386 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Filters that have become clogged prevent operation of the
purge system. Since these fi lters are often neglected, always
assume that the fi lter has not been serviced and check it
for clogging. Filter replacement is covered later in this
chapter.
Other problems are caused by collapsed or kinked
hoses. The most common cause of hose problems are
careless service procedures used by previous technicians.
Often, a hose is improperly routed and kinked or pinched
between other components. Sometimes, a hose on an older
vehicle will swell to the point where the inside is blocked.
Checking Evaporative Flow Control Valves
Flow control valves are often defective. There are
many variations of fl ow control valves. The technician
should refer to the proper service information to diagnose
a defective valve. Using the proper service information,
check the operation of the valve and temperature switch
when the engine is cold and after it has warmed up.
Some systems also contain check valves to prevent
the entry of liquid gasoline into the evaporative canister.
On many older vehicles, the valve is vacuum operated
and controlled through a thermal vacuum valve. On newer
vehicles the evaporative fl ow is controlled by an ECM-
operated valve.
Testing Thermal Vacuum Valves
The service information should be consulted to deter-
mine exact procedures for testing a thermal vacuum valve.
In general, to test the thermal vacuum valve, apply vacuum
to either port on the valve. If vacuum cannot build up when
the switch is cold, it has failed open. If vacuum can build
up when the switch is at the engine’s normal operating
temperature, the switch has failed closed. In either case,
the switch must be replaced.
Testing Purge Valves
Many purge valves are operated by the ECM. A single
solenoid is installed either in the purge line between the
canister and intake manifold or on the canister itself. A
resistance check of the purge valve solenoid can be made,
as in Figure 18-16. The actual operation of the valve can be
monitored by use of a scan tool, Figure 18-17. Scan tools
report the solenoid’s operational state or the percentage of
vapor purge. Some solenoids can be checked by applying
vacuum with a vacuum pump to determine whether the
valve is being opened and closed by the ECM.
If carbon (charcoal) particles are found in the hoses,
tank, valves, or elsewhere in the evaporative system, car-
bon particles are escaping from the evaporative canister.
The canister has failed and must be replaced. In cases of
severe contamination, the entire system must be fl ushed to
remove the carbon particles.
Vacuum or Pressure Buildup
On many vehicles, air can enter the fuel system
through a check valve located in the fuel fi ller cap or
through the emissions canister fi lter. These systems have
a check valve between the canister and tank and rely on
the fi ller cap valve to admit air into the tank. The cap is
designed to allow air to enter the fuel tank as the fuel is
consumed. If the valve sticks closed, the fuel pump may
not be able to draw fuel into the engine. If the canister fi lter
is plugged, the purge system may draw liquid fuel into the
canister. On some vehicles, a separate vent is used, but this
is uncommon.
If removing the fuel cap causes a rush of air into the
tank, check the vent in the fi ller cap to ensure that it is not
stuck closed. Some air rushing out of the tank is normal.
The design of the vented cap allows some pressure to build
up in the tank. After verifying the vent in the fi ller cap is not
stuck, proceed to check for a clogged canister fi lter.
Figure 18-16. Checking the resistance of the purge valve using
a multimeter. Be careful not to touch the test leads together, as
this will give a false resistance reading. A test connector cut from
the harness of a wrecked vehicle would make this test easier.
Purge valve
Figure 18-17. The state of purging the evaporative emission
control system can be monitored by a scan tool. Remember that
purging normally occurs when the engine is running and the
transmission/transaxle is in gear.
Evaporative Emissions Purge Solenoid Operation
State
Percentage
Evaporative
emission canister
solenoid
Evaporative
emission canister
solenoid
Off
On
Evaporative
emission canister
solenoid
Evaporative
emission canister
solenoid
0% 1–100%