Fuel cut on deceleration? - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

Sixbanger · 04-05-2003, 12:11 AM

Hey, I know some cars' EFI systems cut the fuel when you are decelerating, or engine braking down a hill. Does the 240 do this? I'm mainly interested in 96+ obd2 cars...

*sykikchimp* · 04-05-2003, 01:41 PM

I don't believe they did this in the 5-speed, but they might have programmed it in the Auto if you were in a lower gear or something.. I've never heard about it, but that doesn't mean it doesn't exsist.

AKADriver · 04-05-2003, 06:50 PM

I think that's a pretty standard thing for an injected engine to do, since there's no fuel demand when the throttle is closed (except at idle).

*sykikchimp* · 04-06-2003, 12:44 AM

just cause the throttle plate is closed doesn't mean air doesn't get in the combustion chamber..

I would think under engine braking when your RPM's are above normal idle you are consuming more air/fuel due to rpm being higher.. this would require more fuel or else you may cause detonation. Seems that cutting fuel with higher rpms while engine braking could cause engine failure.

Am I completely off base here?

just curious.. what circumstances cause you to ask this question sixbanger?

AKADriver · 04-06-2003, 01:22 AM

A small amount of air is getting in via the idle control circuit, pretty much, but if you have a boost/vac gauge you'll see vacuum shoot up to -14.7 psi more or less with the throttle off. A bit of vacuum is lost to the vacuum operated accessories.

No fuel = no detonation! With no fuel moving through a spinning engine it's just acting as an air pump.

You need fuel to spin the engine at idle because the engine is under the load of... itself. At deceleration the engine is under negative load, the engine is being spun by the momentum of the car.

Someone with something like an S-AFC should chime in here, doesn't that display injector duty cycle?

Sixbanger · 04-06-2003, 03:26 AM

Quote:

just curious.. what circumstances cause you to ask this question sixbanger?

Nothing really, just interested. I tend to coast down a lot of hills and with gas being expensive these days, I was wondering if I save more by staying in gear or keeping the car in neutral and killing my brakes. Not that I can't afford the extra gas or anything.

04-05-2003, 12:11 AM	#1
Sixbanger Leaky Injector Join Date: Oct 2002 Posts: 50 Trader Rating: (0) Feedback Score: 0 reviews	Fuel cut on deceleration? Hey, I know some cars' EFI systems cut the fuel when you are decelerating, or engine braking down a hill. Does the 240 do this? I'm mainly interested in 96+ obd2 cars... Last edited by Sixbanger; 04-05-2003 at 12:13 AM..

04-05-2003, 01:41 PM	#2
sykikchimp WOW addicted Join Date: Dec 2001 Location: the moon Age: 44 Posts: 6,525 Trader Rating: (0) Feedback Score: 0 reviews	I don't believe they did this in the 5-speed, but they might have programmed it in the Auto if you were in a lower gear or something.. I've never heard about it, but that doesn't mean it doesn't exsist. __________________

04-06-2003, 12:44 AM	#4
sykikchimp WOW addicted Join Date: Dec 2001 Location: the moon Age: 44 Posts: 6,525 Trader Rating: (0) Feedback Score: 0 reviews	just cause the throttle plate is closed doesn't mean air doesn't get in the combustion chamber.. I would think under engine braking when your RPM's are above normal idle you are consuming more air/fuel due to rpm being higher.. this would require more fuel or else you may cause detonation. Seems that cutting fuel with higher rpms while engine braking could cause engine failure. Am I completely off base here? just curious.. what circumstances cause you to ask this question sixbanger? __________________

04-05-2003, 06:50 PM	#3
AKADriver Post Whore! Join Date: May 2002 Location: Springfield, VA Age: 43 Posts: 3,517 Trader Rating: (1) Feedback Score: 1 reviews	I think that's a pretty standard thing for an injected engine to do, since there's no fuel demand when the throttle is closed (except at idle).

04-06-2003, 01:22 AM	#5
AKADriver Post Whore! Join Date: May 2002 Location: Springfield, VA Age: 43 Posts: 3,517 Trader Rating: (1) Feedback Score: 1 reviews	A small amount of air is getting in via the idle control circuit, pretty much, but if you have a boost/vac gauge you'll see vacuum shoot up to -14.7 psi more or less with the throttle off. A bit of vacuum is lost to the vacuum operated accessories. No fuel = no detonation! With no fuel moving through a spinning engine it's just acting as an air pump. You need fuel to spin the engine at idle because the engine is under the load of... itself. At deceleration the engine is under negative load, the engine is being spun by the momentum of the car. Someone with something like an S-AFC should chime in here, doesn't that display injector duty cycle?