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Old 10-16-2012, 03:51 PM
 
Location: SF Bay Area
5,996 posts, read 13,830,975 times
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This was a topic on my vehicle specific car forum and since no one could give a concise answers I wanted to see what people here thought.

The question was, is it possible for the ECU to cut fuel (reduce injector pulse width to zero) while the car is coasting under the condition that revs are above a certain point (say 1200 rpm). The momentum of the vehicle and transmission linkage (a manual in his scenario) will rotate the flywheel and keep engine revs at a suitable level for that gear/speed. If the clutch is in, revs drop below 1200rpm and the injectors begin to fire to prevent a stall or if the throttle is pressed, the injectors fire up as well.

The second question was, do any modern vehicles employ this type of fuel cut? (I've heard that some older OBD1 vehicles have done this)

The responses we had ranged from yes its possible, to the ECU drastically shortening injector pulse width yet still firing, to some saying that it would not be possible for the engine to run at all without at least some fuel.

I'm open to being wrong but don't see why this isn't possible.

Last edited by iTsLiKeAnEgG; 10-16-2012 at 04:03 PM..
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Old 10-16-2012, 04:50 PM
 
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Fuel cutoff when decelerating or traveling downhill (in gear, of course) is pretty much standard on most vehicles built with modern electronic fuel injection systems. Hook up a ScanGauge to the OBDII port and monitor the fuel consumption in gallons per hour when driving--it will frequently drop to zero (shown as 9999 on the ScanGauge) when the vehicle is traveling downhill or decleratibng. As the vehicle nears a stop, the EFI will begin injecting fuel again, so that the engine won't die.
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Old 10-16-2012, 05:03 PM
 
Location: SF Bay Area
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One of the members on our Genesis Coupe forum claims that injector pulse width reads very low (0.002) at 3k rpm off throttle which indicates although little fuel is used, the injectors are not completely off. His particular car has an auto trans so I'm wondering if a manual trans would product different results.
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Old 10-16-2012, 05:11 PM
 
Location: Center Township (Pittsburgh), PA
556 posts, read 997,984 times
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Yes. It's called DFCO (deceleration fuel cutoff) and almost every modern car on the road uses it. The injectors are disabled on deceleration.
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Old 10-16-2012, 05:23 PM
 
Location: SF Bay Area
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Quote:
Originally Posted by SK360 View Post
Yes. It's called DFCO (deceleration fuel cutoff) and almost every modern car on the road uses it. The injectors are disabled on deceleration.
Beautiful, I found a thread on our forum using DFCO as a search term. It appears that with the Gen Coupe pulse width and injector duty cycle are drastically reduced without ever fully shutting off.
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Old 10-16-2012, 05:24 PM
 
Location: Texas
203 posts, read 439,487 times
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My 2008 Ford Ranger does this and you can feel when they cut off.
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Old 10-16-2012, 05:45 PM
 
11,256 posts, read 43,368,175 times
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Zero fuel delivery makes for an abrupt "tip in" for the injectors when the engine starts getting fuel delivery again; it's much smoother to have a small amount of fuel going into the engine even when the throttle is at idle and the car is decelerating in gear. All depends upon the fuel management strategy chosen by the manufacturer .....
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Old 10-16-2012, 07:10 PM
 
Location: Sarasota FL
6,860 posts, read 9,282,187 times
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Porsche used injector shut off as far back as 1973. I owned a 1973 911T with mechanical injection. Each cyclinder had an air intake and a in head injector. The was a controller called a 'transducer' about the size of a pack of cigarettes. It was just an electromaget for the tach and a small printed circuit with a few transisters for solenoid control] It controlled the tach and the injector solenoid. When you took you foot off the accelerator, the solenoid would retrack, shutting off the injector pump. When the tach dropped to 1200 rpm [no matter the vehicle speed] the transducer would send a signal to the solenoid to allow the injector pump to operate so the engine would not stall.
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Old 10-16-2012, 08:19 PM
 
Location: U.S.A.
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Each manufacturer is going to have their own method of cutoff or reduction; the thermodynamic properties of different engines require that some maintain a combustion cycle while others do not.

Whatever happened with your "hot start" issue?
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Old 10-16-2012, 10:03 PM
 
Location: SF Bay Area
5,996 posts, read 13,830,975 times
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Quote:
Originally Posted by Lux Hauler View Post
Each manufacturer is going to have their own method of cutoff or reduction; the thermodynamic properties of different engines require that some maintain a combustion cycle while others do not.

Whatever happened with your "hot start" issue?
Still no resolution on hot start. There's a lot of hypothetical fixes but nothing concrete. Some members have claimed that an intake manifold spacer (available for sale) fixes the issue which would make sense if this is a temperature problem. Another idea is that this is simply a tuning issue. I'm going to let it ride out until we have something more concrete rather than throw money at random fixes.
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