Leak Test

The evaporative emission system used on NAS vehicles includes a fuel pressure sensor and a canister vent solenoid (CVS) valve. The system is capable of detecting holes in the fuel system down to 1 mm (0.04 in.).

The test is carried out in three parts:
First the purge valve and the CVS valve closes off the storage system and the vent pressure increases due to the fuel vapor pressure level in the tank. If the pressure level is greater than the acceptable limit, the test will abort because a false leak test response will result. In part two of the test, the purge valve is opened (preferably with the engine idling) and the fuel tank pressure will decrease due to purge operation. In part three of the test, the leak measurement test is performed. The pressure response of the tests determines the level of the leak, and if greater than the limit on two consecutive tests, the ECM stores the fault in diagnostic memory and the MIL light on the instrument pack is illuminated. The test is only carried out at idle with the vehicle stationary. Following the test, the system returns to normal purge operation after the CVS valve opens. The in-tank pressure sensor monitors the pressure build-up to determine whether leaks are present.

Possible reasons for a test failure are listed below:
- Fuel filler not tightened or cap missing
- Sensor or actuator open circuit
- Short circuit to vehicle supply or ground
- Either purge or CVS valve stuck open
- Either purge or CVS valve stuck closed or blocked pipe
- Piping broken or not connected
- Loose or leaking connection

If the piping is broken forward of the purge valve or is not connected, the engine may run rough and fuelling adaptation will drift. The fault will not be detected by the test, but by the engine management ECM detecting that the fuelling adaption is suspended. The evaluation of the leakage is dependent on the differential pressure between the fuel tank and the ambient atmospheric pressure. The diagnostic test is disabled at altitudes above 2,800 meters (9,500 ft).

The fuel tank pressure sensor is included as part of the OBD system. A failure of the fuel tank pressure sensor will not be noticed by the driver, but if the ECM detects a fault, it will be stored in the diagnostic memory and the MIL warning lamp will be illuminated on the instrument pack

Possible fuel tank pressure sensor failures are listed below:
- Damaged sensor
- Harness wiring or connector faulty
- Open circuit
- Short circuit to battery voltage or ground
- ECM fault

Possible symptoms of a fuel tank pressure sensor failure are listed below:
- Fuel tank pressure sensor poor performance
- Fuel tank pressure sensor low range fault
- Fuel tank pressure sensor high range fault

Fault codes associated with the evaporative emission control system are listed below:
- P0171 - Multiplication fuelling adaption (Max.) exceeded lean limit - LH bank
- P0172 - Multiplication fuelling adaption (Min.) exceeded lean limit - LH bank
- P0174 - Multiplication fuelling adaption (Max.) exceeded lean limit - RH bank
- P0175 - Multiplication fuelling adaption (Min.) exceeded lean limit - RH bank
- P0171 - Additive fuelling adaption (Max.) exceeded lean limit - LH bank
- P0172 - Additive fuelling adaption (Min.) exceeded lean limit - LH bank
- P0174 - Additive fuelling adaption (Max.) exceeded lean limit - RH bank
- P0175 - Additive fuelling adaption (Min.) exceeded lean limit - RH bank
- P0440 - Purge valve not sealing
- P0442 - Small leak within system
- P0443 - Purge valve power stage short circuit to battery voltage
- P0444 - Purge valve power stage open circuit
- P0445 - Purge valve power stage short circuit to ground
- P0445 - Large leak within system
- P0446 - CVS valve /filter /pipe blocked
- P0447 - CVS valve open circuit
- P0448 - CVS valve short circuit to ground
- P0449 - CVS valve short circuit to battery voltage
- P0451 - Fuel tank pressure signal stuck high within range
- P0452 - Fuel tank pressure signal short circuit to battery voltage (out of range - high)
- P0453 - Fuel tank pressure sensor signal short circuit to ground or open circuit (out of range - low)