Component Description

Pressure Control Solenoid Valve
PCV controls the fuel tank pressure to be equal to the atmospheric air pressure.

Normally, the solenoid is set to OFF. And the valve opens and closes mechanically in accordance with the pressure difference between tank and atmospheric air, or tank and canister.

The valve is forcibly opened by setting the solenoid to ON at the time of diagnosis.

Fig 1: Identifying Pressure Control Solenoid Valve Diagram
Courtesy of SUBARU OF AMERICA, INC.
Valve Operation and Air Flow
In the figure below, divided by the diaphragm, the part above X is charged with atmospheric air pressure, and the part below X is charged with tank pressure. Also, the part above Y is charged with tank pressure, and the part below Y is charged with canister pressure.

If the atmospheric air pressure port is A, tank pressure port is B, and canister pressure port is C, the air flows according to pressure difference from each port as shown in the table below.

Fig 2: Identifying Valve Operation & Air Flow Diagram
Courtesy of SUBARU OF AMERICA, INC.

Fig 3: Flow Chart
Courtesy of SUBARU OF AMERICA, INC.
When A < B (Solenoid OFF)

Fig 4: Identifying Solenoid Off Diagram (1 Of 2)
Courtesy of SUBARU OF AMERICA, INC.
When B < C (Solenoid OFF)

Fig 5: Identifying Solenoid Off Diagram (2 Of 2)
Courtesy of SUBARU OF AMERICA, INC.
When Solenoid is ON

Fig 6: Identifying Solenoid Is On Diagram
Courtesy of SUBARU OF AMERICA, INC.
CCV
CCV controls the ambient air to be introduced to the canister.

Fig 7: Identifying CCV Controls
Courtesy of SUBARU OF AMERICA, INC.
Tank Pressure Switching Solenoid
One of the atmospheric pressure switching solenoid valves is connected to fuel tank pressure sensor and the other is released to atmosphere. The passage to fuel tank pressure sensor is usually released to atmosphere because the solenoid is set to OFF, but the solenoid is set to ON at diagnosis and the passage open to atmosphere can be closed.

Fig 8: Tank Pressure Switching Solenoid
Courtesy of SUBARU OF AMERICA, INC.
Purpose of this solenoid
Fuel tank pressure sensor detects the difference between the atmospheric air pressure and the tank pressure and the ECM monitors the pressure difference.

Even if the tank pressure is constant, the atmospheric air pressure varies depending on the driving height, and the pressure signal transmitted to ECM will change.

Fig 9: Identifying Fuel Tank Pressure Sensor (1 Of 2)

Courtesy of SUBARU OF AMERICA, INC.

Especially, in the small leakage [0.5 mm (0.02 in)], minute change in the tank pressure has to be detected. This diagnosis period is long (approx. 29 seconds). And if the driving height changes during the diagnosis, the atmospheric air pressure changes. In this case, it becomes difficult to precisely detect the tank pressure variation, causing erroneous diagnosis. Therefore, using the atmospheric pressure switching solenoid, atmospheric air is sealed between the fuel tank pressure sensor and atmospheric pressure switching solenoid, maintaining the air pressure constant and enabling the detection of minute variation of tank pressure.

Fig 10: Identifying Fuel Tank Pressure Sensor (2 Of 2)

Courtesy of SUBARU OF AMERICA, INC.

NOTE: ECM also has the atmospheric air pressure sensor, and always monitors atmospheric air. However, as the monitoring range is large, that is, 53 to 107 kPa (400 to 800 mmHg, 16 to 32 inHg) it is not suitable for detection of minute pressure variation.

In the case of small leakage diagnosis, the tank pressure variation is very small, that is, 0.13 to 0.27 kPa (1 to 2 mmHg, 0.04 to 0.08 inHg) and the fuel tank pressure sensor is equipped.