MOT emmissions fail & rough running when cold

[MJK 24]

1995 N reg Audi Coupe 2.0e 8 valve with 126,000 miles.

Engine also found in Golf, Passat, Corrado, 80 & 100.

Starts immediately hot or cold and idle is perfect hot or cold. When driving with a cold engine, it splutters continually until it reaches operating temperature which is 88-90 degrees on the guage.

Failed its MOT today as the co was at 0.80% at the emmissions test. The maximum permissible value for the MOT is 0.30% so it's quite a bit out. All other values were fine.

Just had oil, plugs and all filters changed. All genuine VAG parts.

Any ideas or advice would be much appreciated!

[mighty kitten]

Lambda correct and hc below 100ppm equals dead cat . Was oil temp taken and did the tester hold the revs above the 2500 rpm window for a bit to get the cat lit?
Tester isn't required to do it but it would have had a full cat test so should have been run for long enough to get it lit .

[MJK 24]

Car was driven 11 miles to the test and then sat idling for 15mins whilst I waited for the appointment (I was early) so should have been hot. Water temp was going to 95 at which point the fans cut in and bring it down to 88.

Does have an oil temp guage - 80 degrees when you're driving along. Takes a while for the oil to warm up from cold though.

Would a dead cat make the car drive spluttery until it warms up?

[MJK 24]

These are the values from the test:

Engine speed: 2755

CO: 0.72

HC: 28

Lambda: 1.015

[Pumaracing]

HC is fine, CO is rich but the calculated lambda value is weak meaning that there is free oxygen in the exhaust. You have an exhaust split or leak which would also explaining the spluttering.

[MJK 24]

The exhaust does seem to fluff slightly when it's cold. It sounds fine when warmed up though.

The blue temp switch I have discovered controls the fuelling at cold start. I disconnected it this afternoon and it made no difference whatsoever to how the car runs when cold or warming up. Is this our culprit?

[Steve H]

Sounds like two faults.

Cat is probably the MOT culprit as has been suggested but that won't cause the cold spluttering, that could be the coolant temp sensor but there are plenty of other possibilities that really would want testing for.

[UKAutospark]

MJK 24 said:

These are the values from the test:

Engine speed: 2755

CO: 0.72

HC: 28

Lambda: 1.015

Every cat failure I have seen has had a slightly high lambda. That is normal so don't worry about an exhaust leak. This car needs a cat but it would be worth checking O2 sensor functionality. I would not expect this car to have a post cat sensor so don't worry about that.