Does fuel get injected upon deceleration?

[gregoron]

My guess is that there is still fuel injected even after you lift off your foot from the accelerator, it just goes through the idle regulator. I read somewhere that if you want to save gas, wait till you almost stop before stepping on the clutch and disengaging the gear as no fuel enters the cylinders. Which is it?

Edit: this is on fuel injected engines.

 

[Jay1981]

If no fuel is entering the cylinders, the engine would not be running.

 

[eljefino]

On my car the injector pulse width goes to zero on decel if it's over 1300 rpm.

The engine is turning b/c inertia is backfeeding through the manual transmission and clutch. So the power steering, brakes, and alternator are all still working. Shoot, the spark plugs are still firing.

When I declutch, the injector pulse width comes back on. This has been pretty much standard since EFI came out. My car's OBD-I by the way.

 

[kr_bitog]

it depends on the ECM, some cars will cut the fuel injected when you decelerate with foot off the pedal for 2 or 3 seconds until the rpm hit 1500 or 2000 rpm. Some will cut for 1 second and start min idle injection amount. If you have the wireless OBD/CAN, you can notice this easily while you are driving

 

[RiceCake]

Most automatics have deceleration cutoff programs in the ECU, the engine is spun by the transmission. In a standard, if you push the clutch, of course its using gas; else how does the engine stay running?

You're right though, idling down, in gear, will use less fuel. Theoretically though, having a way to cut the engine off entirely, coasting down, and then restarting the engine by releasing the clutch would let the car roll further and save even more gas...but that starts getting into the rhelm of ridiculous hypermiling.

 

[sciphi]

It depends on the car and the situation. I've noted the injectors being cut in my Cruze for 2-3 minutes at a time if descending a particularly long, steep hill and it's below 1800 RPM. The engine is still "running" since it's spinning and the electronics are monitoring the inputs to inject fuel as needed.

My old Buick never cut fuel. It scaled the injectors far back, but never fully cut them. It simply went to idle fuel injection rate, even if the engine was moving faster than idle but my foot was off the accelerator.

 

[The motor guy]

It just puts the same amount of fuel as if it were idling.

 

[MinamiKotaro]

Most cars will turn off the injectors at 0 throttle above a certain RPM.

 

[beast3300]

Yes, injectors can shut off during a long enough coasting above a certain RPM threshold. My Buicks, Olds, and Pontiacs all did this, even my 91 Olds. You can see it in the tach. The RPMs will die down towards idle then "catch" and the injectors are back on during the "catch" part.

It's easier to see on a live data stream from the ECM/PCM.

 

[OB4x4]

On my '00 tacoma if throttle input is 0, and RPM 's are above 1500 then the inj pulse width goes to zero.

 

[tinmanSC]

My Lexus SC300, manual, has a pronounced fuel injector cut after 2 seconds or so of coasting with the clutch engaged. If you let off the gas, and the engine is still spinning above 1500 RPM or so, the injector pulse width cuts to 0 after 2 seconds, and you can feel the cut when driving. The fact that I could feel it lead me to investigate what was happening, upon seeing the injector pulse width it was pretty clear what the ECU was doing. The ECU cuts fuel based on the throttle position sensor NOT the mass air flow meter. I'd imagine this is not an uncommon thing for ECUs. Engine tuners can run into a problem when setting the coast fuel cut threshold for TPS voltage.

 

[gregoron]

Good to know this. I always thought there's still combustion going on even if car is decelerating - foot off accelerator.

I'm talking about my 2k VW Jetta 2.0 w/ 5 speed trans. I guess I should get myself an OBD or VAGCOM to see if its injectors cut off at certain engine speeds upon declaration. It's been a habit of mine to keep car in gear when slowing down then press the clutch at around 1500 - 1000 rpm.

 

[Shannow]

Originally Posted By: RiceCake
Most automatics have deceleration cutoff programs in the ECU, the engine is spun by the transmission. In a standard, if you push the clutch, of course its using gas; else how does the engine stay running?


Because at 0 throttle, no load, the engine is idling. When zero throttle, and above idle RPM, the engine is being driven by the drivetrain (term is called over-run), the fuel can, and generally is cut...dip the clutch, and you are back at idle

 

[mechanicx]

Even with deceleration fuel cutoff/idle pulse width when lifting off the throttle with a manual in gear you won't necessarily get better gas milage than declutching and/or shift into neutral because of engine breaking in gear will reduce how far/fast you can coast. That's why for economy many automatics unlock the torgue converter and idle down when lifting off the throttle. Of course how much engine breaking you have depends on how low of gear you are in.

I think leaving a manual in gear while decelerating is for other purposes besides fuel economy.

 

[JHZR2]

Originally Posted By: eljefino
On my car the injector pulse width goes to zero on decel if it's over 1300 rpm.


Agreed. This is what I see, 1200-1300 is the cutout for operating stability.

Some cars show near infinite MPGs above that, others dont show as high (my 98 S-10 for example), for some reason...

 

[JHZR2]

Originally Posted By: mechanicx
Even with deceleration fuel cutoff/idle pulse width when lifting off the throttle with a manual in gear you won't necessarily get better gas milage than declutching and/or shift into neutral because of engine breaking in gear will reduce how far/fast you can coast. That's why for economy many automatics unlock the torgue converter and idle down when lifting off the throttle. Of course how much engine breaking you have depends on how low of gear you are in.

I think leaving a manual in gear while decelerating is for other purposes besides fuel economy.


Yes it is a case by case scenario. If I need to bleed a lot of momentum, staying in gear is sound because of the injector cutoff and the braking effect of pumping air. If the scenario dictates that I dont need to bleed much or any, Ill usually take it out of gear to conserve what I paid for in fuel. Usually MPG meters will show >75MPG in those cases still, so its not bad, and less wear overall I guess...

It really is a case by case thing, being smart with momentum and thinking about next moves. Too many just mash the go pedal and brake, especially in AT cars, because it lets them.

 

[RF Overlord]

Are we talking about manual transmission cars only? Most automatics don't have a rear pump (which is why you can't push-start them), so the engine can't be turned by the drivetrain. The injectors would have to remain minimally active during decel.

 

[RiceCake]

Originally Posted By: RF Overlord
Are we talking about manual transmission cars only? Most automatics don't have a rear pump (which is why you can't push-start them), so the engine can't be turned by the drivetrain. The injectors would have to remain minimally active during decel.


I think he was specifically referring to a standard transmission (hence, clutch), but since then everyone's covered everything

Fun fact if you kill the engine, in gear, while driving in an automatic...you actually can "bump start" the motor since the transmission holds it together and keeps the engine rotating...I'm sure the lockup clutch and whatever else is part of the system in there though is none too fond of this procedure...

 

[mechtech2]

Most cars now cut fuel delivery on decell.
But it is controlled. Below a certain RPM, like around 1,500 to 2,000 RPM, fuel begins to be admitted again.

 

[Shannow]

Originally Posted By: RF Overlord
Are we talking about manual transmission cars only? Most automatics don't have a rear pump (which is why you can't push-start them), so the engine can't be turned by the drivetrain. The injectors would have to remain minimally active during decel.


Part true, they no longer have a rear pump, but when you go from driving to over-run, the whole T/C is/has been spinning at whatever revs,and the pump is still pumping, the clutches and bands are still engage, and the drvetrain still spins the engine.