Evaporative Emissions System: Description and Operation

Evaporative Emission Control Schematic:

PURPOSE
The basic Evaporative Emission (EVAP) control system used on all vehicles 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.


OPERATION
Gasoline vapors from the fuel tank flow into the tube labeled "tank" (See image). These vapors are absorbed into the carbon. The canister is purged by Powertrain Control Module (PCM) control when the engine is fully warmed up, Intake Air Temperature (IAT) reading is over 9.5°C (5O°F), and the engine has been running a specified amount of time. Air is drawn into the canister through the air inlet grid. The air mixes with the vapor and the mixture is drawn into the intake manifold.

The Powertrain Control Module (PCM) operates a pulse width modulated (PWM) solenoid valve which controls vacuum to the charcoal canister.

The PCM turns "ON" the solenoid valve and allows purge when:

^ Engine is warm.
^ After the engine has been running a specified time.
^ IAT is reading above 9.5° C (50° F).

RESULTS OF INCORRECT OPERATION
Poor idle, stalling and poor driveability can be caused by:

^ An improperly connected or faulty EVAP canister purge solenoid valve. This would usually be accompanied by a start that is not as fast as normal (once started, the engine does not build up speed as fast as it normally would) and a puff of black smoke at the tailpipe.

^ Damaged canister.
^ Hoses split, cracked and/or not connected to the proper tubes.

Evidence of fuel loss or fuel vapor odor can be caused by:

^ Liquid fuel leaking from front fuel lines.
^ Cracked or damaged canister.
^ Disconnected, misrouted, kinked, deteriorated or damaged hoses.
^ Air cleaner or air cleaner gasket improperly seated.