DFCO Without VSS

I am trying to get the DFCO to work on my Jeep
swap with a 5.3 manual trans and with no VSS input to the GM P01 PCM. I set the Speed vs Baro to 0mph and the fuel will cut off while on throttle at around 1600ish RPM. So its like it set my rev limiter to that. Is there another setting I need to change to get it to work without a VSS? Only other thing I can think of is setting the TPS values to much lower across the chart? It would be nice to get this work for fuel savings, able coast down hills without riding the brakes and for it to stop hanging on the revs in between shifting.

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Originally Posted by XCRN

manual trans and with no VSS input .

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no speedo output on the trans? what gauges are you using? dakota digital makes a VSS output device. theres several ways to get a vss signal, just nee dmore input of your rig

Turn off abuse mode if the RPM's limit you to 1,600. Raise the settings to 7,000rpm and 0 mph.

So I turned off the abuse mode and it worked better, no cutoff at 1600 while on the throttle. Now I removed the Holley TermX from this vehicle and the DFCO would cut off fuel 100%, but the log is still showing fuel flow during decel which I am seeing a 3% injector duty cycle compared to 10% when cruising. Is there something else I am missing to get it to cut completely or does this system not do that?

Its factory Jeep gauge. The only VSS I have is a 3 wire, and my understanding is the Jeep and GM use different waves to read and are not compatible? But I am not sure on that.

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A GM PCM can deliver ZERO pulsewidth to the injectors during DFCO, just have to adjust the minimum pulsewidth table to Zero.

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https://forum.hptuners.com/showthrea...l=1#post715301


Although I am pretty sure i tried this and still couldn't get it to work correctly, but its been a while and I don't remember if i tried everything (transient fuel minimum mg stuff)

Let me know if you figure it out

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Originally Posted by kingtal0n

https://forum.hptuners.com/showthrea...l=1#post715301


Although I am pretty sure i tried this and still couldn't get it to work correctly, but its been a while and I don't remember if i tried everything (transient fuel minimum mg stuff)

Let me know if you figure it out

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So I tried changing the PW mins to 0 and that did not seem to change much. With the transients is that just a matter of setting the "min fuel milligrams" to 0 as well?

Yeah from what I understand Transient minimum fuel mg is like a limit on how low the injector pulse can go... But I've tried turning it to 0.000 and still DFCO seems like it hangs those injectors open...

On the side note if you notice a smell when it happens you can adjust the injector phase... factory is like 5.55 fully warmed, try 6.2 to 6.5 and it will spray after exhaust valve closed so the smell goes away at least.
Could also give mid-range torque doing that.
But note the factory like to spray to closed intake valve to ensure complete fuel vaporization, if liquid fuel goes into the cylinder because its sprayed so late and poorly atomized by some engines it could wash the oil from the cylinder walls or create droplets which cause engine rough running.

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Originally Posted by kingtal0n

Yeah from what I understand Transient minimum fuel mg is like a limit on how low the injector pulse can go... But I've tried turning it to 0.000 and still DFCO seems like it hangs those injectors open...

On the side note if you notice a smell when it happens you can adjust the injector phase... factory is like 5.55 fully warmed, try 6.2 to 6.5 and it will spray after exhaust valve closed so the smell goes away at least.
Could also give mid-range torque doing that.
But note the factory like to spray to closed intake valve to ensure complete fuel vaporization, if liquid fuel goes into the cylinder because its sprayed so late and poorly atomized by some engines it could wash the oil from the cylinder walls or create droplets which cause engine rough running.

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No 100% shutoff after changing that as well. Its definltly reducing the fuel flow substansially while coasting from where I was at before messing with abuse mode and min PW settings and it does provide adequate engine braking so I can almost live with it. Only issue still is the rev likes to hang for a time when shifting. Not sure if thats a DFCO setting or something else.

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Originally Posted by XCRN

No 100% shutoff after changing that as well. Its definltly reducing the fuel flow substansially while coasting from where I was at before messing with abuse mode and min PW settings and it does provide adequate engine braking so I can almost live with it. Only issue still is the rev likes to hang for a time when shifting. Not sure if thats a DFCO setting or something else.

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Why push so hard for DFCO? The fuel savings are going to be minimum at best

If the engine won't return to idle or doesn't have sufficient braking.. look at the timing while in decel. 5-15 degrees is a good number. Lower will give you pops (all the kids love that now days). Or base running airflow is too high.

Timing for this will be in the base idle spark tables. There is a speed qualifier on some that will make it use the Main spark tables instead. just be aware of that.

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Originally Posted by Alvin

Why push so hard for DFCO? The fuel savings are going to be minimum at best

If the engine won't return to idle or doesn't have sufficient braking.. look at the timing while in decel. 5-15 degrees is a good number. Lower will give you pops (all the kids love that now days). Or base running airflow is too high.

Timing for this will be in the base idle spark tables. There is a speed qualifier on some that will make it use the Main spark tables instead. just be aware of that.

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I have had no issue with the idle dropping below the set point and stalling if that is what you are referring to, its that the revs wont drop quickly when depressing the clutch.
Anything helps MPG wise in a brick but coasting down an icy hill is a lot sketchier without it as I have found out on what I hope is winters last hurrah for the season, and I have a large trip up to the Artic Ocean and the Canadian Rockies where that is a legit concern. Not that it cant be done safely without DFCO, but it helps.