Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
69
Catalytic Converter Service
Catalytic converters are required by law to give at least
50,000 miles of service. However, if the car is properly
tuned and still cannot meet the emissions specifictions,
perform a vacuum test or backpressure test. These
tests indicate only that a restriction exists somewhere in
the exhaust system. Check the rest of the system be-
fore removing and replacing the converter.
Vacuum Test
Connect a vacuum gauge to a manifold vacuum source
to perform the test
To check for a restriction:
1. Run the engine at idle; vacuum should remain
steady and within specifications. If it drops toward
zero, a restriction is indicated. Continue the test.
2. Slowly increase engine speed to 2,000 RPM. Vac-
uum should remain as high as at idle.
3. Quickly close the throttle. Vacuum should rise
rapidly to above idle level. If the reading is 5 in-Hg
or more below normal, and returns slowly and un-
evenly, the exhaust system is restricted.
Backpressure Test
Use this test only on vehicles with dual-bed bead-type
converters:
1. Disconnect the hose at the secondary air manifold
and remove the check valve.
2. Use an adapter to connect a fuel pressure gauge to
the air manifold.
3. With the engine at normal operating temperature
and running at 2,500 RPM, check the backpres-
sure reading on the gauge. If it is above 1.75 psi
(12.06 kpa), the exhaust system is restricted.
For vehicles with other styles of converters, you can
purchase a special tool kit to check backpressure. The
tool requires that a small hole be drilled in the exhaust
pipe upstream from the converter. A gauge is attached
to the hole and the test is performed. When completed
a small rivet is driven in the hole to seal the system.
Temperature Test
Before testing, repair any known problems that may
cause high emissions, as well as any exhaust leaks.
Then, disable the air injection system and connect an
exhaust gas analyzer. Next, precondition the engine by
running it at 2,500 RPM for three minutes. Check the ex-
haust gas analyzer gauges. If CO exceeds 0.3 percent,
O2 exceeds 0.4 percent, and the HC falls between 120
ppm and 400 ppm, expect a faulty converter.
TURBOCHARGER SYSTEMS
A turbocharger is an exhaust driven compressor used to
increase the power output of an engine. The system re-
quires no maintenance other than more frequent engine
oil and filter changes. Turbocharger malfunctions usual-
ly fall into one of the following categories:
1. Lack of proper lubrication—which is caused by the
wrong type of oil, a restricted oil supply line, or a
worn engine that develops low oil pressure.
2. Dirty or contaminated oil system—which is caused
by infrequent oil changes, failure of an engine bear-
ing, piston ring, or other internal component, or an
oil filter bypass valve stuck open.
3. Contamination in the air intake or exhaust sys-
tems— which is caused by a leaking air duct or
missing air cleaner, or a damaged catalytic con-
verter if it is installed between the turbocharger and
the exhaust manifold.
Turbocharger Troubleshooting
Turbocharger problems are generally revealed by:
• Lack of power from the engine
• Heavy black smoke from the exhaust
• Blue smoke from the exhaust
• High engine oil consumption
• Abnormal sounds
Use the troubleshooting charts, figure 3-27, figure 3-28,
and figure 3-29, to isolate the cause of turbocharger
malfunctions.
Fig. 3-26.
Typical exhaust system components.
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