Fuel shutoff on deceleration

[overst33r]

I borrowed my friends scangauge and noticed that my car doesn't shut off the injectors on deceleration. Car is a 2002 Z06. My friends Yaris keeps them off all the way to 1500RPM.

I wonder why this is? I thought this was a technology implemented long ago in cars with EFI...

 

[JCH13]

One thing I have read is that it can be pretty rough on throttle response after fuel has been cut as all of the extra fuel that was sitting in the intake manifold can get drawn into the cylinder. I would imagine the spark gets cut as well, otherwise this extra fuel could make for a very lean burn.

The Corvette engineers were probably more concerned with throttle response than fuel economy, and vice-versa for the Yaris engineers. Just a thought.

 

[cprince]

One thing I have read is that it can be pretty rough on throttle response after fuel has been cut as all of the extra fuel that was sitting in the intake manifold can get drawn into the cylinder. I would imagine the spark gets cut as well, otherwise this extra fuel could make for a very lean burn.

The Corvette engineers were probably more concerned with throttle response than fuel economy, and vice-versa for the Yaris engineers. Just a thought.

I think you're correct. My Pontiac G8 GT will deactivate to 4 cylinders, but never cut fuel off completely on deceleration.

 

[overst33r]

One thing I have read is that it can be pretty rough on throttle response after fuel has been cut as all of the extra fuel that was sitting in the intake manifold can get drawn into the cylinder. I would imagine the spark gets cut as well, otherwise this extra fuel could make for a very lean burn.

The Corvette engineers were probably more concerned with throttle response than fuel economy, and vice-versa for the Yaris engineers. Just a thought.

Could be. Though my S2000 cut them as well, which I'm sure throttle response was a key design spec as well.

 

[imagoon]

How are you testing this? Some cars only cut while in gear (IE idle revving won't cut them) also some cars will show on the scan tool a duty cycle that is actually to low for the injector to fire but still isn't zero. With some vehicles the cut off gets pretty complex with engine RPM, gear, vehicle speed, air and engine temp all playing a part in it.

 

[overst33r]

How are you testing this? Some cars only cut while in gear (IE idle revving won't cut them) also some cars will show on the scan tool a duty cycle that is actually to low for the injector to fire but still isn't zero. With some vehicles the cut off gets pretty complex with engine RPM, gear, vehicle speed, air and engine temp all playing a part in it.

In gear, throttle plate closed. Read out would show 9999 instantaneous if injectors were closed, but it shows up to 20-70mpg depending on the speed. It's very noticeable, because the engine braking isn't as strong as it is in cars with the injectors off. I realize there's other variables, but it's related.

 

[Gillbot]

Your PCM should have DFCO enabled. Has your PCM been altered in any way?

 

[imagoon]

In gear, throttle plate closed. Read out would show 9999 instantaneous if injectors were closed, but it shows up to 20-70mpg depending on the speed. It's very noticeable, because the engine braking isn't as strong as it is in cars with the injectors off. I realize there's other variables, but it's related.

You are likely misinterpreting the gauge then. Instant @ 9999 doesn't automatically mean they are off and there are plenty of cars that will not show 999 or 9999 because the ECU itself either a) limits the number by simply cutting it off at an engineer specified number b) the ecu does some other things that limit the number.

What is the injector pulse with when you are doing this? Is it below the injectors required fire pulse width? If so there is zero fuel going in to the engine at that point and the instant number would not show 9999 because the injector pulse width is not zero. I looked up the DFCO on corvettes and it only activates in specific conditions and less often than an auto vs manual. Unless you are going really fast, the engine speed is low enough in most gears that you would need pop the car in to second or third (auto) to see it and 3rd and 4th in a manual. IE get the engine revs up and holding at 4k etc.

The way you know the vet is in DFCO is to watch it read values from the DFCO cells in the ECU. The car not going in to DFCO when it supposed to could indicate engine trouble also.

 

[overst33r]

Your PCM should have DFCO enabled. Has your PCM been altered in any way?

Not that I know of.

You are likely misinterpreting the gauge then. Instant @ 9999 doesn't automatically mean they are off and there are plenty of cars that will not show 999 or 9999 because the ECU itself either a) limits the number by simply cutting it off at an engineer specified number b) the ecu does some other things that limit the number.

What is the injector pulse with when you are doing this? Is it below the injectors required fire pulse width? If so there is zero fuel going in to the engine at that point and the instant number would not show 9999 because the injector pulse width is not zero. I looked up the DFCO on corvettes and it only activates in specific conditions and less often than an auto vs manual. Unless you are going really fast, the engine speed is low enough in most gears that you would need pop the car in to second or third (auto) to see it and 3rd and 4th in a manual. IE get the engine revs up and holding at 4k etc.

The way you know the vet is in DFCO is to watch it read values from the DFCO cells in the ECU. The car not going in to DFCO when it supposed to could indicate engine trouble also.

Interesting. Thanks. I'll look into it some more, now that I have the proper acronyms. I don't know anything about the injectors activity, I don't have the equipment to monitor them.

 

[JCH13]

Interesting. Thanks. I'll look into it some more, now that I have the proper acronyms. I don't know anything about the injectors activity, I don't have the equipment to monitor them.

If you want to, get or borrow an MSD DashHawk. A very fun little tool/multi-gauge to have, and can display and record these types of things.

 

[overst33r]

Apparently DFCO on a factory tune only kicks in when you are slowing down from above 2100 rpms. Fuell will be turned back on below 1900 rpms. You also have to be above 30 mph.

I'll keep an eye out for these parameters and report back.

 

[Zenmervolt]

Apparently DFCO on a factory tune only kicks in when you are slowing down from above 2100 rpms. Fuell will be turned back on below 1900 rpms. You also have to be above 30 mph.

I'll keep an eye out for these parameters and report back.

Yup, factory settings vary quite a bit. Many even vary between automatic and manual versions of the same car (for example, in a manual S70, fuel cuts off if you're slowing from above 2,100 RPM, but with the same car and an automatic, you have to be above 2,400 RPM). It's all about figuring out how the settings are for the specific car.

ZV

 

[thomsbrain]

The Scangauge also has a parameter designed to change its internal sensitivity to the throttle being closed and the resulting "9999 MPG" display. They say some cars may need this adjusted for the gauge to respond when the fuel is cut. On both my Hondas it reported properly out of the box.