380 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Note: Sometimes, a vehicle fails a state air-
quality test because it was not driven for a
suffi cient period of time before testing or the
driver shut the engine off while waiting for testing.
Using an Exhaust Gas Analyzer
Using an exhaust gas analyzer is the only sure method
of determining how effi ciently the emission control system,
as well as the rest of the engine, is operating. If the air-fuel
ratio is not properly controlled, the engine can develop
driveability problems, poor performance, high exhaust
emissions, and poor fuel economy. Often, an exhaust gas
analysis will give you a good indication of where to look
for a problem. There are several brands of exhaust gas
analyzers and, therefore, a specifi c procedure for their use
cannot be given here. Always exactly follow the equipment
and vehicle manufacturers’ instructions.
After road testing, bring the vehicle directly into the
shop; do not shut off the engine. Begin by starting and cali-
brating the exhaust gas analyzer. Many exhaust analyzers
are very sensitive, so keep the probe away from the vehicle’s
tailpipe during calibration. Then, place the analyzer probe
in the vehicle’s tailpipe. Allow the engine to idle as the
analyzer takes an exhaust gas sample. Note the exhaust gas
readings, Figure 18-8. After making all needed tests with
the exhaust gas analyzer, shut off the engine and remove
the analyzer probe from the tailpipe.
Note: Do not shut off the engine at any
time during an exhaust gas analysis or an
inaccurate reading will result. Also, make
sure the air conditioning system is off during testing.
Understanding Emissions Readings
Before you can diagnose problems in the emission
control system, you must understand how to interpret the
analyzer readings, as well as the readings generated in a
typical state air-quality test. Depending on the type of test-
ing equipment used, emissions can be measured in grams
per mile (gpm) or in parts per million (ppm) and percent-
age (%). Most current state testing programs use the parts
per million and percentage standards. Vehicles with diesel
engines are also tested for particulate emissions.
Figure 18-7. A—A backpressure gauge is installed in the oxygen sensor port. Install the gauge in the port nearest to the engine. B—If
the vehicle is equipped with an air pump system, the backpressure gauge can be installed in the air pipe manifold.
O2 sensor
Remove the
O2 sensor
and install
the gauge fitting
Backpressure
gauge
Exhaust pipe
or manifold
Backpressure
gauge
Remove the check
valve and install
the adapter
Air pipe
Adapter
A B
Figure 18-8. A sample readout from an emissions test.
Depending on the vehicle’s model year, these readings may be
passing or failing. (Ferret)
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