7-2
Diagnostic Technique
Lean Air-Fuel Ratio
•
Lambda is greater than 1.00. If Lambda = 1.10, this means 10%
excess air compared with stoichiometric.
•
High HC is an indicator of excessive leanness and misfires.
•
O
2
is a better indicator of leanness and misfires than HC.
•
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 air for 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 of ignition or by incomplete combustion.
— Common causes include a faulty ignition system, vacuum
leaks and fuel mixture problems.
Miscellaneous
•
When CO goes up, O
2
goes down.
•
When O
2
goes up, CO 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 probe integrity, 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
catalytic converter is O
2
starved.
— Without O
2
it does not “light off,” allowing exhaust
concentrations to be more like readings taken ahead of the
converter.
•
Minimal NO is produced while an engine idles.
•
An excess of O
2
reduces HC and CO emissions.
•
A deficiency of O
2
reduces NO.
•
NO forms in the combustion chamber at peak temperatures
2500°F (1370°C).
•
NO levels increase as O
2
levels increase.
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