Evaporative Emissions Canister 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 should be running smoothly with no warm up enrichment. The Evaporative emissions canister purge valve duty (and flow) is initially ramped slowly because the vapor concentration is unknown (a sudden increase in purge could cause unstable engine running or cause it to stall due to an extremely "rich" air/fuel mixture). The concentration is then determined from the amount of adjustment that the closed loop fueling is required to make to achieve the target air fuel ratio. Once the concentration has been determined, the purge flow can be increased rapidly and the injected fuel can be pro-actively adjusted to compensate for the known purge vapor and the target air control is maintained.
When the purging process is active, fresh air is drawn into the Evaporative emissions canister via the Diagnostic Module Tank Leakage (DMTL) filter and pump on NAS vehicles, or via the vent port on the Evaporative emissions canister of non NAS vehicles.
The Evaporative emissions canister purge valve is located on the left side of the engine, below the ignition coils cover. A vapor pipe is routed from the Evaporative emissions canister purge valve to the inlet of the intake manifold (naturally aspirated vehicles) or the supercharger (supercharged vehicles). The Evaporative emissions canister purge valve is controlled by the ECM and is operated when engine operating conditions are correct to allow purging of the Evaporative emissions canister. The Evaporative emissions canister purge valve is a solenoid operated valve which is closed when de-energized.
A vapor pipe, which runs parallel with the fuel delivery line under the left side of the vehicle, connects the Evaporative emissions canister purge valve to the Evaporative emissions canister.
The Evaporative emissions canister purge valve is operated at 10 Hz by a Pulse Width Modulation (PWM) signal from the ECM. At this high frequency, the pulses of fuel vapor flow into the intake manifold/supercharger in an almost continuous flow. The valve operates between 7% and 100% duty or mark space ratio (percentage open time).
Atmospheric pressure is higher than the pressure at the inlet of the intake manifold/supercharger under all throttle settings and engine running conditions. It is this pressure differential that causes air to flow through the EVAP system to the engine.