1994 Audi 100 Fuel Evaporative Canister Replacement Cost

EVAP system faults are very common, and happen throughout the useful life of the vehicle. The evaporative canister and all other components in the system are linked to the same trouble codes, so all components are inspected when the system has a fault. This includes the filler cap, a.k.a. gas cap. The gas cap is the number one reason EVAP failures are recorded. These codes are preventable with regular cleaning and inspection of the fuel cap.

When the EVAP system is checked, the purge valve and vent valve are closed, and the pressure is monitored by the fuel pressure sensor. If that pressure drops with both valves closed, there is a leak in the EVAP system. If pressure holds properly, the system passes, and will continue to prevent vapor emissions. The sensor is also used in testing vent valve and purge valve operation. When the valves are open, the pressure sensor reads the fuel tank pressure, and must sense a drop in pressure. If not, the purge valve or vent valve may be faulty.

When the identified component is faulty, the technician will remove all electrical and vacuum connectors, clean the mating surface for the faulty part, and install a new part. The evap system will then be allowed to run self-diagnostics to test the repair. If additional leaks are present, or other components have failed, the test will fail, and the process will restart.

To help prevent issues with the EVAP system, especially the charcoal canister, avoid overfilling or topping off the fuel tank. When the tank is topped off, the charcoal canister may fill with fuel, causing system failure, and degradation of the charcoal canister.

Many people like to remove EVAP components, but there is nothing to benefit from removal. The system is there to prevent harmful emissions, and increase fuel mileage in the process. When working with fuel, the battery should always be disconnected, and proper precautions taken against fire hazards.

The EVAP system is best serviced by an experienced DIYer, or a professional technician. Repairing the EVAP system can be a struggle, but with proper understanding the job becomes enjoyable. The biggest mistake is to being changing parts based on hunches and misdiagnosis. That is where this job becomes more expensive than bringing the problem to a professional.