/air mixture to make the engine run as efficiently as possible.

There is another after the cat to also tell the engine how well the cat is working.

With all working well the one before the cat can have transient highs & lows as the engine management system controls everything, the one after the cat should be pretty constant assuming the cat is working correctly.

Your readings on an MOT are obviously taken at the exhaust tip to check the full system out & if these are (like your readings) showing way too much oxygen in the combustion air my best guess would normally be an exhaust leak or an air inlet leak.

With you saying the car idles erratically, that would more so make me look at the air inlet side of things.

Having said that there are other indicators, as in did you also happen to note the HC & CO readings when those fails were registered?

[N1xsta]

Co was 0.003
Hc was 3

So you do not think doing the throttle body would help

[Huff]

That lambda reading is very lean. Looks like a vacuum leak to me - look for split hoses and pipes on the intake side (upstream of the mass airflow sensor if it has one) or an idle control valve sticking open (which would be the bit to clean with carburetor cleaner spray)

The other values are good and show the system overall is probably ok.

[E-bmw]

N1xsta said:

Co was 0.003
Hc was 3

So you do not think doing the throttle body would help

No.

[stevieturbo]

It's pretty obvious there is an exhaust leak.

[N1xsta]

Another mechanic who is a friend has said could be lambda sensor

[stevieturbo]

N1xsta said:

Another mechanic who is a friend has said could be lambda sensor

It could be many things, it could even be faulty test equipment.

But rather than guessing, perform some simple tests based on the evidence in front of you.

[Mignon]

N1xsta said:

Another mechanic who is a friend has said could be lambda sensor

Then he's an idiot. Your lambda value indicates significant excess oxygen in the exhaust system as others have told you which means there's a an exhaust leak.

[N1xsta]

Update

The mechanic done a smoke test today and has said there is 2 air leaks by the inlet manifold and the air mass sensor is not working

Would this be cause

[Mignon]

If either of those were true the CO would be wrong which it isn't.

[stevieturbo]

N1xsta said:

Update

The mechanic done a smoke test today and has said there is 2 air leaks by the inlet manifold and the air mass sensor is not working

Would this be cause

Have you read any of the responses above ?

[E-bmw]

Apparently not, his mate the mechanic knows more than us.