Dear friends:

My 1994 MB c220 has just passed the new California Smog Check II. However, I am worried about its HC emission results, which are pretty close to the maximum allowable limit for HC emission at both 15 mph and 25 mph.

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At 15 mph:

MEAS MAX AVERAGE
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%CO2 15.6 N/A N/A
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%O2 0.0 N/A N/A
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HC(ppm) 74 85 21
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%CO 0.09 0.50 0.06
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NO (ppm) 7 695 150
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At 25 mph:

MEAS MAX AVERAGE
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%CO2 15.6 N/A N/A
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%O2 0.00 N/A N/A
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HC(ppm) 47 50 13
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%CO 0.08 0.47 0.05
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NO (ppm) 2 717 136
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Legend:

MEAS = Measured results on my 1994 c220
MAX = Maximum Allowable Emissions
AVERAGE = Average Emissions for Passing Vehicles
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Here are my comments on the results:

Bad news:

-High HC (ppm) numbers are on the border of maximum allowable HC emission, which may be caused by either incomplete internal engine combustion OR the temperature of catalytic converter is too low to burn off the excess HC in the exhaust gas.


Good news:
- Extremely low CO and NO emissions (nasty gases)
- High %CO2 emission (15.6) = complete combustion OR complete reduction of NOx to NO, with extra oxygen to oxidize CO to CO2.
- %O2 = 0.0 means that either the oxygen sensor = good and/or catalytic converter = good.

My wife took the car to the smog check station, but I did remind her to rev the engine to 2K-3K RPMs several minutes before the smog check began. Unless she did not do what I asked her to do, the high HC numbers seem to indicate that the combustion process is not up to specs. I heard that several generations of MB engines have been rated as "high polluters" (in HC emission?) regardless of their condition, so I would like to ask any other C-class owners to post their smog check results for comparison purposes.

Please post your comments and suggestions!

BTW, I did a new oil change several hundred miles before the smog check.

Best regards,

Eric

Rather strange numbers. Zero percent O2 indicates that all the 02 in the native exhaust and generated from NOx disassociation is being consumed.

High HC and low CO and NOx could indicate misfires, or a rich mixture. In any event the catalyst appears to be operating very efficiently. If the mixture is rich an efficient catalyst could use all the available O2 including that from NOx reduction to oxidize CO and leave significant HC. CO is relatively more reactive than the last remnants of HC.

Another possibility could be an out of calibration test system. A recent test on one of my cars generated numbers than made no sense, but since it passed, I didnt' bother to engage the proprieter in a discussion.

The M103 engines have been identified as a "high emitter profile" by the CARB. Don't know about the status of the C220 engine.

What were your numbers two years ago?

Duke