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Is this for real, and how does it work?

The other day, while browsing discussions, I came across a comment somebody posted which I have been pondering all day and have not been able to figure out, mechanically. It’s nothing new, as I have seen the same statement made by a few other posters, including regulars, since I started posting in 2007, but never gave a tremendous amount of thought to it, but for some reason, it’s bugging me now.

The jist of the statement is this: on any fuel injected car made in the last 15-20 years, during deceleration, the ECM completely shuts down the fuel system to save fuel because the engine does not need fuel to run unless under load. I cannot understand how this can be so. It seems to me that the engine would promptly stall if the fuel were taken away due to the need for combustion to overcome resistance from friction, compression, and the valvetrain. I understand that the momentum of the vehicle could keep the engine turning as long as the clutch is engaged or the torque converter locked, but it seems like this would be difficult for the electronics to manage given such variables as the possibility that the driver may shift to neutral or depress the clutch, which would cause the engine to stall if fuel were not demanded immediately as this happened. It also seems like this would make for a pretty rough ride under deceleration. I considered turning off the ignition in my truck while in gear under deceleration (M/T) to see if I noticed a difference, but chose not to.

So, the question is: is this fact or fiction, and if it’s fact, how does it work while maintaining the driveability of the vehicle and preventing constant stalling?

Can’t reply to your comments but here is another view. Many if not most posters here say that fuel is cut off during deceleration in reply to those who wonder if they would save fuel by coasting in neutral when possible to permit the engine to idle rather than run at a higher speed to use more fuel. During a long period of coasting such as down a mountain, it would seem that some fuel should be burned to keep the cat converter hot and working for when the accelerator is again applied or else a puff of unburned exhaust pollutants of an unknown time duration would occur. A portable oscilloscope to measure fuel injector voltage would answer the question but I don’t have one.

The concept is actually very simple. The EFI system matches the fuel load to the air allowed into the intake system. The throttle valve controls the flow of air. The throttle in most EFI cars actually shuts nearly completely when the throttle is released. The idle speed is controlled with the Idle Air Controller (IAC), which by-passes air around the throttle valve and meters the right amount of air to allow the computer to control the idle speed.

When you are decelerating, with RPMs above 2000, and the throttle valve is closed, the computer will shut the IAC and cut all air going into the intake. The EFI will then cut the fuel going into the cylinders. The IAC will crack back open when the RPMs drop to something less than 2000 RPM, and the fuel will meter in to match the air.

I also suspect that on long down hills, to keep the cylinders and cat warm, it may also crack open to allow some fuel to be burned on an intermittent basis. But, I do know that the fuel cut does work. And so seamlessly, that the engine will still react the instant you apply any throttle.

One thing you have to consider here is, the computers and sensors that control engine functions can monitor and correct things thousands of times a second. The fuel system can respond much more quickly than needed to keep the engine from dropping out.

On my Riviera with the on board touch screen, I used to be able to go into diagnostic mode and watch the readings for the injector pulse width while driving. This is the length of time the injectors are open for each pulse. It would often go down to zero for an instant when you let off of the gas pedal.

If the engine shuts off on deceleration then I ask where the vacuum to operate the power brakes comes from.

In response to ok4450, vacuum is abundant so long as the throttle is closed and the engine is turning. You have a three liter vacuum pump there. The only thing that changes is that It is using the wheels and the momentum of the car to keep it running rather than gasoline…

On Bing’s Riviera, if the fuel injectors really do shut off fuel supply during deceleration, the pulse width should be zero continuously under deceleration. Also, it seems that in the case of an automatic transmission car, coasting should result in either no change in engine speed if the torque converter were locked, or an increase in engine speed if it were not locked before coasting since it would need to be locked to allow the momentum of the vehicle to run the engine.

Bear in mind this is not a requirement. It only does it when conditions allow. The computer programming is smart enough to know when it’s possible. Same as VCM mode on Honda engines. If the conditions are right for doing it, it will, to save on fuel.

This brings up the question of how are the wheels keeping the engine turning through an automatic transmission.
I had posed this scenario in an unrelated question before with no valid response and if I remember correctly, even Transman brought it up also.

Supposedly through the lock up mechanism in the TC.

Even without a lockup, there isn’t much slip in a torque converter beyond a certain RPM. If the car is pulling the turbine at 1,500 RPM, it’ll pull the impeller and the engine along for the ride, with some slip. If the car is pulling the turbine along at 2,500 RPM, the engine speed will be fairly close behind. Many newer autos downshift when on a downhill grade.

