Does fuel get injected upon deceleration?

[TFB1]

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.


Nope once the engine is operating the front pump, the transmission will spin the engine on coast with the injectors shut completely down... Don't believe me, cut the ign off for a few seconds(not to lock), and back on and the engine will be running...

My Marauder shuts down the injectors on coast above approx 1200RPM... All Fords since the '86 SEFI 5.0 have done same...

 

[gregoron]

So, when no fuel is being injected, the ECU would also disable the spark plugs. In a way, this cools down the cylinder chambers considerably since they're just acting as air pumps then. I wonder if oil seeps through the rings at this time when the cylinders and rings contract a little due to drop in temperature.

 

[Kestas]

My Volvo S40 cuts out fuel when coasting under these conditions:
- automatic transmission in Drive
- throttle at 0
- engine rpm above 1200

Even my old 1991 Marquis cuts the fuel down to a low level.

Some cars bleed a little fuel during coasting to keep the catalytic converters warm for emissions. I wish I could diable this feature in the programming.

 

[mechtech2]

Originally Posted By: gregoron
So, when no fuel is being injected, the ECU would also disable the spark plugs. In a way, this cools down the cylinder chambers considerably since they're just acting as air pumps then. I wonder if oil seeps through the rings at this time when the cylinders and rings contract a little due to drop in temperature.


Why would the plugs be disabled?

 

[morris]

the Chrysler Turbine cars 1963-1965 turned the fuel off on deceleration, to try to get some braking from the turbine engine. cause the trans was the pre 1966 trans it had a rear pump that kept the engine turning. and the spark plugs was kept on all the time.

 

[gregoron]

Originally Posted By: mechtech2
Originally Posted By: gregoron
So, when no fuel is being injected, the ECU would also disable the spark plugs. In a way, this cools down the cylinder chambers considerably since they're just acting as air pumps then. I wonder if oil seeps through the rings at this time when the cylinders and rings contract a little due to drop in temperature.


Why would the plugs be disabled?


Actually, I meant to phrase that as a question. Would plugs be disabled by the ECU if no fuel is injected since there's no combustion during the power stroke?

 

[RobertISaar]

depends on the calibration...

all of the GM vehicles using the wastespark DIS setup, when DFCO is entered, there is still spark being commanded, though usually at a significantly different amount of advance. common numbers i see are 10* BTDC when in DFCO.

applications beyond that(especially newer stuff), i wouldn't know.

i tend to leave the torque converter locked on decel for more engine braking, especially once DFCO kicks in... if the torque converter is still locked when DFCO is disabled, you'll usually feel it happen. if it's unlocked by that point in time, you'll only see/hear it.

 

[javacontour]

As an aside, I could push start my Powerglide equipped 66 Nova from a full stop. So some automatics did support push starting.

End of thread-jack!

 

[meep]

powerglide had a rear pump. I had one in the '57. Had to hit 25+ to get it to fire though.

They cut fuel due to emissions. OR they have to boost fuel/mix via IACV.

Idle trim at over-idle rpm results in extra vacuum, minimum compression and very incomplete fuel burn. So in carbuerated vehicles the solution was to throw more fuel/air in to raise the compression for better burning (remember all of those vacuum lines...). That was a crummy way of doing it... FI/ECU allows a more elegant defueling.

M

 

[D189379]

My Frontier cuts fuel, but doesn't cut it right off. I was disappointed to find out that coasting down a hill with the clutch in actually uses less fuel that coasting down it in gear. That's on of my my favorite things about driving a manual, but its too bad I'm actually using more fuel driving this way.

I always thought fuel was completely cut during deceleration in gear. I guess all vehicles are different.

 

[gathermewool]

Originally Posted By: D189379
My Frontier cuts fuel, but doesn't cut it right off. I was disappointed to find out that coasting down a hill with the clutch in actually uses less fuel that coasting down it in gear. That's on of my my favorite things about driving a manual, but its too bad I'm actually using more fuel driving this way.

I always thought fuel was completely cut during deceleration in gear. I guess all vehicles are different.


How were you monitoring this, and at what RPM were you engine-braking?

 

[SteveSRT8]

Originally Posted By: JHZR2
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.


Exactly. Most folks simply press and go. No thought required!

And "case by case" is very informative as programming strategies vary wildly by manufacturer.

 

[D189379]

Originally Posted By: gathermewool
Originally Posted By: D189379
My Frontier cuts fuel, but doesn't cut it right off. I was disappointed to find out that coasting down a hill with the clutch in actually uses less fuel that coasting down it in gear. That's on of my my favorite things about driving a manual, but its too bad I'm actually using more fuel driving this way.

