Purge Valve
The Engine Control Module (ECM) will not operate the Evaporative Emissions (EVAP) purge process until the engine coolant temperature is around 60°C (140°F) and closed loop fueling is operational. Under these conditions the engine will be running smoothly with no warm up enrichment.
The purge valve duty cycle (and vapor flow) is initially increased slowly because the vapor concentration is unknown and a sudden increase in purged vapor could cause unstable engine running or cause it to stall due to an extremely 'rich' air/fuel mixture. The vapor concentration is determined from the amount of adjustment the closed loop fueling is required to achieve the target air fuel ratio. When the concentration has been determined, the flow though the purge valve can be increased rapidly and the injected fuel can be adjusted to compensate for the known purge vapor concentration and the air fuel ratio target is maintained.
When the purging process is active, fresh air is drawn into the Evaporative emissions canister via the canister vent port on ROW vehicles or via the Diagnostic Module Tank Leakage (DMTL) filter and pump on NAS vehicles.
The purge valve is located on the left side of the engine, below the ignition coils cover. The purge valve is a solenoid operated valve which is closed when de-energized.
A vapor pipe is routed from the purge valve to the inlet of the supercharger. The purge valve is controlled by the ECM and is operated when the engine operating conditions are correct to allow purging of the Evaporative emissions canister.
A vapor pipe, which is routed parallel with the fuel delivery pipe along the left underside of the vehicle, connects the purge valve to the Evaporative emissions canister.
The purge valve is operated at 10 Hz by a Pulse Width Modulation (PWM) control ground from the ECM. At this high frequency, pulses of fuel vapor flow into the intake of the supercharger in an almost continuous flow. The valve operates at a duty cycle of between 7% and 100%.
Atmospheric pressure in the Evaporative emissions canister is higher than the pressure at the supercharger inlet under all throttle settings and engine running conditions. This pressure differential allows vapor to be drawn from the Evaporative emissions canister, through the open purge valve and into the supercharger air intake.