Evaporative Emissions System: Description and Operation

Fig 2 EVAP Canister:

EVAPORATION CONTROL SYSTEM
The evaporation control system prevents the emission of fuel tank vapors into the atmosphere. When fuel evaporates in the fuel tank, the vapors pass through vent hoses or tubes to a charcoal canister. The canister temporarily holds the vapors. Through the duty cycle EVAP purge solenoid, the PCM allows intake manifold vacuum to draw vapors from the charcoal canister into the combustion chambers during certain operating conditions. The solenoid regulates vapor flow from the canister to the engine.

The evaporative system uses specially manufactured hoses. If they need replacement, only use original equipment fuel vapor resistant hose.

EVAP CANISTER
All vehicles use a sealed, maintenance free, evaporative (EVAP) canister. Fuel tank pressure vents into the canister. The canister temporarily holds the fuel vapors until intake manifold vacuum draws them into the combustion chamber. The powertrain control module (PCM) purges the canister through the duty cycle EVAP purge solenoid. The PCM purges the canister at predetermined intervals and engine conditions.

The canister mounts to a bracket behind the front fascia on the passengers side of the vehicle (Fig. 2). The vacuum and vapor tube connect to the top of the canister.

Fig 21 Proportional Purge Solenoid:

PROPORTIONAL PURGE SOLENOID
The proportional purge solenoid regulates the rate of vapor flow from the EVAP canister to the throttle body. The PCM operates the solenoid.

During the cold start warm-up period and the hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapors are purged.

The proportional purge solenoid operates at a frequency of 200 hz and is controlled by an engine controller circuit that senses the current being applied to the proportional solenoid and then adjusts the current to achieve the desired purge flow. The proportional purge solenoid controls the purge rate of fuel vapors from the vapor canister and fuel tank to the engine intake manifold.

The purge system starts after closed llop operation begins and after it reaches a specified temperature and the programmed time delay ends. During normal operatioin, the PCM energizes and de-energizes the solenoid.
The PCM varies the vapor flow rate by changing the solenoid pulse width. Pulse width is the amount of time the solenoid is energized.

The solenoid attaches to a bracket near the front engine mount (Fig. 21). To operate correctly, the solenoid must be installed with the electrical connector on top.