Have the EVAP system tested !
FUEL EVAPORATION SYSTEM
NOTE:Not all components listed are used on every vehicle. See VACUUM DIAGRAMS 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 Solenoid Valve (PSV), Fuel Tank Pressure Sensor (FTPS), rollover valve, On-board Refueling Vapor Recovery (ORVR) valve, catch tank, check valve, vapor separator, leak detection pump (Sephia/Spectra), fuel filler cap and Engine Control Module (ECM). ECM controls operation of PSV and CCV. PSV 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, PSV 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 PSV is turned on. When PSV reaches 100 percent duty cycle, CCV is closed. If vacuum buildup, minus compensation gradient, is less than programmed threshold, gross leak is diagnosed. After maximum vacuum buildup, PSV is turned off (closed). If vacuum in fuel tank, minus compensation gradient, decreases to less than programmed threshold, small leak is diagnosed. CCV is opened and monitor is complete. If either gross leak, or small leak, is detected for 2 driving cycles, ECM assumes there is defect in evaporative emissions system and illuminates MIL. For EVAP codes or DTC P1446, P1447 and P1448, see appropriate SELF-DIAGNOSTICS article.
Purge Solenoid Valve (PSV)
PSV is normally closed, duty cycle-controlled valve located in vapor line between EVAP canister and intake manifold. When commanded open by ECM, fuel vapors collected in EVAP canister are allowed to be drawn into (purged) intake system.
Fuel Tank Pressure Sensor (FTPS)
Measures pressure and vacuum levels in fuel tank. ECM can check PSV operation and for leaks in EVAP system by monitoring fuel tank pressure data during different operating cycles. Located on top of fuel pump assembly.
Wednesday, April 8th, 2009 AT 12:33 PM