Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 387
Copyright by Goodheart-Willcox Co., Inc.
If the vent is stuck closed, the fuel pump may be pow-
erful enough to partially collapse the fuel tank. Therefore,
if a venting problem is suspected, check the fuel tank for
signs of collapse.
Sometimes, the OBD II system will place the fuel tank
under a partial vacuum for test purposes. If the tank cannot
hold a slight vacuum, the OBD II system will set a trouble
code for a leak. The fuel tank on vehicles that perform this
test may have a non-vented fuel fi ller cap or a fi ller cap
check valve that opens at a higher vacuum than those used
on other systems. If the cap is opened while the vacuum
test is going on, it will be normal to observe air rushing into
the tank. Check the vehicle’s service information carefully
before deciding that there is a defect in the system.
Leaks
Any leaks of liquid gasoline from the fuel system are
easy to spot as they show up as wet spots or stains on or
near the leaking component. Vapor leaks are harder to fi nd.
Often, the most effective way of fi nding vapor leaks is mak-
ing a visual check for cracked or disconnected hoses, loose
clamps, or damaged components.
Locating leaks may require pressurizing the tank. Use
nitrogen as the pressurizing agent and pressurize the tank
to no more than .5 psi. The tailpipe probe from an exhaust
gas analyzer can then be passed over the fuel system com-
ponents. Any leak will cause an excess hydrocarbon read-
ing on the analyzer.
Warning: Never pressurize the fuel tank
with any gas except nitrogen. Use of
another gas or compressed air may cause
an explosive mixture.
Checking for Leaks in the Evaporative Emission
Control System with a Smoke Machine
A smoke-generating machine, or smoke machine,
can be used to locate leaks in the evaporative emissions
system. See Figure 18-18. Some manufacturers now specify
that a smoke machine be used to test the evaporative
emissions systems. This section gives a general procedure
for checking evaporative emissions systems with a smoke
machine. Consult the proper service information for exact
procedures.
Begin the procedure by making sure the gas tank is
between 1/4 and 3/4 full. A tank that is completely full or
almost empty will affect machine operation. Then, use a
scan tool to set the evaporative system purge and vent valves
to the closed position. This seals the evaporative emissions
system so that it can be pressurized by the smoke machine.
The smoke machine uses nitrogen from an external
tank to pressurize the system. Some machines allow the
use of shop air, but this is not recommended due to the pos-
sibility of fi re and explosion. Connect the nitrogen source
to the machine. Install the pressure line on the gold (.020
inch/.5 mm) or silver (.040 inch/1 mm) orifi ce. This is done
to set the machine to correspond to the suspected leak size,
which was determined by earlier tests using the scan tool.
Then, set the machine’s fl owmeter so the pointer and ball
are aligned, Figure 18-19.
Install the smoke machine’s pressure line on the
appropriate fi tting of the evaporative emissions system. Use
adapters as needed to ensure a leakproof seal. Next, remove
the gas cap and start the fl ow of smoke into the evaporative
emissions system. When smoke begins to exit from the fi ller
neck, reinstall the cap, allow the machine to pressurize the
system, and note the position of the fl owmeter ball. If the
ball has moved upward, a leak is present. If the ball has not
moved or if it moved downward, the system is not leaking.
If the ball movement indicates a leak, carefully observe
the entire evaporative emissions system for signs of smoke.
Areas to concentrate on include the gas cap, tank fi ttings,
charcoal canister, and all hose fi ttings. Use a caged drop-
light or other safe light source to illuminate the suspected
Figure 18-18. A smoke machine, such as the one shown here,
can be used to check the evaporative emission control system
for leaks.
Smoke/air control switch
Smoke on lamp
Flowmeter
Figure 18-19. This smoke machine contains a fl owmeter that is
used to indicate a system leak. Before beginning the test, the
meter’s pointer and ball must be aligned. (Chrysler)
Pointer aligned
with ball
Pressure line