Purpose of this solenoid

WARNING: This page is about a different variant/trim than selected.

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 vehicle altitude, and the pressure signal transmitted to ECM will change.

Fig 1: Detecting Difference Between Atmospheric Air Pressure And Tank Pressure (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 the vehicle altitude change during the diagnosis will lead the atmospheric air pressure change. In this case, it becomes difficult to precisely detect the tank pressure variation, resulting erroneous diagnosis. Therefore, using the fuel tank sensor control valve, atmospheric air is sealed between the fuel tank pressure sensor and fuel tank sensor control valve, maintaining the air pressure constant and enabling the detection of minute variation of tank pressure.

Fig 2: Detecting Difference Between Atmospheric Air Pressure And Tank Pressure (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.