374 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
This chapter discusses procedures used by state emis-
sions-testing programs. It also discusses strategies for diag-
nosing and repairing problems in the emission control and
exhaust systems. While most problems of excessive exhaust
emissions are caused by other engine systems, it is impor-
tant to know how to properly diagnose the emission control
and exhaust systems. The diagnostic procedures discussed
in this chapter concentrate primarily on components in the
emission control system. Refer to Chapter 10 if you need to
review operation of the emission control system.
State Emissions Tests
Emissions tests are performed in various states to com-
ply with federal emissions laws and ensure vehicles do not
exceed acceptable emissions levels. Automotive techni-
cians will be involved in emissions testing, either as part of
state-mandated emissions-testing programs or by diagnos-
ing and repairing vehicles to correct problems in a vehicle
that failed an emissions test. Therefore, the technician must
know how to perform emissions tests, prepare vehicles for
emissions testing, and interpret test results. The test proce-
dures given here can be applied to all testing programs with
minor modifi cations.
Types of Emissions Tests
All emissions tests can be roughly divided into two
main categories:
Enhanced emissions tests.
Static emissions tests.
The most elaborate test is the enhanced emissions test,
sometimes called an IM 240 test or transient test. During
this type of test, the vehicle is operated on a dynamometer
at different speeds and loads to simulate engine and drive
train operation during actual driving. As the vehicle is oper-
ated on the dynamometer, emissions levels are measured.
Enhanced emissions tests often include a check of the
evaporative emissions controls.
During a static emissions test, sometimes called the
basic test or idle-only test, tailpipe emissions are measured
as the engine idles. The vehicle is not operated on a dyna-
mometer during a static emissions test. Some states use
a static test that includes operation at an off-idle engine
speed, often 2500 rpm. This is sometimes called a two-step
idle test. Vehicles are not tested under load in any static
test. In many states, static test procedures include a check
of the vehicle’s gas cap. Checking the gas cap is a simple
test of the most failure-prone part of the evaporative emis-
sions system.
Equipment Used to Perform
Emissions Tests
Various equipment is used to perform vehicle emis-
sions tests, including dynamometers (sometimes referred
to as treadmills), engine speed testers, and automotive
analyzers. If a drive cycle (discussed later in this chapter)
is performed as part of an enhanced emissions test, a scan
tool is also needed. If testing of the evaporative emissions
system is required, a pressure-testing device is needed.
Emissions-testing facilities use computers to perform
the test procedures, record data, and provide target emis-
sions levels. The target emissions levels are determined
when the technician enters the vehicle identifi cation num-
ber (VIN). The computers in a test facility may be attached
to a central computer located at the headquarters of the
state emission inspections authority.
Emissions Test Procedures
Emissions test procedures vary greatly between states
and sometimes between different areas of the same state.
Therefore, the following information should be used as a
general guide to emissions-testing procedures. Consult your
local emissions inspections authority for exact procedures.
Preliminary Preparations
Some preparation is needed before the vehicle can
be tested. Enter the vehicle data (usually the VIN) into the
computer. The computer will use the data to determine the
emissions requirements for the specifi c year and type of
vehicle being tested. Next, start the engine and let it reach
normal operating temperature. Cold or overheated engines
will give false emissions readings.
As the engine warms up, look for obvious problems,
such as excessive exhaust smoke or misfi res. In many states,
a visibly smoking engine will not be tested. Check the gen-
eral engine operation and note whether the MIL is on. If
the MIL is on, the vehicle obviously has a problem and
should not be tested. Also, check for missing, damaged, or
disconnected emission-control devices or other parts that
can affect engine operation. If required, perform the drive
cycle, as discussed later in this chapter.
If an enhanced emissions test is to be performed, make
sure the vehicle can be operated on a dynamometer. Look
for a leaking cooling system, inoperative cooling fans,
worn or defective tires, and low engine oil or transmission/
transaxle fl uid. These defects could cause severe engine
or vehicle damage if the enhanced test is attempted. Also,
check the condition of the fi ller neck, verify for the proper
gas cap is in place, and make sure the fi ller neck restric-
tion is in place. Turn off the air conditioner and any other
accessory that will put an extra load on the engine during
the test.