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Ceed 1.6crdi ('07)
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I am still trying to understand how my Pro Ceed can be using no fuel at all on rundown
You're overcomplicating things but you need to start with a distinction between idling and overrun.

Overrun is simple for either petrol or diesel, where all that has to happen is that no fuel is injected.

The ECU knows what speed the engine is doing by interpreting the timing between pulses from the crank sensor. Too fast, less fuel, too slow more fuel.
Modern fuel injectors offer such fine control that no extra gubbins is needed other than that for a petrol engine there is an air bypass round the throttle that the ECU also needs to tweak.

The injectors are indeed quite remarkable things. If my maths is working, at idle they are delivering something like 60 microlitres during each firing stroke (0.3 gallons per hour). On a 150bhp engine running flat out (say, 5.8 gallons per hour) it rises to about 220 microlitres per stroke. A diesel injector has to be able to deliver this in probably less than half a millisecond. The actual amount delivered will be proportional to the length of electrical pulse issued by the ECU.

That's just a mechanical problem. Creating the electrical pulse for the injector with a resolution of a fractions of a millionth of second is very easy. For instance, for you to be able to read text on your computer monitor, it has to be able to complete the process of going from white-black-white in 12 billionths of a second.
 
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