C4 Audi 100E emissions conundrum

[keithyboy]

The car is my 1993 C4 Audi 100E 2.0 8v.

Last week it failed its MoT on, amongst other things, a massive split in the exhaust just upstream of the cat and emissions.

Last week’s emissions were:

Fast idle test

CO = 0.36 (needs to be no more than 0.3) Fail
HC = 73 (needs to be no more than 200) Pass
Lambda = 1.083 (needs to be between .970 and 1.030) Fail

Natural idle test

CO = 0.5 (needs to be no more than 0.5) Pass

What I didn’t realize at the time was that the idle stability control valve was connected to the hoses but the multi-pin was off. I replaced the cat this week and reconnected the ISCV. After that it was idling rock steady and driving really well. However, my emissions now are:

Fast idle test

CO = 0.6 Fail
HC = 109 Pass
Lambda = 1.028 (Pass

Natural idle test

CO = 03.62 Fail

To sum up, the lambda levels are now within tolerance but the CO level is worse, and much worse at idle.

The MoT tester said he’d usually suspect the lambda but the lambda was now within spec. After that he’d suspect the cat but that’s brand new. The question is where to go next.

There are no other symptoms of a problem, it’s running and idling fine. It is getting up to temperature properly and the cooling fan is cutting in as you would expect.

Any thoughts or suggestions?

[ChasR]

Did you change a genuine cat for a pattern item or was it already a pattern cat on there?

FWIW the Lambda value is there also to check for leaks in the exhaust system, which would have been picked up with your split in the system.

[keithyboy]

The new one is a pattern part (genuine ones for 20 year old audis are hard to find. The old one was the original I think.

[froggy]

with cheap cats they can take a while to light up , if you can get it back on the emmisions tester get the tester to hold it at 4k and see if the hc and co come down as the lambda is working ,i get this a lot with pattern cats .