Curious about decel fuel cut-off, AFM/DOD

[f5journal]

So, I have the LS3 M6 with the decel pop due to fuel cut-off. This got me to thinking about the concept and how it works.

My assumption is that when you let off the gas, a few seconds later the fuel is cut-off to the engine by a solenoid or something. So technically the engine is not running and the back wheels are pushing the engine. Correct?

All 8 cylinders??

Are the spark plugs firing? If so, isn't this drying up/wearing on the cylinders?

This also must be the reason for the odd dragging effect that kicks in a few seconds after you lift on the gas that feels like somebody is pressing the rear brakes just a tad

This condition obviously must also take into account that the car is moving, the clutch engaged and the transmission is in gear. Otherwise the engine would quit running (turning) altogether if you were pop it out of gear or press the clutch. Right? Otherwise the car would not start when it is sitting still.

Giving it is some more thought, the whole deal is not very far removed from the DOD/AFM on the L99...just a little backward in programming.

Any thoughts or corrections to my thinking. Thanks

[Jayrcr3]

The engine is still running. It just goes very lean. When data logging I'm seeing around 25:1 AFR with DFCO active.

[f5journal]

Thanks..that's interesting

[Jayrcr3]

I've never really paid attention to them, but there are some parameters that monitor the clutch pedal position. I'm not sure if they are active in the Camaro though.

[14pilot]

Originally Posted by Jayrcr3

The engine is still running. It just goes very lean. When data logging I'm seeing around 25:1 AFR with DFCO active.

So if the tune was changed to be around 9:1 AFR with DFCO active, that might cool down the engine during decel, coasting? Oh, and eliminate the decel popping too?

Instead of saving fuel, you could use fuel to reduce engine temps and put an end to popping.

[DSX]

I'll clear this up as best I can...

DFCO is a fuel saving technique. It completely shuts down fuel delivery when active. There's no solenoid or anything, the PCM just doesn't command a pulsewidth to the injectors. As a result, you save gas while decelerating, and you also get much more effective engine braking (the dragging feeling you get). This is from the extreme vacuum the engine pulls. Every cylinder shuts down on fuel delivery. The popping you hear are lean pops from fuel boiling off of the back of the intake valves. The spark plugs are still firing, although the timing is retarded back. This doesn't hurt anything.

DFCO does not kick in the instant you let off the gas. There is a slight delay. Certain criteria have to be met to enter DFCO... vehicle speed, engine load, engine speed, etc. DFCO will not be active while the clutch is pushed in, and generally will not be active below a certain rpm. There is a re-enable rpm that will cause fuel to be injected again when the engine speed drops under this speed.

Yes, you can disable DFCO to get rid of the popping completely. Personally, I like heavy engine braking for the minor boost in fuel economy, but also the feel... It is a free way to take some load off of the brakes. You can't really just go in the tune and say "Ok, just run this AFR at this point!" and do what you're suggesting (directed at 14pilot). Personally, I'd never dump fuel like that... EVER. That's a great way to completely foul your spark plugs and wear out your rings (gas is abrasive).

[14pilot]

Originally Posted by DSX

You can't really just go in the tune and say "Ok, just run this AFR at this point!" and do what you're suggesting (directed at 14pilot). Personally, I'd never dump fuel like that... EVER. That's a great way to completely foul your spark plugs and wear out your rings (gas is abrasive).

Everything I posted was in the form of a question and not a suggestion. Asking because I've been involved with the tuning of roughly 400 modern fuel injected sportbikes. We are in Arizona where its 110 degrees today. We always add fuel in into the decel areas of the tunes to all favors of these sportbikes. This stops the decel popping and helps cool the engine. Both my own personal bikes were tuned this way by me. It works and I've measured the lower water temps. My bikes have been running this way for about 4 years, both run great and are on the original spark plugs.

We don't tune an AFR during decel, we tune for water temps. My AFR reference was to demostrate the point, and not to reach to the technical specifics of how this might be achieved.

I also do not understand the statements about how "dumping" fuel will foul plugs and wear out rings. I can see the potenial for this, with the throttle closed, little air coming in comparatively, and the extra fuel causing a "wet" condition in the combustion chambers. But in the practice of doing this on many sportbikes, we have not seen these symptoms on any of our own sportbikes or our approximate 400 customers bikes over the past 4 years of using this tuning method.

I realize that my Kawi Ninja is not a Camaro. A sportbike's combustion chamber would be 1/2 to 1/3 the size of a car engine's and the RPMs would be double or even triple the RPMs of a car engine during decel.

Both bikes and cars have ECU controlled fuel injection systems, are 4-cycle internal combustion engines (spark plugs and rings) so looking to learn here.

Then I'd ask the question, can the Camaro be tuned to pulse the injectors during decel? Thanks...

[DSX]

As you said, these are cars, not bikes. Cars with much larger capacity cooling systems and much larger cylinders.

Yes, you can just turn off DFCO and it will continue to inject fuel during deceleration.

[MichaelM]

If you hate Deceleration Popping, Read This.

I have a 2011 LS3 with a CAI intake and Corsa catback exhaust. Personally, I hate the deceleration popping cause by the Deceleration Fuel Cutoff settings in the computer. In my personal opinion it makes the car sound more like a Harley motorcycle or an old car that is a desperate need of a tune up. Don’t misunderstand, I love a deep powerful sound and a bit of a deep burble when decelerating, not a popping, backfiring type of sound.

I discussed this with my local dealer service manager and he looked into it with GM. He learned that GM is aware of the issues and has received a few complaints from people who upgraded their exhaust. He said GM may consider looking into a solution but no commitment on timing.

I know that a custom tune solves this problem, but I am not ready to sacrifice my powertrain warranty. I would like GM to make adjustments in their tune.

So … if you hate this as much as do, I suggest that you ask your service manager to also check with GM and raise a concern. The more of us who raise the issue, the more chance GM will adjust the computer settings to fix this and we can maintain our powertrain warranty.

Please use this thread to post other ideas on getting our concern to GM including GM phone numbers, email address, etc. and any headway you made on resolving the issue.