Where is my fuel tank pressure sensor

How are EVAP system leaks detected by the ECU?

The most typical system used by GM is a vacuum disintegration system.

The system consists of 3 main components (ie for leak detection). Flushing magnet, venting magnet and pressure sensor. In normal operation, the ventilation magnet is open and, depending on the situation, lets air in and out. The vent magnet is connected to the outlet of a charcoal canister. This canister absorbs gas vapors when air escapes from the tank. While driving, the vehicle opens the flushing magnet and creates a vacuum in the system. If the vent magnet is still open, air will be drawn through it, then through the entire system (including the charcoal canister), and then through the purge magnet into the engine being burned. In this way, gas vapors are captured and burned.

When the system checks for leaks, it closes the vent solenoid. Then use the flushing magnet to draw a vacuum on the system. From here one of two things happens. The system is not lowered into a vacuum as indicated by the pressure sensor. The assumption is that there is a major leak because the system is not holding a vacuum at all. The other scenario is that the system gets into a vacuum. At this point the flushing magnet switches off and the system only monitors with the pressure sensor. If the vacuum stops at less than a certain speed or drops slowly, the system is fine. If the vacuum drops faster than a certain speed, there will be a small hole.

The problem with this type of system is that when you pull a vacuum on gasoline, it evaporates. This looks like a fading vacuum even when there is no leak. Because of this, the rate of vacuum drop is important to tell the difference.