Evaporative Emission Control System Leak Check

About 16-20 minutes after start-up, the PCM goes through an evaporative emission system check. Testing begins only when the engine is idling, the vehicle is stationary and engine coolant temperature is above -7°C (20°F). The test is not run if the barometric pressure sensor detects an altitude over 8,000 ft., if there is a high vapor content in the fuel tank (tank pressure must fall below .2kPa, or 0.03 psi) or the tank is less than 15% full. The evaporative emission control system leak check is completed in the following steps:

1. The CCV closes, blocking the flow of fresh air into the canister.

2. The PCM turns the purge control valve "on", causing engine vacuum to be applied to the canister.

3. With the CCV closed and no fresh air allowed to enter the system, a vacuum begins to build up in the system and the tank pressure begins to drop.

4. The DPS now monitors how well the system holds the vacuum.

5. The PCM sees the decrease in pressure and thus knows that the purge is functioning (the first system test has now been passed).

6. The next test consists of keeping the CCV closed and watching the tank pressure for leaks. If the DPS voltage begins to increase, it means the system has a leak. If the voltage increases rapidly, it is a large leak. If it increases slowly, it is a small leak. If the PCM detects a condition in 2 consecutive driving cycles, a code is set. If the leak corrects itself, there is a program within the PCM to delete the code and turn off the MIL at the end of the third fault free trip.

7. At the end of the test, the CCV opens and the purge control valve returns to its cycling mode. If the CCV is stuck closed, the DPS will see that the system remains under a vacuum and a code will be set. A similar low pressure may occur if the purge control valve is stuck open or a restriction occurs in the vapor line to the tank.