I’ve never seen a vehicle shut the fuel off to the injectors when coasting. Every vehicle I’ve driven goes to idle condition when coasting. First you need engine vacuum in order to operate the power brakes. Second you need the engine to turn fast enough to operate the power steering pump. And third, if the engine were to shut off the pump in the transmission would stop functioning. And on some vehicles it’s not recommended to allow the drive wheels to the transmission to turn the components within the transmission without the pump functioning otherwise damage can occur to the transmission.

Besides, in Minnesota it’s against the law to shut an engine off while driving on the road. Be it intentional or by design.


My 1985 Accord SEi with fuel injection and auto trans had fuel cutoff on decel.
Coasting down when the engine rpms dropped to about 1500 the engine speed would bounce back up a little when the fuel came back on.

I’m with TT, and would add that like most “universal” statements it isn’t really universal. It depends on the programming.

If one’s engine were locked to the tranny and the ECM were programmed to close th injectors when the throttle is closed, the MAF signal drops, the engine temp is at normal, and the crank VSS is still showing speed, it should work great. It could easily open the injector squarewaves if one of those changes. The inertia of the car would turn the crank to operate the alternator, the oil pump, the power steering pump (if applicable), the AC compressor, and the water pump, and all the circuits could stay enabled wnd eerything else run normally. The accessories don;t care what’s turning the crank to drive them, only that they’re being driven by the crank. Explosions or inertia, nomatter.

But I’m not sure all systems are designed that way. It seems like the wind and rolling resistance plus the compression would slow the engine too quickly to keep everything operating unless the engine were to al least be kept at an idle. I know for a fact that my car, a 2005, does not shut the injectors off when I take my foot off the pedal. At highway speed, the tranny disconnects and the RPM drops to a “high idle” position, and at low speeds it just goes to idle. If I’m in 3rd gear or below, it stays engaged with the tranny and I can use th ecompression to slow the vehicle.

There are too many different approaches and designs out there to make any assumptions. Every time I think I’ve seen it all I find out I haven’t. …not even close.

shutting off fuel to the injectors is not the same as shutting the engine off. The engine is still turning, and that turning can still do work such as driving the transmission pump, charging the battery, etc by using the kinetic energy supplied by the car’s motion. Because the engine is still turning, as soon as you put fuel back into the equation, you generate power through normal combustion. Cylinder fuel cut is not illegal, and is nothing close to the same as turning the engine off with the key.

Yup. Although I suspect that the ECU would take engine temp into the equation. For minimum emissions, the ECU will do what’s necessary to get and keep the engine at operating temp as quickly as poaaible and at all times. An ECU that might shut down the injetcors in 80F weather would. I’d expect, probably keep 'em operating at zero degrees all other conditions being the same.

It’s all in the programming. I know for a fact that mine don’t shut down, and therefore I know that the statement that they shut down cannot be applied universally.

I can understand the engine being turned by the driveline in a manual transmission car but I’ve never understood or heard it explained how this would occur with an automatic transmission.
A converter may lock but that doesn’t lock any clutch assemblies or keep them pressured up with fluid.

I think it may be possible that the ECM programming could alternately shut off select fuel injectors during coasting in order to conserve fuel, and this could keep the engine running and probably go unnoticed by the driver, but I don’t think it would be possible to completely shut off all the fuel injectors. I also am not so sure about the idea that an automatic transmission can transmit enough torque from the car’s momentum to keep an unpowered engine turning. If this were possible, it would also be possible to push-start an automatic transmission car (although I have heard this is possible with a Powerglide, but only because it has a pump on the output shaft). As for the idea of alternately shutting off select fuel injectors, I think this is actually how the top speed of newer cars is electronically limited (or “governed”, as some people like to say in my neck of the woods). That one I’m not certain of either, but in my memory, that’s what it felt like when I was a teenager and would get my hands on a late model car and decide to see how fast it would go.

I seem to remember reading a blurb about Chrysler offering something similar to this on some models recently but it involved mostly manual transmission vehicles. However, a few automatics were mentioned.

The part that was unclear to me was that the story stated that the “amount” was varied but the story did not specify exactly what is meant by that word. Length of injector duty cycle, varied fuel pressure, etc?

My Lincoln uses a pump driver module that varies the fuel pump speed and does this based on load, deceleration, etc, etc. This is referred to as Pulse Width Modulation but it does not shut the injectors off; it only alters the pump RPMs to cause subtle tweaks in the fuel rail pressure; all to supposedly aid in fuel economy, etc, etc.