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  1. #1
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    Weird Throttle Issue After Custom Tune

    So after the great results from a Hemifever tune on my 2005 300C, I went ahead and picked up an unlocked PCM and custom tune for my 2015 Challenger R/T. While the car is definitely stronger now, the tradeoff is some annoying drivability gremlins related to the throttle and transmission.


    If I'm cruising down the highway at 60-70 mph I'll have my foot on the gas just enough to maintain speed. As I come up on traffic, I'll take my foot off the gas, the car will 'coast' a bit, then nose dives as though the throttle plate closed. Fairly normal, but when I put my back on the gas, the driveline clunks and the car surges forward.


    I assumed it was related to throttle boost, but I've set that all the way to -20% and it's still there.


    The transmission is also behaving strangely. When I'm pulling up to my driveway and coming to a stop, the car bucks the same way you'd expect on a manual trans if you'd forgotten to push the clutch pedal down.


    I feel like the two problems are related, but when I ran it by HF, he simply replied that it's likely related to torque restrictions and throttle limits being removed, but offered nothing related to a resolution.


    As much as I love the way the car runs and shifts at WOT, the drivability stuff drives me nuts - to the point that I'd rather run the stock tune if I can't get this worked out. Anyone ever deal with anything remotely like this? Any idea's?



  2. #2
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    Almost sounds like its dropping into neutral on the coast then slamming into gear during accel. Also sounds like the TC is staying locked in during braking. Odd for sure.
    No question its tune related if it didnt do it before and thats all that was changed.
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  3. #3
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    Quote Originally Posted by 05300HEMI View Post
    So after the great results from a Hemifever tune on my 2005 300C, I went ahead and picked up an unlocked PCM and custom tune for my 2015 Challenger R/T. While the car is definitely stronger now, the tradeoff is some annoying drivability gremlins related to the throttle and transmission.


    If I'm cruising down the highway at 60-70 mph I'll have my foot on the gas just enough to maintain speed. As I come up on traffic, I'll take my foot off the gas, the car will 'coast' a bit, then nose dives as though the throttle plate closed. Fairly normal, but when I put my back on the gas, the driveline clunks and the car surges forward.


    I assumed it was related to throttle boost, but I've set that all the way to -20% and it's still there.


    The transmission is also behaving strangely. When I'm pulling up to my driveway and coming to a stop, the car bucks the same way you'd expect on a manual trans if you'd forgotten to push the clutch pedal down.


    I feel like the two problems are related, but when I ran it by HF, he simply replied that it's likely related to torque restrictions and throttle limits being removed, but offered nothing related to a resolution.


    As much as I love the way the car runs and shifts at WOT, the drivability stuff drives me nuts - to the point that I'd rather run the stock tune if I can't get this worked out. Anyone ever deal with anything remotely like this? Any idea's?
    The deceleration event you experience is normal; the throttle valve does not go directly to idle when your foot is removed from the pedal. Its a means by which fuel mileage is increased. After a second or two, the PCM decides that you're not going to maintain speed - and immediately goes to idle. This can be modified to more closely mimic a carbureted vehicle and is particularly useful on a road course when engine braking is a distinct advantage. Note also that is you take your foot off the gas pedal and quickly step on the brakes, the PCM interoperates this as you wanting to slow down immediately and will close the throttle valve and commence downshifting the transmission to assist braking effectiveness. With RWDs this has the added benefit of ensuring the vehicle remains pointed where you want to go - if it happened to be a panic stop.

    As to jerking forward as if significantly more throttle has been applied than you requested and quickly loading the drivetrain, that is not normal at all. It can surely be duplicated if the (placebo effect) throttle boost is set to a value that does just that - open the throttle valve way too quickly. This is a tune issue.


    The PCM trying to decide which idle rpm to use (there are a few) during slow speed - is also a tune issue.



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  4. #4
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    I'm deciding on which CMR tuner to use for my car, I'll make note that HEMIFEVER 's reply was minimal.

