6-8
Vehicle Testing Procedures
Testing Tip
X
Some systems may divert air pump output to the air cleaner
housing or the atmosphere under some conditions. This could
lead to incorrect test results. Before replacing components,
understand the system operation.
Test
1. Record exhaust gas readings while the engine is at 1500
RPM.
2. Disconnect the air pump outlet hose while maintaining the
same engine speed, or restrict air pump output by pinching
the air hose with a pliers.
3. Record and compare readings.
Interpretation
•
With the air pump operating O
2
should be about 3–8% vol above
the reading taken when the pump is disconnected.
•
With the air pump disconnected or restricted, O
2
should
decrease and HC, CO and CO
2
reading should increase.
Test results different from these indicate a problem with the air
pump system. Follow manufacturer guidelines for troubleshooting
and repairs.
Carburetor Adjustments
Vehicles Without Feedback Fuel Systems
Adjusting the carburetor using the analyzer requires an
understanding of how each adjustment can affect the air-fuel
mixture.
Adjustments that make an engine run richer than ideal, meaning
more fuel/less air, should increase the HC and CO readings, while
decreasing the O
2
and CO
2
readings in the exhaust. Adjustments
that make an engine run leaner than ideal, meaning more air/less
fuel, should decrease the HC, CO and CO
2
, while increasing the
O
2
reading. When the air-fuel mixture is so lean that one or more
cylinders misfire, HC and O
2
readings increase and CO and CO
2
readings decrease.
Make all carburetor idle mixture adjustments using the
recommended procedures of the engine manufacturer. When the
manufacturer gives a CO setting, use the analyzer to make the
necessary adjustments.
On vehicles where CO specifications are not available, use the
analyzer to ensure that the emission readings are at, or below,
legal limits and consistent with the year, make and model of the
engine being tested.
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