TheWoo

iTrader: (2)

Hi guys,

I searched for this, but no answers were forthcoming, and the nearest downhill stretch long enough for me to test it using our MPG meter is too far away (besides, I always forget).

On most modern cars, there is a deceleration fuel cutoff to conserve fuel. This cuts fuel when the engine is above a certain RPM, and under the effects of engine braking. I scrutinized the FSM, and it states that we have a fuel cut control that
I understand this as that when you shift to neutral and your RPM is >1800, the ECU will eventually cut fuel until RPM drops to 1500, during which fuel flows again to maintain an idle, or near-idle state. However, does the same thing happen when you go down to a lower gear, say, while on a prolonged downhill descent? On a previous car (ok, a truck), downshifting instead of going directly to neutral let me save a decent amount of gas, and I transferred that habit to driving the 350z. Besides, rev-matching is good practice, and I have to justify buying that 380RS pedal, right? Now I wonder if it actually gives no fuel consumption benefits vs. just putting it into neutral when I go downhill.

If you have an A/F meter/readout on your car, DFCO will show as a jump in ratios to as high as your meter will go when engine braking, since it approaches an infinitely lean mix. I really wish A/F ratios was one of the settings on the factory multimeter gauges.

Before this thread degenerates into an "engine braking is bad," "don't expect your car to become a Prius," or "isn't this pretty trivial?" flame-fest, I just want to point out that I'm asking this out of pure engineering curiosity, not to make a statement. Thanks for any helpful replies!

DaveJackson

iTrader: (5)

I'm pretty sure that the cut they are referring to is at no-load 5000+ rpm and it's based on self preservation, not fuel economy.
The car has never felt to me like it's decelerating hard enough that fuel was cut.

TheWoo

iTrader: (2)

I thought the same at first, but the FSM was pretty specific on the 1800 RPM threshhold:



I went to look up the note referring to the MFI system on page EC-25, but all it said was that during deceleration, fuel was "decreased" but did not mentioned it was cut off. Page EC-26 states "Fuel to each cylinder is cut off during deceleration or operation of the engine at excessively high speeds." The second part is obviously the rev limiter, but the first part doesn't say if cut off is dependent on being in neutral or in gear.

It looks like I'm going to have to go find that mile-long downhill and test this out for myself: do one descent in neutral, and another in gear and above 1800 RPM.

PDX_Racer

iTrader: (2)

For years, Nissan has had a "fuel cut" when coasting. But the coasting has to be for more than a few seconds before the fuel cut kicks in.

Cux350z

iTrader: (162)

It does this. Keep it in gear instead of neutral. I have seen it on my 02 gauge.

100% confirmed. It has been discussed on here before but probably not easy to find given the key words are common.

e30cabrio

iTrader: (30)

I have an AFM and every time I take my foot off the gas it goes to 20 which is no fuel.

djamps

iTrader: (1)

There is a full on fuel cut when you're decelerating in gear. Confirmed by my injectors going silent (I have a noisy as hell fuel system) and my A/F meter pegging lean.

TheWoo

iTrader: (2)

Excellent! Thanks for the help, guys.

lolzz

Then what would run the engine, if there is no fuel and no momentum from the wheels (neutral)?
In that case, the rpm would drop to 0.

TheWoo

iTrader: (2)

The rpm doesn't drop to 0 instantly. There is significant momentum retained by the flywheel, which is why going to a lighter flywheel tends to make your engine rev quicker, but also lose rpm quicker when you put it in neutral (less rotating mass = less rotational inertia). The fuel cut outlined in the FSM is when you are at, say, 4k rpm, and put the car in neutral (no load). The ECU would then cut fuel (after a variable amount of time) until the rpm drops to 1800, then it would flow fuel again so it doesn't stall out.

My question was more about whether this function applied while engine braking, which the FSM was unclear about. The gentlemen with the A/F meters gave me an unequivocal answer. Thanks!