Engine braking question. Fuel injector related. EXPERTS INSIDE..

TType84

cookin with propane
Joined
May 27, 2001
I need someone that knows the answer, not someone that thinks they know the answer.

I'm reading a discussion about engine braking, here is the scenario, which we'll apply to buicks because this is a buick tech question and I want to know how our ECMs react to this.

1.pull the car down to 1st gear.
2. accelerate to 6,000rpm
3. let off the gas sharply

Does the ECM shut down all of the injectors until idle? 0% dc, 0ms pulsewidth?

or what?

Can anyone that knows tell me for sure how the ECM handles this situation?
 
TType84 said:
I need someone that knows the answer, not someone that thinks they know the answer.
I'm reading a discussion about engine braking, here is the scenario, which we'll apply to buicks because this is a buick tech question and I want to know how our ECMs react to this.
1.pull the car down to 1st gear.
2. accelerate to 6,000rpm
3. let off the gas sharply
Does the ECM shut down all of the injectors until idle? 0% dc, 0ms pulsewidth?
or what?
Can anyone that knows tell me for sure how the ECM handles this situation?

There are two items that come into play:
Deccleration Enleanment
Deccleration Fuel Shut Off

Over at GNTType in the chip area, they have all the parameters (enables/ disables) listed for both items.

The DE is for *small throttle* lifts, DFCO for the large/ hard throttle lifts.

If you want to look up and read thru the chip stuff over there, then post back with questions, would seem to be the way to go, IMO.
 
Yup. DFCO. Decelleration Fuel Cutoff. If you have a scanmaster and look at the o2 readings, when you let off, it should go to 000 millivolts. The extender chips have a feature that allows you to manipulate this. Given the nature of high stall converters , engine braking isnt very sufficient when you run one. So you can tell it to only use DFCO at higher speeds if you want, where there will be enough "drag" on the converter for engine braking to work. Otherwise, if you run a high stall converter and use the stock DFCO parameters, the engine rpms may dip while you're coasting to a stop, or even stall. The computer sees you're decellerating, and it pulls fuel out completely. But if there isnt enough "drag" through the converter to keep the motor spinning (like it would do with a clutch and manual tranny), this loss of fueling will cause the rpms to dip or just stall all together. I had a high stall non lock converter, but never had any of this stuff happen on my car, i.e. rpm dipping or stalling from DFCO. But, DCFO would only work above 60mph. Now with the stock converter in there, it works down to 50mph. It might be able to work down to a lower speed if my lockup was working. Say Im cruising at 75mph. I let off the gas completely, and the O2s drop to zero, and will stay at zero until I reach 50mph, and then come back.
 
VadersV6 said:
Yup. DFCO. Decelleration Fuel Cutoff. If you have a scanmaster and look at the o2 readings, when you let off, it should go to 000 millivolts. The extender chips have a feature that allows you to manipulate this. Given the nature of high stall converters , engine braking isnt very sufficient when you run one. So you can tell it to only use DFCO at higher speeds if you want, where there will be enough "drag" on the converter for engine braking to work. Otherwise, if you run a high stall converter and use the stock DFCO parameters, the engine rpms may dip while you're coasting to a stop, or even stall. The computer sees you're decellerating, and it pulls fuel out completely. But if there isnt enough "drag" through the converter to keep the motor spinning (like it would do with a clutch and manual tranny), this loss of fueling will cause the rpms to dip or just stall all together.

I had a high stall non lock converter, but never had any of this stuff happen on my car, i.e. rpm dipping or stalling from DFCO. But, DCFO would only work above 60mph. Now with the stock converter in there, it works down to 50mph. It might be able to work down to a lower speed if my lockup was working. Say Im cruising at 75mph. I let off the gas completely, and the O2s drop to zero, and will stay at zero until I reach 50mph, and then come back.

Stalling due to (or while recovering from) overrun, is a bit different then engine braking. :)

Oh, and there's the converter locking:
Keeping the converter locked, vs unlocked will slightly effect engine braking. For me thou, I have mine set up so the converter unlocks at just over the idle TPSv setting.

Geeesh, and I forgot about the throttle follower. That adds some throttle opening though the IAC. A huge IAC count can cause a car for seeminly forever to roll down.
 
If the converter is locked, and if not, but you are going fast enough, it is engine braking. If you slow down so there is not forcefull enough of a fluid thrust in the TC to drive the engine, then you dont have engine braking. Engine braking is just a coupling of the rear wheels to the engine. Below a certain speed, there isnt enough thrust in the TC to make the rear wheels drive the engine.
High stall converter- The computer pulls fuel when you let off the gas at high speeds (highway). With a high stall converter at highway speeds, when you let off the gas, There is not enough of fluid thrust in the converter for the wheels/driveshaft to drive the engine and keep it spinning. It slips too much to couple. The engine dies cause it has no fuel (cause the computer thinks things are coupled and the engine is being forced to spin, so it pulls fuel), and nothing forcing the engine to spin. Engine rpms dip and/or stall.
Given fullthrottlespeed designed a chip feathure to deal with this, they would better know what is happening. I can only guess from what I see happening.
This is how fullthrottlespeed explains it:

DFCO Delay: Vehicles equipped with lightweight or high-stall torque converters have insufficient engine braking at low RPMs for DFCO (Decel Fuel Cut Off) to work properly at low speeds. The result is RPM 'dips' while coasting. By enabling DFCO Delay, the fuel cutoff is only enabled at higher speeds, or when running in Drive instead of Overdrive, improving driveability
 
VadersV6 said:
If the converter is locked, and if not, but you are going fast enough, it is engine braking. [/b]

Yep, but with the converter unlocked you induce converter slippage, and the effects engine braking, which is what the original poster was asking about.

He's talking about a *delay*, and that is in the stock code. What FullSpeed does with a modified chip is mute to the subject anyway. The original poster was asking about what does happen, not what possibly could happen with modified code.
 
thank you guys for the quick and informative responses.

that is exactly what i wanted to know, no further explanation needed :)

as always tb.com delivers.
 
TType84 said:
1.pull the car down to 1st gear.
2. accelerate to 6,000rpm
3. let off the gas sharply

Does the ECM shut down all of the injectors until idle? 0% dc, 0ms pulsewidth?

Assuming stock settings in the chip:

No, the injectors do not shut off immediately. The injectors will not shut off until LV8 drops below 34 for a half a second. Which is usually in the 2000-2500rpm area. They'll come back on just above idle.

There are some other conditions involved, but LV8 is a big one.

Regards,
Eric
 
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