    I do have a question for you though, with the custom tune on your '15 HEMI, is the initial off-idle delay in throttle response eliminated or at least reduced? I'm sure you know what I'm talking about... sitting at idle you press the gas pedal and there is a delay before the engine RPM's increase. Also happens while cruising at low throttle positions.

    While in gear, I can quickly jab the pedal half way down and it will pop back up and NO increase in engine RPM. Drive me nuts.
    2017 300S Hemi, 1969 Cutlass

  5. #5
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    Quote Originally Posted by Mr Nick View Post
    I'm deciding on which CMR tuner to use for my car, I'll make note that HEMIFEVER 's reply was minimal.

    I do have a question for you though, with the custom tune on your '15 HEMI, is the initial off-idle delay in throttle response eliminated or at least reduced? I'm sure you know what I'm talking about... sitting at idle you press the gas pedal and there is a delay before the engine RPM's increase. Also happens while cruising at low throttle positions.

    While in gear, I can quickly jab the pedal half way down and it will pop back up and NO increase in engine RPM. Drive me nuts.
    The throttle delay is definitely reduced, but AFAIC the tradeoff isn't worth it. Every time I've driven a car that has had the throttle tweaked (throttle boost, pedal commander, etc, etc) day-to-day driveability sucks. I know that's a subjective thing, as tons of guys love throttle boost and/or the pedal commander. I'm not one of them.


    In fact, I'm convinced that whatever throttle tweaks that are part of my tune are to blame for my issues. Did a bunch of data logging today and I'm sure it's related to whatever the tune is doing with the throttle. With the stock tune, at idle the throttle plate is showing 2.5% opened. If I press the gas just a bit to 3.5%, I can get the idle from 750, up to 1050. The whole process is fairly smooth. With the custom tune, at idle the throttle plate is showing 6%, and the lowest RPM I can get off idle is 1300, which comes in at 8.0 - 8.5%. That process is very erratic, with the throttle plate slamming shut and opening back up, causing the RPM's to bounce from idle, to 1300+ RPM.


    This is exactly where I'm at on the throttle going down the road when it engine brakes, then when I press the gas a tiny bit to maintain current speed, the driveline clunks and the car speeds back up. I tried doing a throttle recalibration, but it's not having any effect.


    I've put in a request with Sean to create another tune, just like the one I have now, but with nothing altered on the throttle. Hopefully that'll fix the issue.


    Many thanks for all the responses!

  6. #6
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    Something else I've noticed...

    On any platform with no power adders and simply tweaking the stock tune to produce more HP / TQ, timing and fuel can only(!) be altered a relatively small amount to ensure engine longevity. The OEM didn't / doesn't leave much on the table in the first place. Just how such small changes can equate to seat of the pants is truly questionable.

    Fact is, Hemi engines lose or gain 20 or more HP during normal operation - without making any changes whatsoever. The claim folks can feel such a small gain in a tune is likely more psychosomatic. I'm starting to wonder whether to increase customer satisfaction the throttle curve is (also) being tweaked to be non-linear...just like the (placebo) throttle boost / pedal commander.
    Last edited by Hemissary; 03-17-2019 at 04:57 PM.

  7. #7
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    Thanks for the info. I currently have the Pedal Commander, and made the purchase knowing it would be (at best) a band-aid to the throttle lag problem. I have it set at City -3 because the boost feature really isn't what I'm looking for, I just want the delay/lag gone.

    I've contacted Sean and Johan, I'm leaning toward Johan based on his communication and replies to my questions. Hopefully I can sell the Pedal Commander after a CMR tune from Johan. I just have to get over the mental hurdle of spending $1k for an unlocked ECU, i3 tuner, and his custom tune.

  8. #8
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    Quote Originally Posted by Hemissary View Post
    Something else I've noticed...

    On any platform with no power adders and simply tweaking the stock tune to produce more HP / TQ, timing and fuel can only(!) be altered a relatively small amount to ensure engine longevity. The OEM didn't / doesn't leave much on the table in the first place. Just how such small changes can equate to seat of the pants is truly questionable.

