Large Leak Test
This tests for large leaks and blockages in the EVAP system. The PCM will command the EVAP vent valve ON, closed and the purge valve and the vacuum pressure switching solenoid ON, open with the engine running. This allows engine vacuum into the EVAP system. The PCM then commands the purge valve OFF, closed sealing the system. The PCM then monitors the Fuel Tank Pressure (FTP) sensor signal in order to determine if there is vacuum decay. A rise or rapid increase in pressure indicates there is a large leak in the evaporative emission system. If the PCM detects the vacuum level was not achieved and vacuum decay was evident for two consecutive trips, the Malfunction Indicator Lamp (MIL) will illuminate, and DTC P0440 will set.
Small Leak Test
If the large leak test passes the PCM will continue to monitor the fuel tank pressure (FTP) sensor signal in order to determine if there is vacuum decay. A slight increase or rise in pressure indicates there is a small leak in the evaporative emission system. The EVAP control system is required to detect evaporative emission fuel system leaks as small as 0.020" (0.5 mm) between the fuel fill cap and the purge valve. If the PCM detects vacuum decay was evident for two consecutive trips, the MIL will illuminate, and DTC P0442 will set.
On-Board Refueling Vapor Recovery System
The On-Board Refueling Vapor Recovery (ORVR) system is an on-board vehicle system designed to recover fuel vapors during the vehicle refueling operation. (Scheme 1) Instead of allowing fuel vapors to escape to the atmosphere the ORVR system transports the vapor to the EVAP canister for use by the engine. The flow of liquid fuel down the fuel filler neck provides a liquid seal that prevents fuel vapor from leaving the fuel system. The ORVR system architecture varies from platform to platform. Some of the ORVR system components with a brief description of their operation listed below, are optional depending on the platform application. In the following list, numbered items correspond to numbered items in illustration.
- EVAP Canister EVAP canister receives and stores refueling vapor from the fuel system. EVAP canister releases the fuel vapor to the engine through the EVAP control system.
- Vapor Lines Vapor lines transport fuel vapor from the fuel tank assembly to the EVAP canister.
- Vapor Recirculation Line The vapor recirculation line, if equipped, transports fuel vapor from the fuel tank to the top of the fuel filler pipe during refueling in order to reduce the fuel vapor at the canister. The vapor recirculation line can be located inside the fuel filler pipe or outside of the fuel filler pipe.
- Variable Orifice Valve The variable orifice valve, if equipped, regulates the amount of vapor allowed to enter the vapor recirculation line.
- Fuel Filler Pipe The fuel filler pipe carries fuel from the fuel dispensing nozzle to the fuel tank.
- Check valve The check valve limits fuel spitback from the fuel tank during the refueling operation by allowing fuel flow only into the fuel tank. The check valve is located at the bottom of the fuel filler pipe or in the fuel tank filler neck.
- Fuel Sender Assembly The fuel sender assembly pumps fuel to the engine from the fuel tank.
- Fill Limiter Vent Valve The fill limiter vent valve is located in the fuel tank and acts as a shut-off valve. The fill limiter vent valve performs the following functions: Controls the fuel tank fill level by closing the primary vent of the fuel tank. Prevents liquid fuel from exiting the fuel tank through the vapor line and entering the EVAP canister. Provides fuel-spillage protection in the event of a vehicle rollover by closing the vapor path from the tank to the EVAP canister.
- Pressure/Vacuum Relief Valve The pressure/vacuum relief valve, if equipped, provides venting of excessive fuel tank pressure or vacuum. The pressure/vacuum relief valve is located in the fuel filler neck on a plastic fuel tank and in the fill limiter vent valve on a steel fuel tank.
Scheme 1
SELF-DIAGNOSTIC FUNCTION
The Powertrain Control Module (PCM) diagnoses any troubles which may occur in the engine control system when the ignition switch is in the RUN position with the engine running. The PCM indicates a malfunction by illuminating the Malfunction Indicator Lamp (MIL) when a fault occurs in any of the following systems
- Heated Oxygen Sensor 1 (HO2S 1).
- Heated Oxygen Sensor 2 (HO2S 2).
- Engine Coolant Temperature (ECT) sensor.
- Throttle Position (TP) sensor (including the CTP switch).
- Vehicle Speed Sensor (VSS).
- Intake Air Temperature (IAT) sensor.
- Mass Air Flow (MAF) sensor.
- Camshaft Position (CMP) sensor.
- Crankshaft Position (CKP) sensor.
- Knock Sensor (KS) system.
- Evaporative Emission (EVAP) control system.
- Idle Air Control (IAC) system.
- CMP actuator solenoid system.
- Rocker arm oil control system.
- Misfire detection.
- Fuel-trim.
- Catalyst monitor.
- Central Processing Unit (CPU) of the PCM.
When PCM detects a malfunction in one of the above areas, the PCM will illuminate or flash the MIL in order to notify the driver of the occurrence of a fault. PCM will store a DTC when PCM illuminates the MIL. PCM will turn OFF the MIL after 3 consecutive ignition cycles without the malfunction occurring. The DTC will remain stored in the PCM memory after the MIL is OFF.
Clearing Diagnostic Trouble Codes
Note. Do not clear the DTCs unless directed to do so by the service information provided for each diagnostic procedure. The Freeze Frame data which may help diagnose an intermittent fault will be erased from the memory when the DTCs are cleared.
The PCM will begin to count the warm-up cycles when the fault that caused the DTC to be stored into memory has been corrected. The DTC will automatically be cleared from the PCM memory when the PCM has counted 40 consecutive warm-up cycles with no further faults detected. DTCs can be cleared using a scan tool. In order to clear DTCs, use the scan tools CLEAR DTC Information function. Follow the instructions supplied by the scan tool manufacturer.