Evaporative Emission Control (EEC) Systems

The 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.
Removal of the vapors from the canister is accomplished by a solenoid operated bowl vent or vacuum operated purge valve mounted on the canister. In addition to the fuel system vents and canister, the fuel tank requires a non-vented gas cap. The domed fuel tank positions a vent high enough above the fuel to keep the vent pipe in the vapor at all times. The single vent pipe is routed directly to the canister.
These systems commonly use an in-line EVAP pressure control valve as a pressure relief valve. When vapor pressure in the tank exceeds approximately 0.7 psi, the diaphragm valve opens, allowing vapors to vent to the canister. Once the in tank pressure drops below 0.7 psi, the valve closes causing vapors to be help in the tank.
If the EVAP system is not functioning properly, any one of the following conditions may be as result:

1. Poor idle, stalling, and poor driveability can be caused by:
   • Inoperative purge solenoid valve
   • Damaged canister
   • Hoses split, cracked or not connected to the proper tubes
2. Evidence of fuel loss or fuel vapor odor can be caused by:
   • Liquid fuel leaking from the fuel lines
   • Cracked or damaged canister
   • Inoperative canister control valve
   • Disconnected, misrouted, kinked, deteriorated or damaged vapor hoses or control hoses.

If the solenoid valve is open, or is not receiving power, the canister can purge to the intake manifold at the incorrect time. This can allow extra fuel during warm-up, which can cause rough or unstable idle.

Operation

This system reduces the amount of gasoline vapors escaping into the atmosphere. Some models employ a purge control solenoid which is controlled by the ECM, to open and close the EEC system. Other models use a canister mounted vacuum purge valve; when the engine vacuum reaches a certain pressure, the valve opens allowing the gas vapors to be drawn off to the carburetor for burning.
Carbureted models use an exhaust tube from the float bowl to the charcoal canister; fuel injected models eliminate the fuel bowl tube (as no float bowl is used on fuel injection systems). Fuel vapors from the gas tank travel from the tank to the vapor canister, where they are collected. Although the system varies from vehicle-to-vehicle, the operations are basically the same.

Testing Tank Pressure Control Valve

1. Using a hand-held vacuum pump, apply a vacuum of 15 in. Hg. (51 kPa) through the control vacuum signal tube to the purge valve diaphragm. If the diaphragm does not hold 5 in. Hg. at least for 10 seconds, the diaphragm is leaking. Replace the control valve.
2. With the vacuum still applied to the control vacuum tube, attach a short piece of hose to the valve's tank tube side and blow into the hose. Air should pass through the valve. If it does not, replace the control valve.