Fuel Evaporation System: Notes

NOTE: Not all components listed are used on every vehicle. To determine components usage on a specific model. See article.

Fuel evaporation system prevents escape of raw fuel vapor to atmosphere. System components include fuel tank, EVAP canister, Canister Close Valve (CCV), Leak Detection Pump (LDP with Heater), Purge Control Solenoid Valve (PCSV), Fuel Tank Pressure Sensor (FTPS), rollover valve, On-board Refueling Vapor Recovery (ORVR) valve, catch tank, check valve, vapor separator, leak detection pump (Spectra), fuel filler cap and Engine Control Module (ECM). ECM controls operation of PCSV and CCV. PCSV controls when fuel vapors are allowed to be drawn from EVAP canister into intake manifold so that they may be burned. CCV controls EVAP canister's vent to atmosphere.

ECM's On Board Diagnostics-II (OBD-II) runs monitors for large and small EVAP system leaks when vehicle is idling at a stop. ECM uses FTPS output to determine EVAP system pressure and pressure changes. At beginning of monitor, PCSV is closed and CCV or LDP is commanded closed by ECM to measure compensation gradient. Compensation gradient is the increase in fuel tank pressure due to evaporation of fuel in fuel tank. After predetermined amount of time passes, CCV is opened and PCSV is turned on. When PCSV reaches 100 percent duty cycle, CCV is closed. If vacuum buildup, minus compensation gradient, is less than programmed threshold, a gross leak is diagnosed. After maximum vacuum buildup, PCSV is turned off (closed). If vacuum in fuel tank, minus compensation gradient, decreases to less than programmed threshold, a small leak is diagnosed. CCV is opened and monitor is complete. If either a gross leak, or a small leak is detected for 2 driving cycles, ECM assumes there is a defect in evaporative emissions control system and illuminates MIL. For EVAP codes or DTC P0442, P0446, P0447, P0448, P0455 and P0456, see appropriate SELF-DIAGNOSTICS article.