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Introduction...
Evaporative purge system performs two important operations in E34 application.
Primary purpose of the system is entrapment of fuel vapor produced in the gas tank in order to ensure cleaner environment.
Secondary purpose of the system is provision of ventilation for the gas tank by allowing outside air to enter the tank and
take the space left behind by used fuel.
Theory.
The block schematic below shows the currents of outside air and fuel
vapors in evaporative control system.
The charcoal canister provides a way for outside air to enter into
the system to take place within the volume left behind by the used gasoline. The canister has a filter and openings at its
bottom in order to make this action possible. If the blockage occurs along the line from the expansion tank to the charcoal
canister, the travel of air may be precluded resulting in implosion of the fuel tank.
Besides having the filter for the outside air, the canister is filled
with charcoal that traps the fuel vapors to be used by suction of intake manifold when the engine is started.
The evaporative purge valve makes possible for the vapors to enter the manifold. Its
normal state is open, it is switched closed by application of voltage by the engine management computer.
Along with the vapors, air that enters to take place of gone gasoline in the fuel
tank is also allowed to enter the intake manifold.
Faulty evaporative purge valve does not provide closing action at application of
voltage on its electrical input. This results in constant introduction of unmeasured air into the intake manifold through
the charcoal canister and rough idle operation is very likely to occur.
Evaporative Purge Valve.
The valve has very simple construction. Its body consists of two parts
connected at the rim of one part wrapping the other. Inside a flap that restricts motion of fluid is designed to be attracted
magnetically by the induction due to application of voltage at the electrical input of the valve. As the flap is attracted
to the coil, blockage of the path is established resulting in inability of fluid to pass through.
Travel of charcoal from the canister to to the evaporative purge valve
is possible when the canister is old. The charcoal builds up between the outlet and the flap within the valve precluding the
flap from motion to close the passage. Once the valve is not allowed to close, the unmeasured outside air is allowed into
the engine through the bottom opening of charcoal canister, unless in extreme case, the charcoal build up between the flap
and the outlet is so great that all passage is restricted. The passage of unmeasured air implicates rough
idle condition. It may also be possible to hear relatively high pitched resonance from air passing through the canister.
To open the valve...
Tap with a small screwdriver along the seam of the rim between the inlet
and outlet halves of the valve.
Alternatively, it may be possible to clean the valve by simply spraying brake cleaner into
both nozzles.
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