1994 Audi 100 Quattro Fuel Evaporative Canister Replacement Cost

The EVAP system uses the charcoal canister to store fuel vapors that escape the cas tank through a tube. The charcoal (EVAP_ canister empties by engine vacuum sucking the vapors out, to be burned as fuel. This can only be done when the vent valve and purge valve are opened simultaneously. When the vent valve and purge valve are closed, the evaporative canister will store vapors until the vents are opened. This allows environmentally harmful fuel vapors from entering the atmosphere, and increases fuel mileage over a tank of fuel.

The check engine light will illuminate any time there is a failed EVAP system test. This will be indicated by the various on-board diagnostics trouble codes that are stored in the vehicle's memory. Also, in some models, there may be a popping sound accompanying the check engine light, as the leak detection pump continuously attempts to pressurize the fuel tank. The canister may also cause difficulty starting the engine, a rough idle, and poor fuel mileage.

A vehicle with an EVAP system issue can be driven, but may achieve lower fuel mileage. Also, a vehicle with an EVAP system issue will not pass standardized state or federal emission testing. The vehicle may take time to start and idle rough, but driving the vehicle should not be an issue.

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.