Cars / e32 / Engine / M30 Idle Performance

M30 Idle Performance

Idle performance and EVAP Purge Valve

Article by: RAkhmedov

Applies to: All models with the M30 straight six engine.

Erratic idle performance of the engine is often attributed to quality of ignition components, Idle Control Valve, AFM sensor, Throttle Position Sensor, etc. A part of fuel injection system that is often ignored is Evaporative Purge System, consisting of a canister that collects gas fumes from the gas tank, and a valve that directs the fumes into the engine. The valve is subject to blockage by charcoal from the canister, which results in fixation of the valve in “open” mode responsible for introduction of unmeasured air into the engine. The performance of the engine at idle suffers greatly, and fuel economy and drivability problems may ensue. Here I share my experience of opening the valve and cleaning it to restore idle performance.


Evaporative purge system performs two important operations in the M30 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.


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…

Alternatively, it may be possible to clean the valve by simply spraying brake cleaner into both nozzles. Tap with a small screwdriver along the seam of the rim between the inlet and outlet halves of the valve.

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