Evaporative Emission Control: Notes

The Evaporative Emission (EVAP) control system fuel vapors from escaping into the atmosphere. The fuel tank vapors are allowed to move from a sealed fuel tank, due to pressure in the tank, through a 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 vent valve to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them.

At an appropriate time, the Powertrain Control Module (PCM) commands the purge valve ON, open allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent valve OFF, open, fresh air will be drawn through the vent valve and vent line to the EVAP canister. The fresh air is drawn through the canister pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the purge pipe and purge valve into the intake manifold to be consumed during the normal combustion process.

Emissions laws require that the PCM detect any leakage within the EVAP system. The PCM monitors the EVAP system for leaks by controlling and observing the vapor pressures in the EVAP system. The FTP sensor is used to detect any unexpected pressure in the EVAP system. The PCM supplies a 5-volt reference and an electrical ground to the FTP sensor. The FTP sensor contains a silicon based chip that changes electrical resistance when pressure is applied. The sensor converts the positive or the negative pressure into an electrical signal and sends the signal to the PCM. See FTP SENSOR OPERATION  table.