Is it possible to induce DFCO early? - Fuelly Forums

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Old 08-29-2008, 08:30 AM   #1
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Question Is it possible to induce DFCO early?

My truck won't DFCO until after 8-10 seconds of engine braking. This makes it almost entirely useless unless you're descending a mountain or a LONG hill. Until it DFCOs, it uses far more fuel than idle. I was thinking that maybe I can fool it into thinking it's in DFCO conditions earlier, or otherwise trigger DFCO.

What sensor data is used for DFCO?

I'd guess TPS and RPM alone. I'm pretty sure it's a bad idea to monkey with either of those; RPM data is one of the main pieces of data used in fuel and nearly every other calculation, and TPS will probably throw a code.
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Old 08-29-2008, 08:38 AM   #2
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Maybe find a dealer to change the computer programming - bring on DFCO after 3 seconds. My Sierra DFCO's if TPS=0 and RPM> 1500 for more than 3 seconds. The local Chevy dealer advertises that they can put a custom program in your computer. I've been toying with idea of lowering the threshhold for DFCO to 1300 RPM, and the cancel @ 900 RPM so it would DFCO much more often.
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Old 08-29-2008, 08:41 AM   #3
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TPS will most likely throw a code. I wonder, does your truck unlock the tc on decel? My car didn't and I've been playing with it and I can unlock the tc and coast further on DFCO then when I hit the bottom limit of it(1000 rpm) I can re-lock it just before it gets there but after that I'm stuck using engine braking to keep the rpms up(thinking about putting push-on release-off lock/unlock buttons on the steering wheel. (I promise to get that schematic up this weekend; anybody know of a good program I can use?)

What about inducing DFCO manually? Like, a cutoff for the fuel pump to kill the injectors early. Might throw an excessive lean code just like that but you could get one of those dummy O2 sensor gadgets that they use downstream when gutting a cat. Then have the switch activate that and kill the fuel pump?
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Old 08-29-2008, 09:13 AM   #4
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Jay, I was hoping to come up with a low-budget solution to avoid getting reprogrammed. For $200-250, I can get reprogrammed really well by http://www.blackbearperformance.com and I'm pretty sure I could get everything I want -- DFCO, shift patterns, etc.

After reading recently about manual control of TCC locking and possibly of transmission gear choice, I was thinking I could skip that. I don't have the liquid funds to invest in it right now anyway.

dkjones96, I haven't given any attention to the TCC lock status during deceleration. I know sometimes it's locked and others it's not. With a manual control I had already decided I'd take advantage of exactly the strategy you describe, locking just before losing DFCO. I'd also downshift too...

Cutting off the fuel pump would be bad because I'd lose fuel system pressure, requiring a few seconds for the pump to prime the system again before resuming. I was thinking I could cut off the fuel injectors using their common ground wire, but like TPS, that would throw a code. Maybe I could just tolerate a code...I've got the CEL on for my P0440 anyway!
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Old 08-29-2008, 10:31 AM   #5
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I was thinking about the common ground but wasn't sure how far into the wiring harness you were willing to dig to make it happen. If you got that far, might as well get some resistors and a few relays and make a bypass for them so there is no code. It's especially easy if they are batch or bank fire. From what I can tell they didn't go SFI until they went flex fuel but my sources aren't very reliable and I don't have access to our Alldata here at work.

I can see your point behind the fuel pump.
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Old 08-29-2008, 10:55 AM   #6
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That's a good idea...I hadn't thought of that. I'll have to think about that, it sounds like something I can do.
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Old 08-29-2008, 11:19 AM   #7
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Consider this, my VX kicks up the DFCO threshold from 860 to about 1350 RPM when there is any significant electrical load, which also increases the idle speed.

Maybe you could do something to defeat that situation by eliminating the alternators charging when you have your foot off the gas. Something like killing the AC compressor when you are accelerating to eliminate the AC drag affecting your available power.

The components that would be loading the charging system could easily survive on battery voltage for the small percentage of time you would actually be utilizing DFCO.

Try checking your DFCO engagement with absolutely no electrical accessory loads. I know in my case it makes a significant difference.

I actually use the AC when I DFCO to get the evaporator cool with the otherwise wasted energy. Then I kill the compressor at idle or when accelerating but run the fan over the cool evaporator to get cabin temperatures down when it's hot outside. I do have to downshift somewhat earlier and maintain higher engine rpm to keep it in DFCO.

regards
gary
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Old 08-29-2008, 11:25 AM   #8
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Thanks, but under any condition, it just takes a long time. It kicks up to 1250 RPM with A/C on, but no matter what, it takes 8 seconds before it begins to DFCO.
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