MOT CO emissions failure

[Trog_nfs]

Hi chaps,

took my car for its MOT today, and it failed on its emissions.
It only failed on CO though, and only at fast idle.

The results are as follows:

Fast idle Test
Engine speed = 2745 rpm
CO= 0.53
HC = 100
Lambda = 1.020

Second Fast idle test
Engine Speed = 2800 rpm
CO = 0.50
HC = 87
Lambda = 1.020

Natural Idle
Engine Speed = 850 rpm
CO = 0.21

What would cause just the CO to be high on a fast idle?
The catalyst was new last year, but it hasn't had a lambda sensor in a while. Couldn't tell you how old that is, at least 4 years old I would say.

I've done a fault code check, and there's nothing there.
The only thing is is the engine was replaced a couple of months ago, and we didnt do basic settings, and as the engine has been running fine, didnt feel it was necessary.
Would this affect things at all? The idle is a little unsteady, but we are only talking of fluctuations between 850 and 875 rpm.

I found my emissions report for last year:

Fast idle = 2890 rpm
CO level = 0.07
HC = 42 PPM
Lambda = 1.002

Natural idle = 850 rpm
CO level 0.02

Now is it just me or is there a significant difference between all the readings from last year and this year?
The old engine which did the emissions last year died after head gasket failure. I've read that antifreeze can ruin a lambda sensor, and looking at all the results compared with each other, would you say that its the sensor causing the increased emissions?



Thanks
Trog

[SammoVWT]

The lambda looks like its wearing, since the value is starting to go towards the upper limit.

elevated CO & HC is probably from incomplete combustion , so your car may be running a bit rich, a filter service some new plugs and premium fuel may cure it and possibly a new lambda.

Did you give the car a good run to get the cat/engine hot before the MOT?

Or possibly a failing CAT (but they are fairly robust, mine is half dead and still working ok, just some decreased flow and an annoying rattle)

Also does your car have a map or a maf? might be worth using vag com to pull back air intake readings to see if its pulling sensible values. Definately check it before you buy parts. Could be a cheap fix with new plugs & filters.

Edit: does the car missfire at all when revving? you will notice a little skip in the engine rev sound as they increase if it is.

[Trog_nfs]

It's a 1.4 ABD, so we are talking minimal diagnositc capabilities.
I've done the code read etc, and there's nowt to report there.

The cat was new last year, and it had a plug and oil service about a month ago. I've got a K&N filter inside a drilled airbox which is clean, so that can't be it.

To be honest, I didnt get it very hot before the MOT, but the tester was ragging the t*ts off of it trying to get the emissions down. Considering there's a dustbin for an exhaust, he must have a headache today lol!

I've not noticed a missfire, although it really doesn't want to rev first thing in the morning. I try to hassle it when its cold, but sometimes its struggled getting off the driveway which is slightly inclinded, and a small misfire at idle when very cold.
I think the fuel economy is down, although i've only done town driving recently.

Its at the garage today; i think it is the lambda, but its too low to get under myself to test it. Easy when you've got a ramp, not so much without.

I've been digging through all the paperwork I have for it, and i doubt the lambda has ever been changed. The fact the cat is new is probably hiding the car running rich.

Thanks

[SammoVWT]

If your plugs are good, it might be your leads or getting a weak spark from your ignition coil as an idea.

I know when my ignition coil was failing, it was running rich and sluggish, wouldnt rev up easily. (and missfiring quite regularly)

I managed to reach the lambda to change mine from the top, but im not sure whether yours is positioned in the same place. To change it, youre gonna want to WD40 the exhaust/thread and let it soak, they sieze in good. But it will work free after a while.

But like you say probably worth checking the lambda first, then failing that the ignition coil/leads etc(these are around £50-65 for a good one from ECP)

No smoke of any kind im assuming from the exhaust?

EDIT: Also an idea, might be worth checking the coolant sensor too, this will also affect the fuel mixture iirc - worth getting a scan to cross check temperature values, same for air intake and engine temperature. Worth a check, think they are normally around £15 quid for a replacement.

(also remember a fault with a probe doesnt necessarily show up as broken - maybe within range but incorrect if that makes sense)

[Trog_nfs]

Got a call from the garage; they reckon the headasket failure (which i didnt think was a headgasket failure for a few weeks doh!) has ruined the cat and the oxygen sensor, which makes sense as both are knackered by water and antifreeze, and towards the end I was losing alot.

But they reckon if they get it hot enough the can get it to pass this time, giving me time to get this sorted.

Cheers