Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 385
Copyright by Goodheart-Willcox Co., Inc.
The canister can become saturated with fuel or char-
coal may leak out.
Leaks can occur anywhere in the system.
Any vehicle can develop problems in the evaporative emis-
sion system because of leaks in the hoses, tank, or gas cap.
On vehicles equipped with OBD II, leaks or fl ow problems
can set a trouble code.
Warning: Escaping fuel vapors are
extremely fl ammable. Keep the test area
well ventilated and make sure that any
sources of ignition, such as sparks or open fl ames,
are kept well away from the area.
General System Tests
Begin evaporative system service by checking for
system-related trouble codes. Typical codes are shown in
Figure 18-13.
Visually inspect the evaporative emissions system
hoses. Look for hoses that are disconnected, obviously leak-
ing, or collapsed. A common cause of evaporative system
leaks is a defective or missing gas cap. Always check the
cap condition before making more complex leak checks,
Figure 18-14.
Next, attach a fl owmeter to the line between the can-
ister and the engine intake manifold. Start the engine and
observe the purge airfl ow as the engine idles. The minimum
fl ow should be roughly one liter (about a quart) per minute.
Most vehicles with a properly operating evaporative emis-
sions system will have much more fl ow, often as much as
25 liters (7 gallons) per minute during the test. If the fl ow is
not suffi cient, make further tests.
Pressure Test
A pressure test will determine whether the evaporative
emissions system is leaking. On some vehicles, the scan
tool can be used to operate the fuel pump to pressurize
the fuel tank. On other vehicles, a special pump is used to
create a vacuum on the evaporative emissions system.
Locate the evaporative emissions system pressure test
fi tting on the vehicle, Figure 18-15. Next, attach a nitrogen
pressure source and a pressure gauge to the test fi tting.
Open the nitrogen tank valve and pressurize the system to
the required pressure. Then, close the nitrogen tank valve
and wait the specifi ed time while observing the pressure
gauges. If pressure drops, the system is leaking. If the gauge
pressure does not drop, the evaporative emissions system
is sealed.
Flow Restrictions
The most common fl ow restrictions are caused by a
canister vent fi lter that has become completely clogged.
Air must be drawn through the canister to allow the fuel
vapors to be drawn out of the charcoal and into the engine.
Figure 18-13. Typical trouble codes related to the evaporative
emission control system.
Trouble Code Meaning
P0440 Evap emission system (general)
P0442 Small leak detected
P0446 System performance
P0455 Large leak detected
P0496 Flow during non-purge
P1441 Flow during non-purge
Figure 18-14. Check the fi ller cap for damage. Some techni-
cians prefer to replace the cap as part of maintenance on the
emission control system.
Look for damage
or missing part
Figure 18-15. Most pressure-testing fi ttings on the evaporative
emission control system have a green protective cap. The fi tting
may have an identifi cation and caution label as shown here.