Deceleration Valve: Description and Operation

The deceleration device is designed to maintain combustion during engine overrun to prevent the emission of unburned hydrocarbons.

Mechanical Throttle Damper (Dashpot)
The dashpot mechanically delays the closing of the throttle butterfly.

Idle Air Control Valve (IAC Valve)
The IAC valve has been introduced to give:
- a more even engine speed.
- better starting from cold and better performance when driving warm.
- increased idling speed when the air conditioning system is activated.
- compensation for the drop in engine speed due to increased load from the servo pump and generator charging and when driving at high altitude.
- deceleration function.

The IAC valve regulates the air flow that is allowed past the throttle butterfly. The air flow is decided by the opening angle of the valve, which is controlled by signals from the engine control module. This means that the quantity of air required at any moment can be let past in order to maintain a constant engine speed.

The IAC valve is a motor driven sliding valve which is controlled by signals from the engine control module. Because the engine is continuously moving very slightly forwards and backwards, which is experienced as vibration, the opening angle of the slide valve can be varied over very short periods of time (opens and closes within 150-200 ms).

As of M88, the IAC valve has an emergency driving function, "Limp Home", which gives a fixed idling speed of 850 rpm.