Quote:
Originally Posted by TonyS9
Dave - On fuel cut, there is a big hysteresis, it doesn't engage immediately unless you close the throttle above 2000. Around 2000 it will engage after a few seconds of closed throttle. This is the time when it doesn't engage where I keep saying its more MPG efficient to coast in neutral by around +20mpg (say going downhill) as well as losing less momentum.
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I'm aware of the hystersis Tony, i was deliberately trying to keep to the fact that it's above 1200rpm for the purposes of explaining what i think may be happening on this particular engine.
Your idea is just as valid, it may even be a combination of both but as it's modified, we might both be right to a certain extent.
On the original L-Jetronic cars, the fuel cut above 1200rpm was a definite thing, you could feel it. No hysteresis, fuel was either on or off and as the revs dropped below 1200rpm, you felt the fuel start flowing again.
With the LH-Jetronic, a softer cut was implemeted by the use of hysteresis, however the basic principles apply but it is more progressively cut which strangely improves emissions by decreasing oxides of nitrogen (NOx) that can allegedly be toxic, either to us or the atmosphere. These apparently occur due to incomplete combustion so by adding a small amount of fuel during overrun, more complete combustion happens, still with engine braking.
This might explain the increased economy for those few seconds of coasting in neutral.
Also as the engine speed drops, the fuel flow is increased more gradually so you don't feel the jolt as the fuel supply is restored - again we don't know if this was altered (or even removed) on the chipped version of the ECU that he has.