Evaporative Emissions System: Description and Operation

EVAP Canister And Leak Detection Pump:

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.

LEAK DETECTION PUMP
The leak detection pump is a device used to detect a leak in the evaporative system.

The pump contains a 3 port solenoid, a pump that contains a switch, a spring loaded canister vent valve seal, 2 check valves and a spring/diaphragm.

Immediately after a cold start, when the engine temperature is between 40°F and 86°F, the 3 port solenoid is briefly energized. This initializes the pump by drawing air into the pump cavity and also closes the vent seal. During non-test test conditions, the vent seal is held open by the pump diaphragm assembly which pushes it open at the full travel position. The vent seal will remain closed while the pump is cycling. This is due to the operation of the 3 port solenoid which prevents the diaphragm assembly from reaching full travel. After the brief initialization period, the solenoid is de-energized, allowing atmospheric pressure to enter the pump cavity. This permits the spring to drive the diaphragm which forces air out of the pump cavity and into the vent system. When the solenoid is energized and de-energized, the cycle is repeated creating flow in typical diaphragm pump fashion. The pump is controlled in 2 modes:

PUMP MODE:
The pump is cycled at a fixed rate to achieve a rapid pressure build in order to shorten the overall test time.

TEST MODE:
The solenoid is energized with a fixed duration pulse. Subsequent fixed pulses occur when the diaphragm reaches the switch closure point.

The spring in the pump is set so that the system will achieve an equalized pressure of about 7.5 inches of water.

When the pump starts, the cycle rate is quite high. As the system becomes pressurized, pump rate drops. If there is no leak, the pump will quit. If there is a leak, the test is terminated at the end of the test mode.

If there is no leak, the purge monitor is run. If the cycle rate increases due to the flow through the purge system, the test is passed and the diagnostic is complete.

The canister vent valve will unseal the system after completion of the test sequence as the pump diaphragm assembly moves to the full travel position.