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Chapter
D
EMISSION CONTROL SYSTEMS DIAGNOSIS
AND REPAIR
Reducing the exhaust emissions of a modern internal-
combustion engine to an acceptable legal level is a com-
plex task. A number of engine subsystems are used to
break down and reduce exhaust pollutants. These may
include:
• Positive crankcase ventilation
• Idle speed control
• Exhaust gas recirculation
• Exhaust gas treatment
• Catalytic converter
• Inlet air temperature control
• Intake manifold temperature control
• Fuel vapor control
Emission control system use and design vary by engine,
manufacturer, and model year. Not every vehicle uses all
of the subsystems listed above. However, all of the sub-
systems that are used on a particular engine are de-
signed to interact with each other to lower exhaust emis-
sions. Most emission control devices are electronically
regulated by the engine management system. Before at-
tempting to diagnose what appears to be an emission
control problem, make sure the engine is in sound me-
chanical condition and good running order. Isolating a
faulty component is a process of elimination that may in-
clude electronic and dynamic troubleshooting.
POSITIVE CRANKCASE VENTILATION
(PCV) SYSTEMS
A flow restriction in the PCV system can result in drive-
ability problems. Improper venting, or scavenging, of the
crankcase can result in oil dilution and sludge formation.
In extreme cases, oil collects in the air cleaner and even-
tually clogs the filter element. Symptoms of a clogged or
damaged PCV system include:
• Increased oil consumption
• Diluted or contaminated oil
• Escaping blowby vapors
• Unstable engine operation at idle and low speed
PCV service includes inspection, testing, and compo-
nent replacement. There are no adjustments possible.
System Inspection
Conduct a thorough visual examination. Be sure to:
1. Inspect hoses for proper routing and tight connec-
tions. Also, check hoses for signs of cracking, de-
formation, and clogging.
2. Check the air filter element for signs of excessive
blowby deposits.
3. Check the crankcase inlet air filter, if equipped, for
blockage.
4. Check for deposits that may restrict passages in the
intake manifold.
5. Check the PCV valve for deposits or clogging.
The plunger inside a PCV valve must move freely to
open and close the vent port, figure 4-1. A quick way to
check a typical PCV valve is to shake it and listen. Ex-
pect to hear the plunger move inside the valve. A valve
that does not rattle is probably clogged and should be re-
placed. A good valve can be cleaned with carburetor
cleaner spray. Drain the cleaner from the valve and let it
air dry.
System Testing
Use the following method to test the PCV system:
1. Connect a tachometer and start the engine.
2. Disconnect the PCV valve and the vent line from the
crankcase. Engine speed should increase slightly.
THIS END OF THE PCV
VALVE IS SUBJECT TO
CRANKCASE PRESSURE
THAT TENDS TO CLOSE
THE VALVE
THIS END IS SUBJECT
TO INTAKE MANIFOLD
VACUUM THAT TENDS
TO CLOSE THE VALVE
THE SPRING FORCE OPERATES TO
OPEN THE VALVE TO MANIFOLD
VACUUM AND CRANKCASE PRESSURE
Fig. 4-1.
Cross section of a typical PCV valve.
Summary of Contents for ASE-A8
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