I’ve heard people say that fuel injection automatically turns off when you let the car coast in gear, but if I completely let off the accelerator in first gear, the car will only slow down to 3-5mph (depending on the car) and will then stay at that speed indefinitely like a lil’ cruise control, suggesting that the car was always injecting fuel at engine idle ratio?
I understand this kind of question has been asked before, but as this element doesn’t check out, I don’t believe it has actually been completely answered.
In: Engineering
GM calls it “Deceleration Fuel Cut-Off” (DFCO) and uses these parameters with values unique to the vehicle model:
RPM: above 1400 rpm
Speed: above 40 mph
Coolant Temperature: above 30f
Engine Load for activation: below 30 kpa
Engine Load for deactivation: above 32 kpa
Amount of spark retardation: 20 degrees
Delay: 1000ms
KPA is from the Manifold Absoute Pressure (MAP) sensor that measure the vacuum/pressure in the intake manifold. MAP generally increases as the throttle is opened. Each engine model with have a specific MAP value at idle speeds, such as 35 kpa, which increases to around 100 kpa at full throttle (equal to atmospheric pressure outside of the engine).
Deceleration with the throttle closed creates high vacuum (low pressure) because the throttle is closed so the engine is having suck in what little air it can. This produces a MAP value in the 20 KPA range.
For example, If the vehicle is moving faster than 40 mph, the RPm is above 1400 rpm, and the MAP value is under 32 KPA, then the computer shuts off the fuel injectors (pulse width goes to 0.0 milliseconds).
When any of the conditions is no longer met, the computer resumes regular fuel delivery.
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