    Fact is, Hemi engines lose or gain 20 or more HP during normal operation - without making any changes whatsoever. The claim folks can feel such a small gain in a tune is likely more psychosomatic. I'm starting to wonder whether to increase customer satisfaction the throttle curve is (also) being tweaked to be non-linear...just like the (placebo) throttle boost / pedal commander.
    I think there's enough empirical data out there showing a good tune to be worth 20'ish horsepower over a peak running stock tune. The factory tune is both a touch fat, and a bit conservative relative to timing curves to allow for a wide range of gas quality.

    I go for the custom tune to account for the fact that I always run 93 (so may as well run as much timing as 93 will allow), to get rid of torque management, quicken/firm up the shifts, along with a change of fan temps for the cooler stat. Whether or not the better feel is just the removal of torque management, any HP gains, or placebo, is certainly open for debate. But I definitely do not want/need funky throttle calibration changes to trick me into thinking the car is running better than it is. I've been at this long enough - I know what to expect from slight changes to timing and fuel curves.

    For anyone interested, I'll follow up once I get an updated tune from Sean.
    Likes Mr Nick liked this post

  9. #9
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    I'm trying to figure out how sensitive the "seat sensor" is to detect a delay in throttle to warrant pedal commanders or throttle boosts in tunes.

    We're talking milliseconds here, and very few of those at that.

    Sent from my XT1650 using Tapatalk

  10. #10
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    Quote Originally Posted by punisher69 View Post
    I'm trying to figure out how sensitive the "seat sensor" is to detect a delay in throttle to warrant pedal commanders or throttle boosts in tunes.

    We're talking milliseconds here, and very few of those at that.

    Sent from my XT1650 using Tapatalk
    Agreed; there is no throttle lag. Throttle valve response to pedal position changes occurs at the rate it takes the electrical signal to travel from the pedal to the servo motor. There is no signal processing involved that creates a delay. If this were actually a problem it would have become painfully evident the very first day at a / any 1/4 mile track. I've never read or heard of any drag racer complaining about throttle response.

    If anyone wishes to dispute this, set up a mirror to observe the throttle valve action while standing with one foot on the gas pedal. Get back to us with the results...

  11. #11
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    My 05 has pretty much zero throttle lag - at least, none that I can detect. But the 15? Definitely. Come to a complete stop, goose the throttle (just a quick blip) and the car does nothing. Do the same thing, this time feather the throttle and the car will lunge forward. It is 100% there on the 15, and not on the 05. Something changed between those years. When and what, I don't know.

    For me, it's not that big a deal as I don't need to operate the throttle like that. But I can take a quick vid of the effect and post a link to it if anyone has any doubts?

  12. #12
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    Quote Originally Posted by 05300HEMI View Post
    Come to a complete stop, goose the throttle (just a quick blip) and the car does nothing.
    Same thing on my 17 300S Hemi. I actually hesitate pulling out into traffic because the delay from when I touch the gas pedal to when the car responds and begins to move, is enough the traffic traveling 45+ MPH is now too close to comfortably pull into. I'm actually surprised that this is "allowed" by the DOT or whatever organization would oversea something like this.

    There IS throttle lag.

  13. #13
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    Will a dealer software update help with any of this?

  14. #14
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    Well I got an updated tune from my tuner and while it helped a bit with the throttle on/off at highway speed, it made the bucking worse coming to a stop. When I got to work I decided to drop the custom PCM tune in favor of the canned 91 tune, but kept the custom TCM tune. Car runs perfect now. No bucking coming to a stop, no clunking at highway speed going on/off the throttle.

    I'm convinced it's something to do with torque management. I know most of the custom tuners remove all of the TQ management, whereas the canned tunes leave some of it in place. I asked my tuner what he though about it yesterday morning, but he hasn't gotten back to me yet so I don't know for sure. For now, I'm fine running the canned tune with the custom TCM tune until I hear back from HF.

  15. #15
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    Interesting, hopefully he's able to fix the the issue when coming to a stop. That is, assuming you're willing to put the time into trying more tunes.

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