Chapter
D
: Emission Control Systems Diagnosis and Repair
81
Purge Tests
On late-model vehicles the PCM controls purging of the
stored vapor with a duty cycle purge solenoid figure 4-
14. The PCM does not allow purge during the cold start
warm-up period or during hot restart time delay. Once
the vehicle enters closed-loop operation and delay times
have elapsed, the PCM energizes the solenoid. The so-
lenoid is energized approximately five to ten times per
second, depending upon the throttle position, engine
speed and engine load. The PCM varies the pulse width
signal to the solenoid to control the quantity of vapor
drawn into the intake manifold.
When the purge is in process the resulting air-fuel ratio
enrichens. The PCM monitors HO2S signals during can-
ister purge and sets a DTC if no change is seen in the
values. Use a scan tool to check for DTC's. If present,
refer to the appropriate Service or Diagnostic Manual for
further information service procedures.
Older systems, those without PCM control, use a vacu-
um-operated purge valve to open and close the purge
line, figure 4-15. The purge valve also may shut off the
vapor vent line to the canister whenever the engine is
running. Check valve operation with a vacuum gauge
and pump.
Whether an electronic solenoid or a vacuum valve is
used, a canister that does not purge properly can cause:
• Incorrect air-fuel ratio
• An increase in exhaust emissions
• Loss of fuel economy
• Driveability complaints
EVAP SYSTEM LEAK DETECTION
Inspection
Check all EVAP system hoses for damage or deteriora-
tion and replace as needed. Replacement hose must be
Fig. 4-13.
In order to monitor catalyst operation HO
2
S sensors are mounted before and after the converter.
FILTER
FILTER
CARBON
AIR FLOW
DURING PURGE
GRID
VAPOR FROM
FUEL TANK
VAPOR PURGE
TO CARBURETOR
PURGE CONTROL
SOLENOID
ELECTRIC PURGE VALVE CANISTER
Fig. 4-14.
A PCM-controlled solenoid operates the canister purge
valve on late-model applications.
designed for fuel system use. Securely tighten all con-
nections.
Check that the vapor vent hose leading to the canister is
properly routed and positioned. Normally, the hose is
routed in a downhill position to prevent liquid gasoline
from accumulating in the hose and seeping into the can-
ister. If there is liquid in the line, the canister cannot urge
roperly. This also creates a potential fire hazard.
Summary of Contents for ASE-A8
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