Operating Instructions - Snap-on Equipment

InterpretationData Analysis GuidelinesUse the following information when analyzing data to isolate a specific problem. It isimportant to remember that by-products of combustion depend on the air-fuel ratio.Stoichiometric Air-Fuel Ratio• Lambda = 1.00 at stoichiometric air-fuel ratio.• CO and O 2 are equal at the stoichiometric air-fuel ratio.• CO 2 is an indicator of combustion efficiency that peaks at or near thestoichiometric air-fuel ratio, and decreases with a lean or rich air-fuel ratio.• NO in the range of 1700 to 2500 ppm is typical on an engine running atstoichiometric without EGR.—With EGR, NO levels are usually between 500 and 1000 ppm.Lean Air-Fuel Ratio• Lambda is greater than 1.00. If Lambda = 1.10, this means 10% excess aircompared with stoichiometric.• High HC is an indicator of excessive leanness and misfires.• High O 2 indicates an excessively lean air-fuel ratio.• NO is highest when CO and HC are lowest.✓ Air injection systems should be disabled to prevent false readings.Rich Air-Fuel Ratio• Lambda is less than 1.00. If Lambda = 0.90, there is only 90% of the required airfor stoichiometric.• Low O 2 indicates a rich mixture.• High CO readings usually indicate a fuel mixture richer than stoichiometric.• High HC readings usually indicate excessive unburned fuel caused by lack ofignition or by incomplete combustion.— Common causes include a faulty ignition system, vacuum leaks and fuelmixture problemsMiscellaneous• When CO goes up, O 2 goes down.• O 2 combines with HC to form CO 2 and H 2 O.• O 2 combines with CO to form CO 2 .• O 2 and CO 2 are indicators of exhaust system integrity, sample hose and probeintegrity, or both.• O 2 is essential for proper operation of the catalytic converter.— O 2 is essentially unchanged by the catalytic converter, providing a “window”through the catalytic converter to the engine.— O 2 readings are higher on vehicles with properly operating air injection systems.• With the air injection system disabled and the CO above 1%, the catalyticconverter is O 2 starved.— Without O 2 it does not “light off,” allowing exhaust concentrations to be morelike readings taken ahead of the converter.5-2