376 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Evaporative Emissions System Test. Many enhanced
and some static emissions tests include a test of the evapo-
rative emissions system. Some tests on older vehicles mea-
sure fl ow into the engine from the evaporative emissions
system as other tests are being performed. See Figure 18-3.
The minimum fl ow should be roughly one liter (about a
quart) per minute during the test. Most vehicles with a
properly operating evaporative emissions system will have
much greater fl ow than this.
On OBD II vehicles, the evaporative emissions sys-
tem can be pressure checked using nitrogen, Figure 18-4.
Once the nitrogen has been used to pressurize the system,
you must wait a specifi ed time to determine whether the
system loses pressure. A pressure drop indicates a leak in
the system. On some vehicles, the scan tool can be used to
operate the fuel pump to pressurize the fuel tank.
Warning: Nitrogen is used as a pressurizing
agent to reduce the possibility of fi res. Never
use a gas other than nitrogen for this test.
OBD II System Test. OBD II information is becoming
an integral part of the emissions-testing process in some
states. It is possible to eliminate the traditional tailpipe tests
by reading information from the OBD II system using the
diagnostic link connector.
Removing the Vehicle from the Test Area
After the testing process is complete, apply the brakes
to ensure the drive wheels are no longer spinning. Place
the vehicle in park or neutral and set the parking brake.
Raise the dynamo meter drive-on plates to release the drive
wheels from the rollers. Next, remove the vehicle-secur-
ing devices and all other test equipment. Then, drive the
vehicle off the dynamometer and to the customer-waiting
area of the test facility.
Completing the Test Paperwork
At the end of the test, the vehicle’s owner should
receive a pass/fail report that compares the actual read-
ings to the desired values and reports whether or not the
vehicle has passed the test. In most cases, the test computer
automatically generates the report. These emissions are
typically checked and reported:
Unburned hydrocarbons (HC).
Carbon monoxide (CO).
Oxides of nitrogen (NOX).
Free oxygen (O2).
Carbon dioxide (CO2).
Flow or pressure results from testing the evaporative emis-
sion control system are also given when applicable. In some
states, the readings and pass/fail status are directly trans-
mitted to the central computer at the state motor vehicle
department or the emissions inspection headquarters.
If the vehicle passed the emissions test, inform the
driver or owner and give them the completed test results
and other paperwork. If a windshield sticker is used in your
state to indicate the vehicle is in compliance, apply it at
this time.
If the vehicle failed the emissions test, the testing
computer will usually print out a repair diagnostics report
in addition to the pass/fail report. The repair diagnostics
report lists possible causes of the particular failure(s). This
report can be used as an aid to diagnosis and repair of the
vehicle.
Inform the driver or owner of the problem and answer
any questions. When applicable, inform the vehicle owner
of the repair cost–waiver program. A repair cost–waiver
Figure 18-2. Typical acceleration trace for performing one par-
ticular enhanced emissions test.
20 40 60 80 100 120 140 160 180 200 220 240 0
60
50
40
30
20
10
Vehicle
Speed
(mph)
Phase 1 Phase 2
Time (sec)
acceleration
Figure 18-3. In some vehicles, condition of the evaporative emission control system can be determined by connecting a fl owmeter
in the line between the engine and the charcoal canister.
Filler cap
Filler neck
Rollover valve
Fuel tank
Charcoal
canister
Flowmeter
Engine