MOT emissions help

[Krikkit]

Hi chaps.

Took my car for its MOT, unfortunately failed on a few points, emissions included.

Results were that the CO was about 20% too high, HC was mid-range and Lambda reading was 1.03. I have a tubular manifold that moves the lambda probe right down towards the joint with the under-floor cat, unfortunately this joint has always proved a bh to get sealed up fully, and there's still a small leak.

My question is, will the air leak about an inch downstream of the lambda probe be enough to affect its reading, causing the ECU to rich out the mixture? I know it will affect the tester's reading at the tailpipe, but is the scavenging/pulsing effect in the exhaust strong enough to pull the air through far enough to the probe?

The engine is otherwise in fine fettle, and runs very sweetly. The MOT tester apparently gave it a good warming (I wasn't there to see it unfortunately) before the test. The cat should have been able to take care of a standard level of emissions, it's a cheap one but has only seen a couple of hundred miles since it passed the test last year. Lambda is also recent.

[stevieturbo]

Krikkit said:

Results were that the CO was about 20% too high

More detailed emissions info needed.

Limit is approx 0.3%, so are you 20% of 0.3, or do you mean actually 20% CO ?
Which I'd actually be amazed if the engine still ran at 20% CO

Accurate info is essential to have any hope of a diagnosis

[Krikkit]

My memory isn't great, so some proper numbers from the report.

Idle test was CO 0.53 (Fail).

Fast idle was 3kRPM, results:

• CO: 0.72 1st test, 0.56 2nd
• Lambda: 1.02 and 1.02
• HC: 137 1st, 125 2nd

So, as before, it looks like it's running rich with a poor cat to me... Even so, marked improvement between the 1st and 2nd idle test, so warming it for a couple of minutes has helped.

[Disco_Biscuit]

http://www.cataclean.com/

[paintman]

You might find this thread of interest. Another cheap cat & looks like the CO is similar to yours.
http://www.pistonheads.com/gassing/topic.asp?h=0&a...

[Steve H]

If the air leak was making the lambda sensor run the engine rich your lambda reading at the tailpipe would generally be below 1, not just above it.

I'd go with catalyst and having done only 200 miles since the last MOT makes it less, not more, likely to pass.

[Krikkit]

You're probably right Steve, chances are I'm clutching at straws.

Another issue that's cropped up (which is MOT-relevant), is that if you hold the throttle constant, rather than being constant revs you get 3-4s of good running followed by what feels like a misfire. -200RPM shows on the tach then it goes straight back to normal, rinse and repeat.

A Google suggests throttle pot, but that's smooth as silk on the multimeter.

[Steve H]

I wouldn't have gone for throttle pot, you wouldn't normally have got the 3-4 seconds of good running at that position first.

Not sure I understand what you mean by "200rpm shows on the tacho" ?

What kind/spec of car are we on?

[Krikkit]

Steve H said:

I wouldn't have gone for throttle pot, you wouldn't normally have got the 3-4 seconds of good running at that position first.

Not sure I understand what you mean by "200rpm shows on the tacho" ?

What kind/spec of car are we on?

Peugeot 106, GTI engine.

I meant the needle drops 200RPM when it happens, then goes straight back up again for a few seconds. Rinse and repeat. Also tried the MAP with no difference, tonight will be coilpack from the spares collection.

[Steve H]

If it's moving up and down by 200rpm when you try to hold it at fast idle my bet would be on the lambda sensor switching slowly, worth taking a look at the signal.