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Throttle Position Sensor

EFI Tuning Fundamentals

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Hi, When fuel injection is disabled at high RPM and throttle is closed, how combustion process works if the engine is still working and moving? May be a dumb question but I would like a better appreciation of how it works, please. Any help is appreciated.

When the RPM is above idle, and throttle closed, then engine is typically in an "overrun" condition. Often with the transmission driving the engine (like going down a hill). So there is no need for combustion to keep the engine turning. Many ECU use what's called "Decel Fuel Cut-Off (DFCO)" to disable all injection after a short delay when the throttle is closed and the RPM is above a certain minimum. Of course injection needs to be re-enabled in order for the engine to idle and not stall.

And how that resumption of injection is tuned makes a big difference to the drivability of the vehicle.

Hi David and Black Rex, so typically this happens when the car is in gear or D in an automatic right? But what happens when it is in neutral in a downhill, Fuel Injection, and combustion still working?

Without a gear driving the engine, the engine speed (RPMs) will quickly return to idle, where injection is still occurring. DCFO would likely not be active since it should return very quickly (like blipping a throttle).

What is your concern?

It would also be a bad practice as the gearbox wouldn't be splash lubricating it's internals.

The gearboxes I'm familiar with would have the input shaft turning when in neutral, and of course output shaft would be turning of the drive wheels are turning. So I think the gearbox would be lubricated as long as things are turning, regardless of whether gears are engaged or not. There should be very little load to generate heat anyway...

Yup, you're correct, mind fart time as I was thinking of the engine off and coasting in neutral*, which isn't the scenario in question.

An idling engine, with the clutch engaged, certainly should be turning the layshaft/2nd motion shaft, and hopefully providing sufficient splash lubrication to the free spinning output shaft gears.

*The problem with towing, or coasting in neutral with the engine off, is the output shaft is spinning inside the output gears and not receiving any lubrication - worse, most will use plain bearings which are dependent on the dynamic wedge and boundary layer lubrication. This may not be a problem for short periods and/or distances, but as soon as metal to metal contact is made, it can get messy in a hurry! Also a reminder of the importance of ensuring the geargox oil level is correct, with the correct oil, and any leaks are remedied promptly, as if the level drops too far...