Bosch Motronic - Component Datasheet
Carbon Canister Frequency Valve (N80)
In order to minimise evaporative fuel emissions, the S2 like many other Audis, collects fuel vapours in a carbon canister that is located at the foot of the drivers side A-pillar hidden behind the front inner wheel arch. This carbon canister contains an activated charcoal compound that absorbs vapour from the fuel tank and prevents them being released directly to the atmosphere (which all cars did until EU & US environmental legislation was mooted).
The rate at which fuel vapours are channelled from the carbon canister into the engine is determined by the ECU. This carbon canister frequency valve (N80) uses a solenoid to open and close a valve which determines when fuel tank vapors, collected by the carbon canister, are permitted to enter the intake manifold for combustion.
When the engine is not running, a spring-loaded check valve prevents fuel vapors from entering the engine. Should N80 fail, or be disconnected, whilst the engine is running, then manifold vacuum pressure is utilised to open the check valve such that a controlled quantity of fuel tank vapors are allowed to enter the engine for combustion. Hopefully the diagram below helps with the understanding...
Last Updated 6th August 2002