Diagnosis of Fuel Tank Ventilation System In USA Vehicles

Diagnosis Of Fuel Tank Ventilation System In USA Vehicles

System Overview







System description

In order to prevent fuel vapours from entering the atmosphere, an EVAP canister is connected to the tank ventilation line.
Fuel vapours are collected in the EVAP canister. As the EVAP canister can only accommodate a certain amount of fuel vapours, these must be taken in and combusted during driving.
For this to occur, the EVAP canister purge valve (1) opens. Due to the vacuum predominating in the intake manifold, air is sucked up via the purge air line (3) through the EVAP canister. The air sucked up in this way draws the collected fuel vapours with it.
American law requires the tank ventilation system to be checked for leaks. For this purpose, the tank pressure sensor (5) and shutoff valve (6) are required.

Function of Tank Ventilation Diagnosis

In order to check the system for leaks, vacuum is built up in the tank, the system sealed and a test performed to see how quickly the vacuum diminishes again. Pressure conditions are influenced by the vaporisation of fuel. Therefore, prior to diagnosis the pressure increase due to fuel vaporisation is examined.

Tank ventilation diagnosis is divided into three steps:

1. Measuring the pressure increase due to fuel vaporisation.
- The EVAP canister purge valve (1) and shutoff valve (6) are closed. The increase in pressure is measured by the tank pressure sensor (5).

2. Vacuum build-up in the system.
- The EVAP canister purge valve is opened and the shutoff valve remains closed. This causes vacuum to build up in the tank. If no vacuum builds up, a large leak is present, e.g. the tank cap is not screwed on.

3. Vacuum reduction in the system.
- At a particular vacuum (approx. 8 mbar), the EVAP canister purge valve is closed once more. The system is now sealed. Depending upon the speed at which vacuum is reduced, leaks in the system may be determined.