Evaporative Emissions Testing Subaru:

Enhanced Evaporative Emissions Test Part One

When the car is first started and you begin to drive, the pressure control solenoid is turned ON after a few seconds. At this time, the ECM is looking for some pressure change in the fuel tank to indicate that the system has been sealed and can be purged.

If the ECM sees a change in pressure, the pressure control solenoid will turn OFF in about 40 seconds, and the vehicle will pass the first part of the Enhanced Evaporative Emission System Test.

If the ECM doesn’t see any pressure change, the pressure control solenoid will remain ON for the remainder of the drive cycle.

Enhanced Evaporative Emissions Test Part Two

If the first part of the test passes, the ECM will wait until the vehicle reaches cruising speed to perform the next three steps of the test. If the vehicle is cruising over 45 mph but under 80 mph and has less than a half a tank of fuel, the next part of the test will begin.

The second part of the test is the only time when the pressure control solenoid and vent solenoid will turn on at the same time. When the pressure control solenoid turns ON, the ECM is looking for a pressure change in the tank. If it sees one, the second part of the test will pass.

The vent solenoid then turns ON to shut off the vent to outside air. Since the canister purge valve is also open at this time, the entire evaporative system is drawn to a low pressure. If the pressure in the tank can be lowered below -1.338 kPa, the third part of the test will pass and the vent solenoid shuts off to reopen the vent to outside air.

The low pressure in the tank should begin to recover toward atmospheric pressure at this time. If the pressure rise in the tank is sufficient to satisfy the DTC formula, the fourth part of the test will pass, the pressure control solenoid shuts OFF and the vehicle emissions system is judged to be functioning normally.

The graph representing fuel tank pressure for this vehicle indicates the fuel tank could not be drawn to a negative pressure. This problem was caused by a pressure control solenoid valve that was sticking open, which prevented the fuel tank from developing negative pressure.