General Rules of Emission Analysis

• If CO goes up, O2 goes down, and conversely if O2 goes UP, CO goes down. Remember, CO readings are an indicator of a rich running engine and O2 readings are an indicator of a lean running engine.

• If HC increases as a result of a lean misfire, O2 will also increase

• CO2 will decrease in any of the above cases because of an air/fuel imbalance or misfire

• An increase in CO does not necessarily mean there will be an increase in HC. Additional HC will only be created at the point where rich misfire begins (3% to 4% CO)

• High HC, low CO, and high O2 at same time indicates a misfire due to lean or EGR diluted mixture

• High HC, high CO, and high O2 at same time indicates a misfire due to excessively rich mixture.

• High HC, Normal to marginally low CO, high O2, indicates a misfire due to a mechanical engine problem or ignition misfire

• Normal to marginally high HC, Normal to marginally low CO, and high O2 indicates a misfire due to false air or marginally lean mixture

To verify that the exhaust readings are not being diluted in the exhaust system or analyzer sampling point, combine the CO reading with the CO2 reading. An undiluted sample should always have a sum of greater than 6%. Remember, the secondary air system may be diluting the sample if it is not disabled during analysis. In fact, engines with secondary air injection systems will have relatively high oxygen concentrations in the exhaust because of the extra air pumped into the exhaust, post combustion.

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