Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 379
Copyright by Goodheart-Willcox Co., Inc.
muffl ers, resonators, and converters to check for internal
damage. If the exhaust system should have a catalyst moni-
tor, make sure it is present and the connector is tight and
properly routed.
Make sure that all exhaust components are in place.
Add-on components, such as chrome exhaust tips, are
legal in most areas so long as the original emissions and
exhaust components remain in place and are not bypassed
or defeated. Some states permit aftermarket exhaust system
components that enhance performance. However, these
components must allow all original-equipment emissions-
control devices to be connected and must meet or exceed
the emissions performance standards for the vehicle.
Many emission testing programs include a visual
inspection for the presence of a catalytic converter. A
common form of exhaust tampering involves remov-
ing the catalytic converter from the exhaust system and
replacing it with a standard exhaust pipe. Sometimes, the
converter is removed, hollowed out, and the shell rein-
stalled. These illegal procedures are done to gain better
performance by reducing exhaust system backpressure. If
the converter rattles or sounds hollow when you tap on it
or testing shows an unexplained excessively rich mixture,
lower-than-normal backpressure, or excessive emission
levels, the catalyst element is either damaged or may have
been removed.
Checking for Excessive Backpressure
While it is normal for all engines to have some back-
pressure, excessive backpressure can cause many drive-
ability problems:
Loss of power.
Excessive fuel consumption.
Overheating.
Severe engine damage if left uncorrected.
Exhaust backpressure can be checked in one of two ways.
One method uses a vacuum gauge and the other uses an
exhaust backpressure gauge.
Vacuum Gauge
The vacuum gauge is an established device for deter-
mining the internal condition of the engine. With the engine
running, the vacuum gauge reading determines the overall
condition of the engine and may be useful in determining
the condition of individual cylinders. The vacuum gauge
can also be used to check for a restricted exhaust system.
Attach the vacuum gauge to the intake manifold. Also
attach a tachometer. Then, start the engine. Record the vac-
uum at idle. Put the transmission/transaxle in park or neutral,
set the parking brake, and raise the engine speed to 2500
rpm. Allow engine speed to stabilize and read the vacuum
gauge. The reading should be steady and slightly higher
than it was at idle, Figure 18-6. If the reading is unchanged
or lower, the exhaust system is probably restricted.
Exhaust Backpressure Gauge
Using a backpressure gauge is more accurate for
checking exhaust backpressure than using a vacuum gauge.
Some backpressure gauges are stand-alone tools. However,
many standard pressure gauges have available adapters for
checking exhaust backpressure.
Some backpressure gauges are designed to thread into
the oxygen sensor port. See Figure 18-7A. On vehicles with
multiple oxygen sensors, use the sensor nearest to the exhaust
manifold. Remove the oxygen sensor and install the adapter.
Use anti-seize compound on the gauge adapter fi tting. Some
vehicles have air-injection tubing. The gauge can be installed
in this tubing after removing the check valve, Figure 18-7B.
Start the engine and note the reading on the backpres-
sure gauge. The reading should not exceed .5–1.25 psi
(3.4–8.6 kPa). Increase engine speed to 2500 rpm. The gauge
reading should not exceed 2–4 psi (13.8–20.7 kPa). If exhaust
backpressure is excessive, there is a restriction in the exhaust
system. Inspect the exhaust pipes, resonators, muffl ers, and
catalytic converters for a restriction. If backpressure is normal,
remove the gauge adapter, coat the oxygen sensor threads
with sealant or anti-seize compound, and reinstall it.
Exhaust Gas Analysis
As stated earlier, an emissions problem is sometimes
brought to the driver’s attention when the vehicle fails a
state-mandated air-quality (emissions) test. Since most com-
ponents in the emission control system are not replaced as
part of routine maintenance, most drivers do not notice the
function of this system until there is a problem.
If the exhaust system has normal backpressure and
your visual inspection revealed no problems with any of
the parts in the emission control or exhaust system, thor-
oughly road test the vehicle. This is to bring the engine and
the exhaust system up to the proper operating temperature
so an exhaust gas analysis test can be performed.
Figure 18-6. A vacuum gauge can indicate the internal condi-
tion of the engine. The exhaust is restricted if the gauge shows
normal vacuum at idle, but drops several inches below the
idle reading when the engine speed is raised to 2500 rpm. A
severely restricted exhaust may cause the vacuum reading to
drop to almost zero.
Normal vacuum at idle
Restricted exhaust
(compare at idle
and 2500 rpm)