Chapter
D
: Emission Control Systems Diagnosis and Repair
79
PULSE-TYPE SYSTEMS
Pulse-type, or aspirator, AIR systems use the natural
low-pressure pulses in the exhaust system to draw fresh
air in through a reed-type check valve. Some systems
use a valve assembly that has a separate check valve
for each cylinder. Other designs direct fresh air into the
exhaust through a single aspirator valve.
Single Aspirator Systems
If an aspirator valve fails, noise from the exhaust system
will increase at idle. Hot exhaust gases traveling back
through the valve may cause the valve-to-air cleaner
hose to harden and crack.
Check the aspirator air system as follows:
1. Inspect hose connections for leaks.
2. Check the aspirator tube and exhaust manifold con-
nection for leaks.
3. Start the engine and stabilize the idle speed.
4. Disconnect the aspirator inlet hose.
5. Intake air pulses should be felt at the aspirator inlet,
air cleaner side of the valve. If hot exhaust gas is
felt, replace the valve.
Pulse Air Valve Systems
Diagnose a pulse air valve assembly which contains in-
dividual check valves as follows:
1. Listen for a brief hissing noise at the valve assem-
bly. A noise indicates either the connections are
loose at the manifold or the valve assembly is de-
fective. Check the connections first, then the valve
assembly.
2. Poor engine performance and surging can result if
one or more of the check valves has failed. This al-
lows exhaust gas to enter through the air cleaner.
3. Burned paint on the pulse air valve assembly, or a
charred or cracked rubber hose to the air cleaner,
indicates the valve is allowing hot exhaust gas to
flow through the valve.
4. Poor driveability may indicate that the rubber hose
has chipped inside and that some of the chips have
been drawn into the intake air charge. To repair,
clean the carburetor or throttle body, then replace
both the hose and the pulse air valve assembly.
CATALYTIC CONVERTER SYSTEMS
The harmful emissions can be processed in a threeway
catalytic converter to help ensure compliance with gov-
ernment mandated standards, figure 4-12. Like the en-
gine, the three-way catalytic converter is sensitive to the
air-fuel ratio. Maximum conversion efficiency is obtained
when the stoichiometric air-fuel ratio is supplied to the
engine.
Converter Operation
The purpose of the three-way converter is to help clean
the NO
x
, HC and CO emissions in the engine exhaust
(hence the term "three-way"). Three metals (platinum,
palladium, and rhodium) are used as
catalysts
to pro-
mote the conversion of the emissions. The process oc-
curs in two stages. In the first stage, the catalyst en-
courages NO
x
to break down into free nitrogen (N2) and
free oxygen with the oxygen portion encouraged to fur-
ther oxidize CO into harmless carbon dioxide (CO2).
In the second stage, the oxygen-enriched emissions
pass over a second catalyst that encourages further ox-
idation of HC and CO into water vapor and carbon diox-
ide. Some earlier systems used a "mini Ox" converter
upstream of the main catalyst.
Effective catalytic control of all three pollutants is possi-
ble only if the exhaust gas contains a very small amount
of oxygen.
Converter Testing
Onboard Diagnostics II (OBD II) regulations require
monitoring the functionality of the catalyst in the catalyt-
ic converter. When the catalyst system has deteriorated
to the point that vehicle emissions increase by more than
1.5 times the standard, OBD II requires malfunction in-
dicator lamp (MIL) illumination.
FREE FLOW
NO FLOW
VALVE PLATE
INLET
OUTLET
TUBE
BODY
BACKUP PLATE
SPRING (HIDDEN)
VALVE DISC
CHECK VALVE
AIR MANIFOLD
ASSEMBLY
Fig. 4-11.
Check valve operation and location.
Содержание ASE-A8
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