Chevrolet TrailBlazer, Service Manual, Page: 204 / 677

Emission Control Systems
In order to help reduce emissions and improve fuel economy in the 1980s, manufacturers
started equipping vehicles with three-way catalysts and electronic fuel metering systems.
By 1988, the California Air Resources Board (CARB) began regulation of the OBD
systems sold in California. These OBD systems were designed to monitor fuel, ignition,
and emissions system components to determine if they were operating correctly. When a
system was found to be operating out of specification, a fault code was stored in the
Engine Control Module (ECM). In some cases, a "check engine light" would illuminate.
Technicians could connect to the ECM through a Data Link Connector (DLC) and
download fault codes.
The new emission control systems were a significant departure from traditional engine
systems. Instead of using mechanical systems to control key engine components, such as
the carburetor and ignition system, these components are controlled by the on-board
computer known as the ECM. Control of emissions is now geared toward the precise
tuning of combustion for each set of operating conditions as determined by the input from
specific sensors. This minimizes engine emissions while improving performance.
Three-way catalysts are effective in simultaneously reducing emissions of HC, CO and
NO
X
. Proper operation of a three-way catalyst requires precise control of the fuel metering
system. If there's too much air, the converter will not reduce NO
X
emissions. If there's too
much fuel, the converter will not reduce HC and CO emissions.
To achieve this precise control, electronic fuel metering systems began incorporating
oxygen sensors in the exhaust to provide feedback to the ECM on whether the air/fuel
mixture was rich (too much fuel) or lean (too much air). When these electronic fuel
metering systems read the input from the oxygen sensor, they are said to be running in
"closed loop". "Open loop" describes the mode of operation when these electronic fuel
metering systems disregard the oxygen sensor signal.
A closed loop fuel control system precisely controls the air/fuel mixture. The vehicle's ECM
maintains the air/fuel mixture at the optimum conditions for minimizing emissions, while
maximizing performance.
The fuel system and catalytic converter must have the proper balance of air and fuel in
order to maintain low emissions. The stoichiometric 14.7:1 air/fuel ratio is the proper
reference point in which catalyst efficiency is greatest in uniformly reducing all emissions.
The carbon monoxide emissions will be lower at a fuel mixture leaner than 14.7:1, but a
sacrifice is made with an increase in hydrocarbons and oxides of nitrogen.
The fuel program that the engine uses is based on an air/fuel ratio of 14.7:1 for optimum
catalyst efficiency. This balance is difficult to maintain under normal circumstances
because of the changing variables such as RPM and engine load. To overcome the
difficulties of maintaining balance, the fuel management system forces the system rich for
approximately 300 milliseconds and then forces the system lean for the same amount of
time. If the system stayed rich longer then it stayed lean, the system is correcting for a
lean condition and is still considered to be in "closed loop" fuel control. The carbureted fuel
control systems of this era were only capable of making approximately 10 changes in a
second.