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I'm trying to drive economically and my understanding is that as long as there is enough momentum in the car to keep the engine spinning while in gear at above idle the engine won't inject more fuel, however, it seems that with certain cars the engine still injects fuel even at say 1000 RPM when the idle is maybe 750 RPM.

Is there a clear way to tell at what RPM the car starts to inject fuel again?

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    Your question suggests that "with certain cars, the engine still injects fuel..." How are you able to determine this supposition?
    – fred_dot_u
    Apr 27, 2023 at 21:30

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There is no blanket answer for all cars, since the manufacturer decides how their vehicles behave. But you can use an OBDII code reader to measure instantaneous fuel consumption of your car.

For safety, let someone else drive while you observe RPM, road speed and fuel consumption. Go to Mode 1 readings (current data) and read PID 5E, engine fuel rate. Let your driver accelerate to an arbitrary speed, then get off the gas pedal and allow the car speed and engine RPM to drop. If the PID drops to zero, the engine is consuming no fuel. If that happens, you can look for the exact RPM at which fuel starts to be consumed again. Note the RPM. That's your answer for your car.

On my car (a diesel, manual trans) the PID remains zero all the way down to idle speed.

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  • I drive a Seat Ibiza 1,4 petrol 2004 and there is no option to see instant fuel consumption on my OBDII, only basic live data like speed, ECT, long term and short term fuel trim etc...
    – Aden
    Apr 28, 2023 at 12:52
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GM EFI engines use a term, deceleration fuel cutoff, describing coasting from speed with foot off the throttle pedal; EFI programming determines throttle at closed position while above 10-15 mph or 16-24 kph, disabling fuel injectors until below aforementioned speed when injectors are enabled. Most drivers using manual transmissions can see the tachometer drop but remain above idle (emissions regulation to maintain catalytic converter temperatures) while coasting in neutral until speed drops down when high idle drops to normal idle. The combination of emissions regulation and programming determines when fuel cutoff occurs and then resume fuel injection. Automatics will have this programming too but not easily felt or seen since the xmission is always engaged thru the torque converter but may be seen if coasting from speed in neutral. Each manufacturer determines how their EFI systems operate based on emissions requirements in nations requiring emissions control. I don't know if SEAT uses DFCO but service manuals can answer this.

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