Diagnostic Test Sequence
The diagnostic is divided into different phases in which the various components are tested. If a fault is detected in any of the phases the diagnostic is interrupted and the DTC for the component identified is stored. The diagnostic test works as follows:
- The fuel tank pressure sensor is checked for an unstable signal. See Figure. DTC ECM-4308 (faulty signal) is stored if the signal oscillates more than 5 times± .145 psi during 5 seconds. The fuel tank pressure is checked so for stability and that the short-term fuel trim is not too low.
- EVAP canister shut-off valve is shut and an evaporation check is carried out by gauging how much fuel tank pressure increases. See Figure. This provides a value for how much fuel evaporates, and this value is used later to calculate leakage flow. If fuel tank pressure sinks, this indicates that the Canister Purge valve is leaking and DTC ECM-4008 canister purge valve open is stored.
- EVAP canister shut-off valve is opened, the tank system is open. canister purge valve starts to pulse. As a result of the vacuum in the intake manifold the engine starts to suck in air through the EVAP canister. Because EVAP canister shut-off valve is open, the fuel tank pressure drops slowly. If fuel tank pressure sinks rapidly this indicates that the EVAP canister shut-off valve is clogged and DTC ECM-4019 EVAP canister shut-off valve shut is stored.
- EVAP canister shut-off valve closes the canister purge valve which pulses with a duty cycle of approximately 17 percent. Fuel tank pressure sinks to -.145 psi. DTC ECM-4308 large leak (signal too low) is stored if this pressure is not reached within ten seconds. This indicates a larger leak in the fuel tank. DTC ECM-4308 large leak (faulty signal) is stored if the fuel tank pressure does not change within 2 seconds. This indicates a defective fuel tank pressure sensor or clogged piping.
- Canister Purge valve closes, EVAP canister shut-off valve is shut. There is now a negative pressure in the fuel tank. This negative pressure will decrease slowly. The decrease rate depends on fuel level, fuel evaporation and any leaks. Leakage flow is calculated by comparing pressure increase speed with the pressure decrease speed from stage 4 and by compensating for the evaporation measured in stage 2. DTC ECM-430D small leak is stored if the calculated leakage flow exceeds a certain level. This indicates a smaller leakage in the fuel tank system.
- EVAP canister shut-off valve is opened, the diagnostic test is concluded.
During the different phases when the system gauges whether the fuel tank system pressure acts normally or not, there are a number of circumstances which are taken account of, for example:
- Amount of fuel in the tank
- Height above sea level
- Fuel temperature and evaporation