Lambda NCT fail - what to do?

antiskeptic

Hi there,

Audi A4 1.8t failed on hi idle Lambda.

Actual: 1.13

Required: between 0.97 and 1.03

CO was 0.02 vol% (fail at 0.2)

HC 0 ppm (fail above 200ppm)

I drove the car for 15 minutes to the test centre from cold but burbled along rather than driving hard (I see folk talking of warming the catalytic conv)

Engine oil temp was 80C

Guy at the NCT said it was usually a leaky exhaust that caused this.

Anyone any advice on what action to take?

Cheers.

cruizer101

I'd a fail on high lambda before that was leak in exhaust alright. Thats not to say that is definitely the cause.

You might be able to get under car and look at exhaust and spot the leak, would normally occur around joints.

If you cover the end of exhaust pipe it can make it easier to spot as will for it out the leak, don't get under car with it running but up on jack stands and lie down beside it and get someone to hold rag over end of exhaust.

If it is a leak can maybe be fixed with gun gum.

antiskeptic

Cheers. It's due tyres so I'll get the fast fit lads to get it on the jack

smartarsery

Last time I failed on emissions I just threw some Dipetane in with the fuel, and finally went to the bother of fixing a little crack in the vacuum line on the turbo, and it passed next time, no problem.

But from my science days I think I remember "lambda" meaning "difference", so it's maybe the difference in emissions between the upper and lower oxygen sensors in the exhaust? So a hole in the exhaust would probably make more sense, or maybe one of the sensors is faulty or dirty.

Oops!

The stones need to be reddened out of that car on the way to the test centre, at least a 20-30 min drive.... Different results then.

John.G

Does look very like a exhaust leak, putting it up on a ramp is the best way to check.

blackbox

https://www.boards.ie/discussion/comment/117924166#Comment_117924166

If the lambda is high, it means there is too much oxygen in the exhaust. No amount of hard driving will fix this.

https://www.boards.ie/discussion/comment/117941532#Comment_117941532

Clean the MAF and check the vacuum leak. Never use electrical contact cleaner. Too many people seriously thinking it does the same job and that is very wrong. I understand them too, MAF cleaner cost several times more money and hard to find it on Ireland. Vacuum leak detection I suggest to use the smoke machine. Yes there is possible to find the spot by soapy water or brake cleaner but for DIY it can lead to catastrophe or burn entire car down. I was thinking to share possible cool smoke machine project, but then I released it can cause inexperienced DIY enthusiast death. So I do not share that. Only what you need to know the baby oil makes a smoke on specific temperatures. Use your imagination and be careful.

blackbox

I've never heard of a car being burnt down with soapy water.

https://www.boards.ie/discussion/comment/117943965#Comment_117943965

Brake cleaner can burn it down, too much soapy water can lead to hydrolock. So, select wisely your tools.

antiskeptic

https://www.boards.ie/discussion/comment/117925462#Comment_117925462

Had it up on a ramp down at fast fit and couldn't detect anything amiss with the exhaust. Unfortunately. But cheers..

antiskeptic

https://www.boards.ie/discussion/comment/117941532#Comment_117941532

That's what I just read a min ago on an informed sounding post. Lambda should be 1.00(000).

Sounds like I'm running well lean. What kinds of thing cause this? Besides an air leak (which poster Local Car Mechanic has pointed to)

antiskeptic

https://www.boards.ie/discussion/comment/117942471#Comment_117942471

Thanks for the tips. I'll see can I track down cleaner. Get what you say about hyrolock. Although DIY I'm handy so know not to over cook it with soapy water. But what vacuum am I looking to test - the intake side of things generally or something specific?

Cheers..

antiskeptic

Can I not just oil some of the air filter to block it and richen up the mix a tad?

😀

Start from visual inspection, look the cracks, listen the noise. Check every hose what is connected to intake manifold. If you have access to live data you can block hose by hose clamping/blocking these off. Check PCV valve. Load of things is made by plastic, it can crack over the time. To rule out MAF problem you can disconnect it and car use default settings. Load of thing to do but you can to do it if you really want it.

BrianD3

If the car is running well and the CO/HC emission are otherwise good (very good in this case) IMO the main suspect is still the exhaust. If the car was actually running lean or had an "upstream" issue would the engine management system not try to compensate for that? How thoroughly was the exhaust examined, I would carefully run my hand over the entire length of it to feel for leaks.

Hellrazer

https://www.boards.ie/discussion/comment/117948423#Comment_117948423

Agreed BrianD3- High idle CO failure is in my experience almost always an exhaust leak. With these readings Id imagine its probably a minor leak in between one of the joints.

Best way to find it is to block the exhaust with a cloth covered hand see if you can hear or see a leak.

antiskeptic

https://www.boards.ie/discussion/comment/117948423#Comment_117948423

I only had a few minutes when the car was up on the ramp. I did run my hand over everything I could access but I was in hurry so if the leak was very slight I'd have missed it. And I could only really access from endpipe to where the exhaust disappeared up into the engine bay. The fast fit guy held his hands over both ends of the exhaust.

Out of interest, how does a leak "lean" the exhaust?

I'd have thunk the exhaust leaking would see fumes coming out rather than air being sucked in (so as to "lean" the results)

The car is running fine otherwise.

antiskeptic

https://www.boards.ie/discussion/comment/117948584#Comment_117948584

You mean High Idle Lambda?

Question: if I'm doing this on my own would it suffice to block one of the two exhaust outlets (twin pipe audi a4). I don't imagine I could block both pipes for very long without causing an issue somewhere?

Hellrazer

https://www.boards.ie/discussion/comment/117948676#Comment_117948676

It leans the exhaust emissions because its not always "blowing" out the back - there is a certain amount of back pressure and if you have a hole in the exhaust then its this back pressure that causes it to run lean ie too much air mixed with the fuel.