Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 377
Copyright by Goodheart-Willcox Co., Inc.
program grants an emissions waiver after the owner has
spent a certain amount of money, even if the vehicle has
not been restored to the desired emissions levels.
Preparing a Vehicle for Emissions Testing
The technician may be called on to prepare a vehicle
for an emissions test. Many owners want their vehicles
checked out before going to the offi cial inspection station.
In addition, the technician must check for proper emissions
levels on vehicles that were repaired after failing a state-
mandated emissions test. The following are a few methods
for improving the chances of passing an emissions test.
Make sure the engine and vehicle are in good operat-
ing condition. Diagnose and repair any obvious engine
misfi res, rough idle, or other performance problems.
Always locate and correct the cause of an illuminated
MIL.
Change the engine oil and fi lter. This removes
unburned hydrocarbons that are present in the old oil.
New oil also does a better job of sealing the piston
rings, resulting in a cleaner-running engine.
Try to take the test on warmest day possible. This will
ensure that the engine stays at its normal operating
temperature during the test.
Do not fi ll the gas tank before getting the test. A full
tank may overload the evaporative control system and
cause the appearance of a system defect.
Take a highway drive of about 5–10 miles before the
test. A long, highway-speed drive will tend to remove
deposits from the internal engine parts.
Do not turn off the engine while waiting for the test.
A running engine removes unburned fuel from the
crankcase and evaporative emissions control system.
Preparing to Diagnose the Emission
and Exhaust Systems
Most problems in the emission control system have
little or no noticeable effect on driveability. Emission prob-
lems sometimes only become known to the driver when
the vehicle fails a state-mandated emissions test. If the
vehicle has failed a state emissions test, try to obtain the
test readout from the driver.
Some emissions complaints are related to the condi-
tion of the exhaust gas as it leaves the tailpipe. The driver
may complain about the exhaust gas having an unusual
color or the presence of an offensive odor. Some drivers
may even complain about water exiting the tailpipe, which
is a normal part of the catalyzing operation.
It is important to remember that most problems related
to exhaust emissions are not caused by a defect in the
emission control system. Usually, a defect in the engine,
ignition, fuel, electrical, or computer control systems
causes a problem for which the emission control system
cannot completely compensate, resulting in high exhaust
emissions, Figure 18-5. Before inspecting the emissions or
exhaust system, you should eliminate these other systems
fi rst.
Most complaints related to the exhaust system are not
about vehicle performance, rather excessive or unusual
noise. Noise is usually caused by leaks or defective or mis-
aligned parts in the exhaust system. However, the exhaust
system can become restricted, which will affect vehicle
performance. Also, an exhaust leak before the oxygen sen-
sor and/or catalyst monitor will affect ECM input, possibly
creating an artifi cially lean condition.
Figure 18-4. Setup for using nitrogen to pressurize an evaporative emission control system.
Filler cap
Filler neck
Rollover valve
Fuel tank
Charcoal
canister
Engine
Nitrogen cylinder
Pressure
gauge