378 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Caution: Do not road test a vehicle with a
restricted exhaust. Road testing a vehicle
with a restricted exhaust component could
result in severe engine damage, fi re, or explosion.
A restricted exhaust will show the symptom of a
lack of engine power. If an exhaust restriction is
suspected, perform a backpressure test before any
road test.
Excessive exhaust noise will usually show up on
engine start-up, making an extensive road test unnecessary.
If the exhaust system sounds okay, bring the vehicle into
the shop for visual inspection. As you learned in earlier
chapters, you should locate the appropriate service infor-
mation before starting the visual inspection.
Avoid road testing the vehicle at this time. If a prob-
lem with the emissions system can be found by visual
inspection, you do not want the engine heated to operating
temperature. Some parts of the emissions control system,
such as air-injection pipes, can get extremely hot. These
parts must be allowed to cool down before they can be
removed.
Visual Inspection
Inspection and testing of the fuel, ignition, electrical,
and computer control systems should reveal signs that the
problem is in either the emission control or exhaust system.
You may also be checking the emission and exhaust sys-
tems as part of follow-up for a repair performed on one of
the other engine systems.
When visually inspecting the emission control system,
check for evidence of tampering. This includes making
sure that all emission-control devices are installed in their
proper places and in good condition. Make sure that all
electronic connectors and hoses are connected. Inspect the
exhaust manifolds for cracks and leaks around the manifold
and its gaskets. Leaks are usually indicated by a line of dis-
coloration on the manifold.
Raise the vehicle on a lift to inspect the exhaust system.
Make sure that all heat shields and hangers are in place
and tight. There should be plenty of clearance between
the exhaust system and the vehicle components and heat
shields. Inspect the system itself for damage from debris
and corrosion. There should be no openings in the pipes,
muffl ers, resonators, or catalytic converters. Tap on the
Figure 18-5. These are the most common problems that occur in the emission control and exhaust systems. However, most emis-
sions problems are caused by a malfunction in an engine-related system.
Emission Control System
Component or System Problem
PCV system Hose clogged or disconnected, valve stuck, wrong valve
Thermostatic air cleaner Vacuum line disconnected; misadjusted, stuck, or defective part
Air pump Belt loose or disconnected, defective pump or valves, hoses clogged or disconnected
Spark timing controls Disconnected, defective
EGR valve
Disconnected, stuck, delay valve defective or removed, position sensor defective or
disconnected
Evaporative emissions
control system
Filter or canister clogged, hoses disconnected
Emissions fi lter Filter clogged or missing
Computer control system
Disconnected or defective sensors or output devices, defective or incorrect ECM or
replacement memory
Exhaust System
Component Problem
Exhaust manifolds Cracked, loose bolts or oxygen sensor, leaking or missing gaskets
Exhaust pipes Bent, leaking, collapsed, corroded
Muffl er Clogged, leaking
Catalytic converter Clogged, removed, hollowed out, contaminated, worn
Resonator Clogged, leaking
Tailpipe Bent, collapsed, plugged