Does anyone else get fed up with Decel Enleanment kicking in? (Page 1/3)
Patrick JUL 01, 02:19 AM

I imagine most of you with a manual transmission 2.8 Fiero will be familiar with the sensation that occurs while engine braking (I don't believe the 2.5 has this issue), where once a factory set minimum RPM (or speed?) is reached while slowing down in gear, the engine will all of a sudden "kick" and increase RPM, even though your foot is still off the gas. (It might be occurring around 1500 RPM in 3rd gear.) It's annoying as hell.

When I'm engine braking, I want the engine braking to continue until I decide to step in the clutch and stop.

My '88 Formula has the 1227170 ECU and factory AKYN3442 PROM.

Who has the ability to burn a custom PROM with just this setting disabled and/or altered?

Any advice and/or discussion on this topic would be welcome.
fieroguru JUL 01, 08:16 AM
I think what you are describing is when Decel Fuel Cut Off (DFCO) is engaged during engine braking (to improve fuel economy). While DFCO is active, the injectors stop flowing fuel. There is a specified window of parameters when DFCO will be active and when it won't. At the lower limit of RPM or vehicle speed, DFCO will disengage and the injectors start firing again to avoid stalling out the engine. It will also disengage when you give it more throttle or push in the clutch while DFCO is active.

In either scenario, when DFCO disengages, all the injectors start firing again, the engine braking stops, and engine power generation starts, which is likely the power/rpm surge you are feeling.

It is frequently turned off during the tuning process, and can be disabled entirely in the tune, but you need to find an OBD1 programmer to do it.

[This message has been edited by fieroguru (edited 07-01-2023).]

Patrick JUL 01, 06:07 PM

quote
Originally posted by fieroguru:

I think what you are describing is when Decel Fuel Cut Off (DFCO) is engaged...



While googling this issue, I came across a couple of different terms/settings and what they do. I ran with the term I used due to the following post.


quote
Originally posted by opm2000 Here:

...from the customefis site: How GM Electronic Fuel Injection Works

Decel Fuel Cut Off - Decel Fuel Cut Off means exactly that. When you take your foot off the gas pedal going downhill or decelerating quickly, the ECM will cut off the fuel entirely. The difference between this term and the next one, Decel Enleanment, is that some fuel is allowed to reach the engine during Decel Enleanment. As to when one or the other occurs, I don't know. I do know that on my TPI system, I noticed a lunge from the engine right around 1500 RPM, so I think this is where the Decel Enleanment takes over from the Decel Fuel Cut Off. The lunge was significant and could easily cause you to run into the something if you weren't expecting it. I also believe a speed sensor is required for these two modes to operate period. So this is another reason to have a speed sensor.

Decel Enleanment - See Decel Fuel Cut Off above.



From what I can determine in this post, Decel Fuel Cut Off is operating just fine with my Formula, but it appears it's the Decel Enleanment that's activated (in the quote above, it's described as a "lunge") that I'd like to have adjusted/eliminated via a custom PROM.

Fieroguru, after reading the above quoted passage, do you still believe that it's the Decel Fuel Cut Off parameter (DFCO) that needs adjustment, or is it the Decel Enleanment parameter (DE) that needs attention? I appreciate your feedback!

[This message has been edited by Patrick (edited 07-01-2023).]

fieroguru JUL 01, 08:25 PM
My understanding of decel enleanment is it reduces fueling during the "transient" of lifting the throttle to avoid overly rich conditions at drop throttle. It lasts a second or two.

If the issue is happening at the end of your coast down and not the start, then it likely is DCFO disengaging and the engine going back to power mode.
Patrick JUL 01, 08:36 PM

quote
Originally posted by fieroguru:

If the issue is happening at the end of your coast down and not the start, then it likely is DCFO disengaging and the engine going back to power mode.



Okay, if I understand you correctly, then DCFO and DE are two parameters which may both be involved during deceleration, but are completely independent of each other. I got the impression from the source I quoted that DCFO was being disengaged when DE kicked in. It sounds from what you're saying that the disengagement of DCFO actually has nothing to do DE. Interesting. Thanks for the info.

Of course, now I realize I've got the wrong parameter mentioned in the topic heading.

[This message has been edited by Patrick (edited 07-02-2023).]

A_Lonely_Potato JUL 01, 09:29 PM
almost daily lol. theres 2 roads that lead to my house, and one of them is a very steep hill. going down in 2nd gear and riding the brake, when i turn left at the Y at the bottom i usually clip the RPM cutoff and the car lurches a couple times from the clutch springs.
Patrick JUL 02, 12:53 AM

quote
Originally posted by A_Lonely_Potato:

almost daily lol ...the car lurches a couple times from the clutch springs.



I'm glad I'm not the only one that finds this damn annoying!

I don't understand the point of the DCFO disengaging, and the engine springing back to life, while the vehicle is still in the midst of decelerating.

I'm hoping someone here can offer either a solution, or a suggestion as to where to find a solution, with a custom PROM.

[This message has been edited by Patrick (edited 07-02-2023).]

A_Lonely_Potato JUL 02, 01:19 PM
i think overall its worth having though. if you were to be idling through a parking lot in first or second gear, and touch the brake kicking on DFCO or deleanment or whatever the proper term is, its going to die almost immediately from braking + engine braking if it leaves fuel off. yes you could clutch in, but for the majority of people it makes sense how it is. its just annoying lurching when engine braking up to a stop(especially with a passenger!)
sdgdf JUL 02, 03:14 PM

quote
Originally posted by Patrick:

I'm glad I'm not the only one that finds this damn annoying!

I don't understand the point of the DCFO disengaging, and the engine springing back to life, while the vehicle is still in the midst of decelerating.

I'm hoping someone here can offer either a solution, or a suggestion as to where to find a solution, with a custom PROM.




The 3rd gen Camaro guys: https://www.thirdgen.org/fo...4190-turn-d-f-c.html


quote
3rd Gen Camaro forum:

ok, heres what i see in tuner pro with my 7747 ecm.---DFCO IAC DECAY COEF, -----DFCO UPPER MAP,THRES, AND LOWER MAP,--DFCO UPPER RPM THRES AND LOWER , ---DFCO MAP, THRES [O/L],--DFCO RPM THRES[O/L],-----------AND my egr is set at temp on, 0.00[thats where i was told to keep it] [i thought it would should be at highest setting ?]--------what is ccp, and highway mode.




They're also talking about surge on decel. Aren't these settings in the Fiero ecm? I know the Fiero DFCO disengages extremely early, like 1500RPM when on newer standard trans vehicles of mine it goes all the way down to idle. On my S10 it disengages real early too and its annoying, I rev it low with the 4.3 and makes no sense for it to stop engine braking at 1500.
pmbrunelle JUL 02, 06:23 PM

quote
Originally posted by Patrick:
I don't understand the point of the DCFO disengaging, and the engine springing back to life, while the vehicle is still in the midst of decelerating.



You have to bring the engine back to life at some point before it returns to idle, to avoid the risk of the engine stalling due to lack of fuel.

I don't think that anyone wanted the return of fuel to be intrusive; that's just the sophistication of the era.