Evaporative Emissions System: Description and Operation

Fig. 1 EVAP Canister and EVAP Purge Solenoid:

NOTE: The evaporative system uses specially manufactured hoses. If they need replacement, only use fuel resistant hose.

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 filled evaporative canister. The canister temporarily holds the vapors. The Powertrain Control Module (PCM) allows intake manifold vacuum to draw vapors into the combustion chambers during certain operating conditions.

All engines use a duty cycle purge system. The PCM controls vapor flow by operating the duty cycle EVAP purge solenoid. Refer to EVAP Purge Solenoid.

EVAP CANISTER
All vehicles use a sealed, maintenance free, charcoal filled EVAP canister. The canister mounts to the radiator support behind the front fascia (Fig. 1).

Fuel tank pressure vents into the canister. The canister temporarily holds the fuel vapors until intake manifold vacuum draws them into the induction system. The PCM regulates vapor flow by switching the EVAP purge solenoid On and Off. The EVAP purge solenoid supplies the vacuum signal to the EVAP canister. The PCM purges the canister at predetermined intervals and engine conditions.

DUTY CYCLE EVAP PURGE SOLENOID
The duty cycle EVAP purge solenoid regulates the rate of vapor flow from the EVAP canister to the throttle body. The Powertrain Control Module (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.

When purging the PCM energizes and de-energizes the solenoid approximately 5 or 10 times per second, depending upon operating conditions. The PCM varies the vapor flow rate by changing solenoid pulse width. Pulse width is the amount of time the solenoid energizes.

Fig. 2 Pressure Vacuum Filler Cap:

PRESSURE-VACUUM FUEL FILLER TUBE CAP

CAUTION: Remove the fuel filler tube cap to relieve fuel tank pressure before servicing the fuel or emission systems.

The fuel filler tube cap seals the fuel tank (Fig. 2) when threaded onto the filler neck. For safety, the cap contains pressure-vacuum relief valves. A malfunction in the system or damage to the fuel lines could cause excessive fuel tank pressure. The relief valves prevent possible excessive pressure or vacuum build-up in the fuel tank.

Removing the cap breaks the seal, relieving fuel tank pressure.

If the filler cap needs replacement, only use a similar unit.