Snap-On Diagnostics DGA 1000, Manual, Page: 111 / 116

7-1
Diagnostic Technique
Interpreting gas concentrations under a given set of conditions
becomes a matter of:
• Understanding engine operation and the system or systems
under test,
• Understanding the relationships of exhaust gas concentrations
to each other, to the engine operation and to the catalytic
converter, and
• Applying logic to the possible causes, while eliminating those
that are not probable, and selecting the most likely of the
possible causes.
Applying logic becomes the diagnostic technique used to
determine the cause of unacceptable readings, together with a
course of action that includes one or more of the following
variables:
• Change engine speed, load, or both,
• Defeat air management,
• Enrich the fuel mixture with propane,
• Create a vacuum leak to lean the fuel mixture, and
• Use other test equipment such as an oscilloscope, vacuum
gauge, or scanner.
Data Analysis Guidelines
Use the following information when analyzing data to isolate a
specific problem. It is important 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 the stoichiometric 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 at stoichiometric without EGR.
— With EGR,
NO levels are usually between 500 and 1000
ppm.
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