EVAP Control System Description

EVAP Control System Description

General Description

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP canister vent solenoid valve to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the ECM will command the EVAP canister purge valve ON, open, allowing engine vacuum to be applied to the EVAP canister.

With the EVAP canister vent solenoid valve OFF, open, fresh air will be drawn through the valve and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air / fuel vapor mixture continues through the EVAP canister purge pipe and EVAP canister purge valve into the intake manifold to be consumed during normal combustion.

Component Description

- EVAP canister (1):
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.

- EVAP canister purge Valve (2):
The EVAP canister purge valve controls the flow of vapors from the EVAP system to the intake manifold. This normally closed valve is pulse width modulated by the control module to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.

- EVAP canister vent solenoid valve (3):
The EVAP canister vent solenoid valve controls fresh airflow into the EVAP canister. The valve is normally open. The ECM will command the valve closed during some EVAP tests, allowing the system to be tested for leaks.

- Fuel tank pressure sensor (7):
The fuel tank pressure sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The ECM provides a 5-volt reference and a ground to the fuel tank pressure sensor. The fuel tank pressure sensor provides a signal voltage back to the control module that can vary between 0.1 - 4.9 volts. As fuel tank pressure increases, the output voltage of fuel tank pressure sensor decreases, high pressure = low voltage. As fuel tank pressure decreases, the output voltage of fuel tank pressure sensor increases, low pressure or vacuum = high voltage.