Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

SYSTEM DESCRIPTION
The Evaporative Emission Control System (EECS) is used to prevent raw fuel vapors from escaping into the atmosphere from the fuel tank. The system vents fuel vapors from the tank and directs them to an activated charcoal storage canister, where the vapors are condensed and stored. This retains the fuel 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.

Vapor Canister:

EVAP Canister And Pressure Control Valve Location:

VAPOR CANISTER
The vapor canister is filled with "activated" charcoal, and has a one way diaphragm/air vent that allows air into the canister to mix with the stored fuel when the canister is being purged. Fuel vapors enter the canister through a port on the bottom of the canister. Fuel and air exit the canister through another port, also located on the bottom of the canister.


EVAPORATIVE CONTROL - PURGE SOLENOID
A computer controlled vacuum solenoid valve regulates purging of the vapor canister. The valve is normally closed and opens when the ECM energizes it, to purge fuel vapors from the canister. The solenoid gets battery power when the ignition is turned "ON" and becomes energized when the ECM completes the circuit to ground.

OPERATION
The ECM will energize the purge solenoid under the following conditions:
- Coolant temperature greater than 65°C (149°F)
- Vehicle speed greater than 10 mph.
- Throttle position sensor indicates throttle opening greater than 3.5%.
- System is not in deceleration fuel cut-off mode.
- Fuel system is operating in "closed loop".

NOTE: For testing, the ECM will also energize the purge solenoid when the diagnostic test terminal of the ALDL connector is grounded.

Fuel Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE
The fuel tank pressure control valve, located under the right rear of the car, is a biassed spring diaphragm valve, normally closed. The valve regulates vapor pressure in the fuel. When the tank pressure excedes about 5 kPa (0.7 psi), the valve opens venting fuel vapors to the canister. Vapors are vented through a small restriction so that pressure escapes the tank side of the valve slowly, preventing the valve from buzzing. When fuel tank pressure drops below 5 kPa (0.7 psi) gauge pressure, the valve closes, retaining fuel vapors in the tank.