Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

The basic Evaporative Emission Control System (EECS) used is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

The solenoid used with this canister uses Pulse Width Modulation (PWM) to control purge. The Electronic Control Module (ECM) opens and closes the solenoid to control purge. The ECM operates a Normally Closed (N/C) solenoid valve which controls the vacuum to purge the charcoal canister. Under cold engine or idle conditions, the solenoid is not energized by the ECM, which blocks vacuum to purge the canister. The ECM energizes the solenoid valve and allows purge when:

^ Engine is at normal operating temperature.
^ Engine rpm is greater than 1500 rpm.

VAPOR CANISTER

Vapor Canister And Hose Routing:

Gasoline vapors from the fuel tank flow into the tube labeled tank. Any liquid fuel goes into a reservoir in the bottom of the canister to protect the integrity of the carbon bed. These vapors are absorbed into the carbon. The canister is purged when the engine is running or commanded by ECM. Ambient air is allowed into the canister through the air tube in the top. The air mixes with the vapor and the mixture is drawn into the intake manifold.