Fuel Tank Vacuum Sensing

Fuel Tank Vacuum Sensing
The evaporative system as defined by federal regulation includes the following components:

^ fuel tank
^ evaporative emission canister vent solenoid
^ fuel tank vacuum/pressure sensor
^ fuel pipes and hoses
^ vapor lines
^ fuel cap
^ evaporative emission canister
^ purge lines
^ purge valve solenoid

The evaporative diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay. At an appropriate time, engine vacuum is used to draw a small vacuum on the entire evaporative emission system by mechanically sealing off the designed vent path. After the calibrated vacuum has been achieved, the purge source is turned off and the system is sealed. A leak is detected by monitoring for a change in vacuum level over a given time period, all other variables remaining constant.