Fuel cutoff

Fuel cut-off, sometimes called overrun fuel cut-off or deceleration fuel shut-off, is an engine-management feature that completely stops fuel injection when the driver lifts off the accelerator while the car is in gear and still moving. In that situation the engine does not need any fuel of its own: it is being turned by the wheels through the transmission, so the management system simply switches the injectors off and lets the car's momentum keep the engine spinning. The feature exists to save fuel and reduce emissions during the many moments of everyday driving when no power is being demanded.

The mechanism is governed entirely by the engine control unit, which constantly monitors throttle position, engine speed and road speed. When it detects a closed throttle combined with engine revolutions comfortably above idle and the vehicle moving in gear, it ceases injecting fuel altogether. Because the drivetrain is mechanically coupled to the wheels, the engine continues to rotate without combustion, and the car coasts forward while burning literally no fuel. The instant the driver presses the accelerator again, or the engine speed falls towards idle, injection resumes seamlessly to prevent the engine from stalling.

The practical benefit becomes clear when descending a hill or approaching a junction with the car in gear. Throughout that phase the engine consumes zero fuel, which is why a real-time consumption display often reads 0.0 L/100 km or shows infinite mpg during overrun. This is genuinely more economical than coasting in neutral, a habit some older drivers still favour, because in neutral the engine must be fed a trickle of fuel to keep idling. Staying in gear and letting fuel cut-off operate therefore saves more fuel than freewheeling.

Fuel cut-off is intimately bound up with engine braking. With the throttle closed and no fuel being burnt, the engine's internal pumping losses and compression provide a retarding force that helps slow the car, the effect being stronger in a lower gear. The two phenomena are really two sides of the same condition: the engine both saves fuel and provides braking effort during overrun. Selecting a lower gear on a long descent thus enhances both control and economy simultaneously.

There are sensible limits to the feature. Injection is reintroduced as engine speed drops towards idle to avoid stalling, and modern systems blend the transition smoothly so the driver feels nothing. On vehicles with automatic transmissions the same principle applies through the management of the torque converter and gear selection.

The feature is a natural by-product of modern electronic fuel injection, which gives the management system precise, instantaneous control over each injector. It relates closely to engine braking, to fuel injection generally, and to overall fuel consumption, contributing quietly but consistently to the efficiency of every journey.