ECM Purge Control

The engine management ECM controls the output signals to the purge valve and the canister vent solenoid (CVS) valve, and receives an input from the fuel tank pressure sensor. The system will not work properly if there is a leakage or clogging within the system, or if the purge valve cannot be controlled.

When the engine is running, the ECM decides when conditions are correct for the vapor to be purged from the canister and opens the canister purge valve. This connects a manifold vacuum line to the canister and fuel vapor containing the hydrocarbons is drawn from the canister's charcoal element to be burned in the engine. Clean air is drawn into the canister through the air inlet port to fill the displaced volume of vapor.

The purge valve remains closed below preset coolant and engine speed values to protect the engine tune and catalytic converter performance. If the charcoal canister was purged during cold running or at idling speed the additional enrichment in the fuel mixture would delay the catalytic converter light off time and cause erratic idle. When the purge valve is opened, fuel vapor from the charcoal canister is drawn into the plenum chamber downside of the throttle housing, to be delivered to the combustion chambers for burning.

The purge valve is opened and closed in accordance with a PWM signal supplied from the ECM. Possible failure modes associated with the purge valve failure are listed below:
- Valve drive open circuit
- Short circuit to vehicle supply or ground
- Purge valve or pipework blocked or restricted
- Purge valve stuck open
- Pipework joints leaking or disconnected.

Possible symptoms associated with purge valve or associated pipework failure is listed below:
- Engine may stall on return to idle if purge valve is stuck open
- Poor idling quality if the purge valve is stuck open
- Fuelling adaptation forced excessively lean if the charcoal canister is clear and the purge valve is stuck open.
- Fuelling adaptation forced excessively rich if the charcoal canister is saturated and the purge valve is stuck open.
- Saturation of the charcoal canister if the purge valve is stuck closed.

To maintain driveability and effective emission control, purging control must be closely controlled by the ECM, as a 1% concentration of fuel vapor from the charcoal canister in the air intake may shift the air:fuel ratio by as much as 20%. The ECM must purge the fuel vapor from the charcoal canister at regular intervals as its storage capacity is limited and an excessive build-up of fuel pressure in the system could increase the likelihood of vapor leaks. Canister purging is cycled with the fuelling adaption as both cannot be active at the same time. The ECM alters the PWM signal to the purge valve to control the rate of purging of the canister to maintain the optimum stoichiometric air:fuel mixture for the engine.

See EMISSION CONTROL, Description and operation.