Evaluate combustion-related problems with exhaust gas readings

Getting to know your gas readings.


A rich air-fuel mixture will increase CO readings, but may not increase HC readings significantly unless the engine misfires from the rich condition. Also, because of the cooling effect of the rich mixture, NOx levels are likely to be lower than when the mixture is closer to stoichiometric (14.7:1).

A lean air-fuel mixture will cause lower CO readings, but HC levels may rise dramatically if the engine misfires as a result. In addition, since lean mixtures tend to cause combustion chamber temperatures to soar, NOx levels will increase.

Ignition timing advanced beyond its normal range results in higher NOx and HC levels due to the increased combustion chamber temperature. CO, on the other hand, may stay the same or increase depending on the amount of timing advance.

Conversely, ignition timing retarded beyond its normal range increases CO because combustion is likely to still occur once the exhaust valve opens. Since cylinder pressures and temperatures are reduced at this time, HC and NOx emissions drop.

A neglected cooling system can cause NOx readings to increase, while having only a slight impact on HC and CO levels. Deposits around combustion chambers can result in hot spots, causing combustion chamber temperatures to skyrocket.

A weak ignition coil can't sustain the proper spark duration to continue igniting air-fuel molecules. When this happens, HC readings increase, CO readings may drop slightly and NOx readings will drop.

Interpreting exhaust gas readings goes beyond “poke and hope” methods. With a thorough understanding of what triggers each gas when it’s formed, you’ll be better prepared to deal with the root cause of combustion-related problems.

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