Novak Conversions Jeep Wrangler TJ radiator

Tuning DFCO for better engine braking and fuel economy

SkylinesSuck said:
I have the same behavior as Jugs with my JTEC PCM with the hanging idle, but I can also get it to stall at low speed maneuvering. Interesting additional symptom, it will idle normally, but if I push the clutch in and coast down my driveway, the idle will frequently kick up to the hanging idle speed without touching the gas. Been chasing this for a while and playing with the throttle follower settings to no avail. Subbed for the learnings 👍
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@Steel City 06 was right that starting by checking for vacuum leaks is critical before proceeding with any settings or you'll be chasing ghosts. Then you've got to get the idle stable first before you can do any other tuning.

You're still running a stroker, with a bigger cam if I recall?

If so you may need to adjust some of the idle settings up. Larger cams and higher compression usually require a higher idle or they can lope into stall. To start with, I'd probably get idle setpoint on P/N to ~1000rpm and see if it will stabilize the idle. Then under airflow in the Throttle Follower - Decay (RPM) section, I'd set the values below your working idle to 0 as I don't think you want it to decay below. Manuals only use the P/N idle setting.

If you can get a stable idle at 1000rpm, then I would try these same paired adjustments at ~900, then 800... until you reach the point where it starts to act like it's going to stall and go back up ~50 rpm. It doesn't let you pick just any rpm, so go with the nearest. ie I had to go from 800 to 824 when I wanted to add a smidge for AC compensation.
 
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I’m interested to see Skylines’ tps voltage at closed throttle and IAC count. He may need to open the throttle blade because the IAC can’t keep up.
 
From what I've seen in my logs, the IAC can open enough if commanded with the stock cam setup to sit above 2000rpm with the throttle position at 0.0%
 
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Juggernaut said:
I’m interested to see Skylines’ tps voltage at closed throttle and IAC count. He may need to open the throttle blade because the IAC can’t keep up.
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PXL_20251115_220216939.jpg


PXL_20251115_220227649.MP.jpg
 
I'll try and get to it today or tomorrow. If you mean it isn't a ton of vacuum at idle, this is my cam for reference...


Also, I'll probably DM you so we can discuss that way as not to bugger up SteelCity's thread 👍
 
JTEC specific-For hanging idle on decel- 95% of the time - throttle decay /dashpot preposition by rpm will address this. This is almost mandatory if you are using a big throttle body AND OR you are running an aftermarket camshaft. Cheap Throttle Bodies ( Ebay, BBK) will not be consistent in their behavior. I normally have to lower this by 20% at minimum , and sometimes more.

NGC3 , it is a function of PT/MBT/Min Spark. No airflow adjustments to speak of in HPT.

Beyond that- timing being commanded on decel can also play into this.

Just my .02
 
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Steel City 06 said:
Other notes:

This is on a @Wranglerfix PCM, but the factory PCM is exactly the same.

I also have a @Flyin' Ryan Performance canned tune on this PCM, but his tune does not change DFCO from factory settings. (Also note this is why I won't just post the tune file - FRP's spark and VE tables are proprietary. Buy his tune, don't pirate it. It's worth the $$.)

The other way to get better engine braking (without DFCO) is simply to retard the spark to a significant amount at MAP values that you'd only get with a closed throttle. This works, but it isn't nearly as much engine braking. (It also arguably wastes fuel and increases CO emissions.)

What actually seems to happen with the TJ is that it defaults to the idle spark advance during normal (non-DFCO) deceleration rather than the spark values in the part-throttle spark tables. So it generally drops to the default of 10 degrees advance (14 degrees for automatic transmission), which provides the mild engine braking effect. That's also why the throttle seems snappy when you're idling along in 1st gear and then tap it ever so slightly - it's not adding much fuel, it's simply snapping to a higher spark advance.

With DFCO enabled to do the engine braking duty, that actually removes one of the limitations on advancing the idle spark. I've experimented with raising it as high as 24 degrees, and have settled on 20 degrees as a happy medium. Advancing the idle spark significantly cuts idle fuel consumption, although it eats into the spark torque reserve, meaning the engine becomes a little bit more prone to stalling and may not be able to maintain the idle RPM as closely as it would at lower timing advance. Above 20 degrees and it seems to become a bit unstable and easy to stall from my testing.

Increasing the idle spark also counteracts the engine braking you'd get without DFCO enabled, and may make the transition into and out of DFCO feel a bit more harsh. (Personally I like it - it feels a lot like regenerative braking on an EV or hybrid.)
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I have a wranglerfix PCM and when i bought ir they said it couldnt be tuned. Im curious as to why they would say that. Also did you see a noticeable difference in mpg?
 
Ajwatuc said:
I have a wranglerfix PCM and when i bought ir they said it couldnt be tuned. Im curious as to why they would say that. Also did you see a noticeable difference in mpg?
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I think it's mainly because they weren't sure and they didn't want people to buy it and then be dissatisfied because it couldn't be tuned. People have repeatedly tested it and found that HP Tuners can tune it. You'll need to buy new credits to be able to do so.

You may need a jumper to tune it if you have a manual transmission, but other than that it generally tunes without issues.

I did notice significant improvements in fuel economy, but AI have been doing a lot of modifications with that intent in mind, not just tuning. Tuning is a big part of it though
 
Steel City 06 said:
You may need a jumper to tune it if you have a manual transmission, but other than that it generally tunes without issues.
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Steel City 06 - How did you figure out the jumper trick? I've seen you post on that before, and I've been meaning to ask.
 
Steel City 06 said:
Kept searching and eventually found it on a Dodge SRT forum. No idea how that poster figured it out...
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Wow. I'll bet that's in the FSM, or another dealer reference manual, in the section on flashing the PCM for that vehicle. Nice find!
 

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