CO too high (year 1996, 993)

johnw7884 · 07-06-2011, 12:30 PM

Hi,

My 1996 993 carrera (Euro engine) failed its annual emissions test due to high CO readings.

The readings were as follows:
IDLE:
- Lambda 1.030
- CO 0.679% (fail, limit 0.50)
- CO2 13.77%
- HC 136
- O2 1.21%

AT 3000rpm
- Lambda 1.014
- CO 0.779% (fail, limit 0.30)
- CO2 13.99%
- HC 52
- O2 0.84%

After running at 5000rpm for 5s, HC reduces to zero, and stays there even on idle as catalysts are hot. By doing the same, I managed to get the CO for a while to 0.369, which fails the test only by a whisker.

Engine is running smooth and strong. Any suggestions on how to pass the test? Would removing the breather hose during the test help? I got only few days to sort the thing out before the valid test expires, so any quick and temporary fixes on how to pass the test appreciated.

Joe Bob · Joe Bob's Garage

HC goes up, CO goes down. Get to an indy shop and put it on an analyzer. Adjust from there.

matt demaria · 07-06-2011, 02:49 PM

By the lambda you are where you should be (+/- 2 or 3%). I would suspect catalysts are not working well. Especially when the statement is made about warming the cat up to reduce hc. Could this be a high mileage car with worn valve guides?

johnw7884 · 07-18-2011, 09:34 AM

Quote:

Originally Posted by matt demaria

By the lambda you are where you should be (+/- 2 or 3%). I would suspect catalysts are not working well. Especially when the statement is made about warming the cat up to reduce hc.

I replaced the catalyst, and immediately both CO and HC went literally to zero. Now exhaust gases are probably more pure air than the engine sucks in.

Thanks for the advise. It was spot on

.

Nine9six · Nine9six's Garage

Quote:

Originally Posted by matt demaria

By the lambda you are where you should be (+/- 2 or 3%). I would suspect catalysts are not working well. Especially when the statement is made about warming the cat up to reduce hc. Could this be a high mileage car with worn valve guides?

If Matt is correct, as he was with his suspicions on the cat; you could be ignoring the root cause, which may be worn valve guides. A compression, leakdown, and diagnostics may be in order, to confirm or refute the suspicion.
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