I always thought fuel was completely cut during deceleration in gear. I guess all vehicles are different.


How were you monitoring this, and at what RPM were you engine-braking?


I have a computer programmer that monitors fuel flow. Anywhere from 2500-3500rpm going downhill/coming to a stop at a light. I tested it many times as I didn't think it should use more fuel, but sure enough, it did.

 

[gathermewool]

Originally Posted By: D189379
Originally Posted By: gathermewool
Originally Posted By: D189379
My Frontier cuts fuel, but doesn't cut it right off. I was disappointed to find out that coasting down a hill with the clutch in actually uses less fuel that coasting down it in gear. That's on of my my favorite things about driving a manual, but its too bad I'm actually using more fuel driving this way.

I always thought fuel was completely cut during deceleration in gear. I guess all vehicles are different.


How were you monitoring this, and at what RPM were you engine-braking?


I have a computer programmer that monitors fuel flow. Anywhere from 2500-3500rpm going downhill/coming to a stop at a light. I tested it many times as I didn't think it should use more fuel, but sure enough, it did.


That is strange. I haven't checked the exact cut-off point, but my Subie will go into OL and IDC will go to 0 at anything > 2k RPM, IIRC. I had my accessport hooked up while decelerating downhill tonight, but I forgot to check.

 

[Cujet]

There are a number of Ford cars that DO NOT cut fuel under throttle lift. This means that matching speed in congested, high speed traffic sometimes requires brakes, ugh.

One thing you can do, if you don't have some form of ECM display, is to shift to neutral. Cars that don't cut fuel will often run at 1500 or so RPM while you decelerate.

 

[meep]

^^^ good point... i've had 2 jeeps that seem to boost fuel on coast. both idle up until below ~7mph or so. the minivan is hard to tell.... it seems to "coast" forever at 35 mph... making me wonder if if also boosts idle some.

the tundra may cut fuel. a couple seconds after starting to coast, you can feel engine braking increase.

 

[TFB1]

Originally Posted By: Cujet
There are a number of Ford cars that DO NOT cut fuel under throttle lift. This means that matching speed in congested, high speed traffic sometimes requires brakes, ugh.

One thing you can do, if you don't have some form of ECM display, is to shift to neutral. Cars that don't cut fuel will often run at 1500 or so RPM while you decelerate.


Then there a problem with your Ford's, no FI engine Ford I've owned in the last 20 years(something like 35 vehicles) did so unless, the TPS, IAC had issues or it had a vacuum leak... The RPMS do kick up slightly on the older EEC-IV systems just prior to stopping, but still if it's requiring extra braking effort there is something amiss...

 

[Rickey]

"Does fuel get injected upon deceleration?"

Please understand that the following is based on my experience with late model gasoline powered GM passenger vehicles.
I can't comment on other brands.

The fuel IS cut off on in gear coast or whatever you would like to call it.

However this has potential emissions consequences.

If the fuel is cut off for an extended time as in the case of an extended downhill grade; the catalytic converter(s)can fall below their ideal operating temperature range.

To address this GM programs their ECM to intermittently fuel the engine just enough to keep the catalyst "lit-up" and ready to go.

Good day.

Rickey.

 

[SteveSRT8]

Many people confuse the cars programming for an issue.

The elevated idle speed many experience during coast is an effort to alleviate emissions and/or stalling issues while decelerating. The IAC or the actual throttle are not allowed to slam shut by the ECU deliberately.

In most cases this is not a fault and you will quickly set a code if the commanded idle does not match up to actual.

 

[Cujet]

Originally Posted By: TFB1
Originally Posted By: Cujet
There are a number of Ford cars that DO NOT cut fuel under throttle lift. This means that matching speed in congested, high speed traffic sometimes requires brakes, ugh.

One thing you can do, if you don't have some form of ECM display, is to shift to neutral. Cars that don't cut fuel will often run at 1500 or so RPM while you decelerate.


Then there a problem with your Ford's, no FI engine Ford I've owned in the last 20 years(something like 35 vehicles) did so unless, the TPS, IAC had issues or it had a vacuum leak... The RPMS do kick up slightly on the older EEC-IV systems just prior to stopping, but still if it's requiring extra braking effort there is something amiss...


Wrong. Fact: Many fords don't cut fuel under certain deceleration conditions, using modest throttle and fueling to keep the catalytic converters at operational temperatures.