dtc 1491 (vacuum cut valve bypass valve) diagnosis - Lyberty

Classification: Reference: Date
EC02-008 NTB02-033 March 21, 2002
DTC 1491 (VACUUM CUT VALVE BYPASS VALVE) DIAGNOSIS
APPLIED VEHICLE(S):
1998 – 2002 All Models
SERVICE INFORMATION
This bulletin is an aid to the diagnostic procedures in the Service Manual for DTC P1491.
If one of the applied vehicles has the MIL “ON” with DTC P1491 (Vacuum Cut Valve
Bypass Valve) stored in Self-Diagnostic Results, use the service procedure in this bulletin
and the appropriate Service Manual to help you diagnosis the incident, if it should occur.
SERVICE PROCEDURE
1. Confirm the Vacuum Cut Valve is installed correctly (See Figure 1).
EVAP
Canister
side port
Fuel tank
side port
When the valve is installed
correctly, this cap is on the
bottom side as shown.
Vacuum Cut Valve - BOTTOM VIEW
Figure 1
When installing the valve,
match the key-way on the
mounting bracket with the
key-way on the valve.
NOTE:
• The Vacuum Cut Valve will not work correctly and a DTC will be stored if it is
installed upside-down.
TP020097
• The Vacuum Cut Valve is a white plastic valve near the fuel tank and EVAP canister.
2. Connect CONSULT-II and turn the ignition key to “ON” (do not start the engine).
3. On CONSULT II, select ENGINE - DATA MONITOR - SELECTION FROM MENU -
EVAP SYSTEM PRESSURE SENSOR.
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4. Note the EVAP System Pressure Sensor Voltage. Confirm that it is reading in a normal
range for a key ON and engine OFF condition.
NOTE:
For 2001 models and earlier:
• Voltage in step 4 should be near 3.36 volts.
• The normal range is between 3.32 - 3.40 volts.
For 2002 models:
• Some 2002 models use a new type EVAP System Pressure Sensor with different
operational characteristics.
• Refer to the EC section of the appropriate Service Manual to determine which
type sensor your vehicle is equipped with.
• Models that use the new type EVAP System Pressure Sensor will have a voltage
reading that varies with changes in the “barometric” pressure (atmospheric air
pressure that changes with altitude and/or weather conditions).
• The usual voltage range obtained in step 4 with the new type EVAP System
Pressure Sensor is between 3.8 and 4.2 volts. Because this range will vary with
barometric pressure, it is best to compare readings with a known good vehicle.
• Some 2002 models use the earlier type EVAP System Pressure Sensor (same
as used on 2001 and earlier models). The normal range for the earlier type is
3.32 to 3.40 volts and does not vary with barometric pressure.
5. Start the engine. Observe the EVAP System Pressure Sensor voltage.
• If the value has reduced 0.04 volts or more from the reading obtained in step 4,
the EVAP Canister Purge Volume Control Solenoid Valve may be leaking.
• Refer to the EC section of the appropriate Service Manual for inspection
procedures of the EVAP Canister Purge Volume Control Solenoid Valve, DTC
P1444.
NOTE: The EVAP Canister Purge Volume Control Solenoid Valve is located in
the engine compartment.
6. Perform the DTC Confirmation Procedure for DTC P1491. Refer to the EC section of
the appropriate Service Manual for confirmation procedure steps and driving conditions.
7. If the DTC confirmation results are NG, do the following:
a. Refer to the EC section of the appropriate Service Manual to determine if the
vehicle is equipped with On-Board Refueling Vapor Recovery (ORVR).
• If the vehicle has ORVR, go to b.
• If the vehicle does NOT have ORVR, go to step 8.
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. Place a clamp on the Refueling EVAP Vapor Line near the fuel tank. The
Refueling EVAP Vapor Line is the larger hose (see figure 2). Use hose clamp
(3M P/N # 08556-1), found in Fuel Injector Cleaning Kit J-45701, or equivalent.
c. Retry DTC Confirmation Procedure
d. If the result is now OK, the Refueling Control Valve may be leaking and should
be inspected. Refer to the EC section of the appropriate Service Manual for
Refueling Control Valve inspection procedures.
8. If the DTC confirmation is still NG after step 7, or the vehicle is not equipped with
ORVR, do the following:
a. Disconnect two (2) lines from the Vacuum Cut Valve. One that goes to the fuel
tank and one that goes to the EVAP Canister (see figure 1).
b. Use a hand vacuum pump to check each of the two (2) disconnected hoses for
free flow. Make sure they are clear into the fuel tank (vent line) and into the
EVAP Canister (see figure 2).
NOTE:
Figure 2
• Figure 2 includes ORVR system. Refer to the appropriate service manual to
determine if your vehicle has ORVR or not.
• The lines between the Vacuum Cut Valve and Fuel Tank, and between the
Vacuum Cut Valve and EVAP Canister may go through a metal line. Make
sure the lines are clear between the Vacuum Cut Valve and Fuel Tank, and
between the Vacuum Cut Valve and EVAP Canister, including any metal
lines.
c. If any blockage is found, determine the cause and repair as necessary.
Blockage in either of these lines will cause a DTC.
9. If all of the above checks do not locate the source of the incident, refer to the
appropriate Service Manual for further DTC P1491 diagnosis.
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