392 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Checking Exhaust System Parts
The most common driveability problem caused by the
exhaust system is lack of power or low top speed caused
by restricted exhaust components. If the exhaust system is
restricted, much of the power developed by the engine is
used to force the exhaust gases past the restriction instead
of propelling the vehicle.
If you suspect a restricted exhaust, start by checking
the manifold vacuum or exhaust backpressure at low and
high engine speeds. This is covered earlier in the chapter.
In most cases, problems with the exhaust system
are related to exhaust gas leaks caused by corrosion of
components. Figure 18-30 shows a muffl er that has been
severely damaged by corrosion. Corrosion usually starts at
the exhaust component furthest from the engine, where the
exhaust system is cooler. As the exhaust gas cools, the water
and acids in the exhaust gas have a tendency to condense.
If the vacuum or backpressure test establishes that the
exhaust system is restricted, the problem can be isolated
by removing exhaust system components and searching for
the source of the restriction. As a quick check, an exhaust
system joint can be loosened ahead of the suspected pipe
and the vehicle test driven a short distance. If the vehicle
performs normally, the exhaust system is restricted some-
where past the loosened joint. Continue to loosen joints
until you isolate the part.
Note: Loosen the exhaust pipe connected
to the catalytic converter inlet fi rst. A
restriction is more likely in the catalytic
converter or downstream of it than it is upstream of
the converter.
Several parts of the exhaust system can cause restric-
tions to exhaust gas fl ow, Figure 18-31. While all parts of
the exhaust system can cause a restriction, the catalytic
converter is the most frequent cause of problems with
exhaust fl ow. After long-term use, the active elements in the
catalytic converter can become coated with deposits. The
converter stops working, cools off, and becomes plugged.
Usually, a converter that quits working can be identifi ed
Figure 18-28. Point the infrared thermometer at the inlet
pipe just ahead of the catalytic converter. Be sure to aim the
thermometer at the pipe and not any shields surrounding the
catalytic converter
Target
Figure 18-29. Point the infrared thermometer at the outlet pipe
just beyond the exit of the converter shell.
Target
Figure 18-30. Corrosion is the exhaust system’s worst enemy.
Corroded exhaust parts will change the exhaust gas content at
the tailpipe, which will alter any readings by an exhaust gas ana-
lyzer. On some vehicles, a leak before the catalytic converter
can change the air-fuel ratio.