Fuel Injector shut-off while Coasting

[kamesama980]

Originally Posted by franko422

I have a 2009 Ford Focus five speed and when I coast, the rpm bounces around between 1000 and 2000. This happens most of the time under all weather conditions. A Ford tech took it for a ride with his computer and his determination was that the catylitic converter was telling the engine that it needed more oxygen to burn off pollutants, so the engine was speeding up and running very lean to pump more oxygen. I'm not satisfied with that answer. If that is true, it still ticks me off that it does that. So, if I never coasted, the pollutants would never get burned off? And what would keep the converter from telling the engine to speed when the car is in gear? Anyone else experience this?

BS the cat can't tell the computer anything. the post cat-O2 sensor is the only thing that could and part of drivability testing/engineering is that cars DONT do what you're saying the RPMs are doing. besides, the ECU won't bring the revs up at idle to get the cat "more oxygen" go to a different place to get a diagnosis. IME most dealer techs aren't any better than the mom n pop shops except they have the fancy mfr tools and a patch that says "trained" (I worked in a dealer for a while, that's part of why I left...no standards)

As to the OPs question of if DFCO is only on older cars, in 4 years at school for automotive tech (and a few more working in shops outside of school and on my own) I haven't yet found a fuel injected car that didn't DFCO. Some have a delay or more criteria to be met before it'll happen, but they will all do it. Air going through the engine, even without fuel, picks up a LOT of heat, while it's not the 1000+f with load, it doesn't cool the cat off fast enough to be a problem. the SG does register DFCO on my fiancee's 03 protege and it'll DFCO from 80 till you can't keep the revs up (including downshifting the auto to keep them up). coolant drops 3-4 degrees on a cool day but it still DFCOs.

[slurp812]

2002 Civic Si, and the 2004 Monte Both do it. The automatic in the Monte makes it mush less likely to happen tho. Not really coasting, it only happens under engine braking.

[vfrjim]

I have a 2012 Mazda 3i (skyactiv) with 6sp MT and I can confirm that my fuel cuts off when coasting. I am using the APP Torque on my Tablet with a BT OBD-2 sensor. At idle (not coasting), my fuel rate is ~0.24 gph and it goes to 0.00 when in gear and I release the throttle.

[theholycow]

Cool. Have you detected any conditions where it does not do it so readily, such as a recent shift or electrical load from wipers and headlights? How low does RPM get before it cancels fuel cutoff?

[vfrjim]

Originally Posted by theholycow

Cool. Have you detected any conditions where it does not do it so readily, such as a recent shift or electrical load from wipers and headlights? How low does RPM get before it cancels fuel cutoff?

The only time that I noticed that it does not cut off is when you are shifting and you are in the wrong gear, for example, your doing 34 mph and lugging in 6th gear, then shift to 5th, but since the engine is still not under a proper load, you need to shift to 4th gear, then it will cut off, BUT if you shift to 5th and crack the throttle for a second, it will cut off. Another thing I noticed about the cut-off was when you are coasting down a steep hill (doing a high rate of speed such as 64 mph) and you push in your clutch (not in any gear) the flow rate goes from ~0.24 gph to 0.00 intermittently, the only reason I did not want to be in gear was that there was a hill in front of me that I needed the momentum to go up, really did not want to reduce my speed at all (love the 0.27 drag coefficient of this car!, it even has a belly pan built in from the manufacturer!) I never saw the cutoff go off, even in gear, when I was about to stop at a stop sign, it would stay off till I would stall if I did not push in my clutch!


Without the information from the OBD2 and the Torque app, I would never find out this info, it is a great way to get the info you need to drive your car to get the best mileage and recommend it (instead of the scan guage ) to anyone interested finding out more info about their car to optimize mileage.

Jim

[VanWylder]

Originally Posted by mtbiker278

I saw on a couple of threads that the fuel injectors on some autos shut down while caosting in gear. Does this apply to most newer cars? I've been coasting to stop lights and such in neutral cause my rpms stay at idle, but if the injectors shut-off while coasting in gear this would be pointless.

Anyone know more on this subject?

Its called over-run. Fuel is not injected. While I'm not sure which vehicles specifically this applies to, it definitely applies to my A4 TDI. Just to prove it to myself, I occasionally coast in neutral, and coast in gear to compare on the Scanguage II. Coasting in gear returns higher MPG due to no fuel being injected. YMMV

[Draigflag]

I find coasting neutral uses less fuel, due to the fact that on a hill you will travel faster and therefore further as the engine does not brake and slow the car down like it does in gear. There are plenty of large long hills here so the savings have been apparent for quite some time!

[theholycow]

Originally Posted by VanWylder

Its called over-run. Fuel is not injected. While I'm not sure which vehicles specifically this applies to, it definitely applies to my A4 TDI. Just to prove it to myself, I occasionally coast in neutral, and coast in gear to compare on the Scanguage II. Coasting in gear returns higher MPG due to no fuel being injected. YMMV

Mostly I've heard of it called DFCO (Deceleration Fuel Cut Off).

During the time in which you're doing it (which is technically engine braking, not coasting, even if it's so little braking that you can't feel it) you will use zero fuel. However, depending on the road ahead, neutral coasting can often beat DFCO when you look at more than just instant MPG.

DFCO needs the engine at higher RPM with more friction loss, more reciprocating loss, and a bunch more pumping loss. That energy comes out of the vehicle's inertia. Further on up the road you'll need to burn more fuel to make it up unless you were already going to brake anyway. Depending on the vehicle, the road ahead, and whatever other conditions, that energy can be more than the amount of fuel required to idle the engine or less. Very often it takes more fuel to re-accelerate the vehicle after DFCO than it would have to idle the engine for neutral coasting.

That's merely a narrow-view technical analysis of the fuel usage at stake using math and experience; if there are other issues at stake like legality or safety, those concerns come first.

[Draigflag]

No I get what you mean, the need to accelerate arrives sooner in gear, as the car will slow quicker, and accelerating uses more fuel than idling in neutral for sure.

[block]

Idle jumping around and mention of lean sounds like the focus has a vacuum leak, which is not unusual for them (I'm struggling to fix my V6 Contour which is also known for vacuum leaks).

If the FI shuts down, the engine is off - you ought to be able to hear/tell when your engine is off. I glided with my engine off but in gear (Contour V6 with 5 speed manual) a few times (to slow down at the bottom of a hill that has a stop sign) instead of staying in neutral like I usually do)) and it's obvious that the engine is off...the car slows down faster and you can hear the lack of detonation.

I have a long hill in a 45MPH zone on a country road on my way to work. Car keeps going right around 45MPH while gliding, so I often shut off the engine and glide in neutral..brakes and steering work fine (I turn it to "On" after turning the engine off).