VR6 MOT CO emissions fail - any ideas?

[was8v 0]

Bit of history:

 

Standard VR6, runs well, no miss fires or other issues. No faults logged on VAG-COM.

 

I promptly forgot about this for this year, of course the MOT tester this time wasn't prepared to fiddle around and try lots of times to get it through.

 

So results:

 

Fast Idle
RPM: 2598   Pass
CO:  0.571  Fail (max 0.3)
HC:  39 Pass
Lambda: 0.988 Pass

2nd Fast Idle Test
RPM: 2616 Pass
CO:  0.504  Fail (max 0.3)
HC:  32 Pass
Lambda: 0.989 Pass

Natural Idle test
RPM: 704   Pass
CO:  0.429 Pass (max 0.5)

 

 

I watched as he tested, and the CO fluctuated in and out of the PASS zone each time a test was run at constant RPM.

 

On VAG-COM the Lamba value fluctuates between 0.9 and 1.1 at constant RPM. The idle speed also fluctuates slightly, I thought this was normal.

 

First question - should Lambda flutuate at a constant RPM that much?

 

Second question - How do I fix it? :lol:

 

The MOT tester was ex-VW and seemed to think a change of oil and filter would make a difference, otherwise who knows because theres so much that will imapct it, so he wasn't much use (last oil change 2k miles ago).

[was8v 0]

Hmmm

 

After some research it seems the Lambda from MOT of .988 / .989 being less than 1 may indicate slightly rich running, leading to high CO.

 

Or my Lambda could be "sluggish" i.e. not switching fast enough.

Or my CAT could be burnt out.

 

Anything could cause slightly rich running. Theres no other indication of rich ruunning - no black smoke on WOT and fuel consumption is very good I'd say.

CATs and Lambas are expensive to just "try".

 

:brickwall:

 

A good service and italian tune may bring it just in line I suppose and thats the first thing I'll try. Anyone got any input? Cheers.

[was8v 0]

http://www.petercoopercarrepairs.co.uk/new_page_3.htm

 

Thios has been useful. I shall use an oscilloscope to check the pre cat lambda and possibly rule out rich running / duff lambda.

[marf497 0]

You could try one of those fuel additives into about half a tank of petrol give it a good run and make sure the engine is nice and hot for the retest. Only other thing is a new Cat and lambda sensor if they are still the original then thats probably what you need.

[Roger Blassberg 0]

Could it be the blue water temperature sensor over-richening the mixture at normal running temperature, or maybe a very dirty air filter. It's a cheap fix if it is either ( or both) of those..

 

Best wishes

 

RB

[was8v 0]

Could it be the blue water temperature sensor over-richening the mixture at normal running temperature, or maybe a very dirty air filter. It's a cheap fix if it is either ( or both) of those..

 

Best wishes

 

RB

 

 

Cheers, temp reported in vag com appears to follow the dash display, AFAIK they default to 77 degress if they are faulty? Although i suppose it could be under-reading too.

 

New air filter on the way and i've done an oil service, so just to get it nice and hot for the retest on weds. Failing that KADVR6 on here has a low miles cat and lambda I can put on.

[dr_mat 0]

Also double check for air leaks post-maf and pre-lambda, they can throw off the real fuelling, even though the ECU will report no faults.

It's not normal for a VR6 idle speed to oscillate, but it is normal for a VR6 lambda reading to oscillate in the range you describe.

I would very much doubt that changing oil and plugs will help in the slightest - this is most likely caused by faulty sensors, or sensors reading the wrong things. And buying a 2nd hand MAF to swap out would be a good start ...

[was8v 0]

Just about passed!

 

I reseated all vacuum hoses on the inlet side, checked the exhaust over pre-cat, fitted new dizzy cap & rotor, new air filter, new oil and filter, half a tank of V power and a damn good thrashing of the engine must have had some effect as it passed.