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Thread: Surge + Engine RPM's not dropping. Car wants to drive itself!

  1. #1
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    Surge + Engine RPM's not dropping. Car wants to drive itself!

    I'm hoping someone will be able to assist me and my tuner in locating the source of the issue I have been experiencing with my tune...

    If I gently pull away in 1st or 2nd, at about 1,500RPM the car surges forward, often causing wheel spin....like someone put 200 shot of nos into it.

    I also found out today that if when this surge happens, I take my foot completely off the accelerator the engine continues to hold that RPM (often the surge takes it to about 2800rpm) for at least 30 seconds or more even if i'm in 1st or 2nd, and would normally be expecting engine braking to slow me down pretty quickly.

    The issue of the RPM's not wanting to drop can also be seen if I accelerate hard and then let complete off the gas. If I get beyond about 4500rpm the engine just wants to keep going even though I'm in a low gear. Its put me in a couple of dangerous scenarios where I was expecting engine braking, but instead had to fight the engine with the brakes.

    Attached is a short log showing the issue.

    I'm beginning to wish this car just had a carburetor!


    tune11-a-61621.hpl

  2. #2
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    Can you post a tune file? What vehicle and engine?

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    Here's the tune file I was running during that log. The tuner switched to SD tuning at my requests as I was getting fed up and wanted to try something different.

    Car is a 2014 M7 C7 Stingray, fully forged LT1 , Stage 3 cam, ported heads, 95mm tb, Procharger D1X..25PSI, Decat long tubes and x pipe, meth, XDI injectors, LT4 HPFP, DSX secondary low-side fuel pump, sheet metal intake manifold.

    James 2014 C7 M7 Procharged cam meth XDI SPEED DENSITY CR tune 11.hpt
    Last edited by jimxms; 06-15-2021 at 03:47 PM.

  4. #4
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    Quote Originally Posted by jimxms View Post
    Here's the tune file I was running during that log. The tuner switched to SD tuning at my requests as I was getting fed up and wanted to try something different.

    Car is a 2014 M7 C7 Stingray, fully forged LT1 , Stage 3 cam, ported heads, 95mm tb, Procharger D1X..25PSI, Decat long tubes and x pipe, meth, XDI injectors, LT4 HPFP, DSX secondary low-side fuel pump, sheet metal intake manifold.

    James 2014 C7 M7 Procharged cam meth XDI SPEED DENSITY CR tune 11.hpt
    If this is all low pedal angle issues, try putting the driver demand rows with low pedal angles back to stock. They look different than a stock file when compared. You shouldn't need to modify any of the driver demand table rows below 75% or so.

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    Also, make sure you don't have a vacuum leak and that the bov on the Procharger is opening properly.

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    OK I'll see if I can find the stock file for my car and copy over the tables from there. Can you think of any reason why someone might have changed them? I've been through several tuners so its hard to know who changed what and why.

  7. #7
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    Quote Originally Posted by jimxms View Post
    OK I'll see if I can find the stock file for my car and copy over the tables from there. Can you think of any reason why someone might have changed them? I've been through several tuners so its hard to know who changed what and why.
    Most likely they did not fully know what they were doing. Obviously they didn't drive it after they tuned it otherwise they would have fixed it before giving it back to you, or they didn't care.

  8. #8
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    @cadillactech - I looked at someone elses stock file from a M7 C7 Z51 and my driver demand rows seem to be exactly the same. Were you maybe thinking of the A8 tables as I downloaded a tune from that too and those are very different.

    @Jason - I capped off everything behind the throttle body today to eliminate any chances of a vac leak. I also removed my BOV completely so the car wasn't making any boost, thus eliminated any kind of boost surge from the equation and the issue is still there.

    Is it normal for the tune to keep the throttle body 25% open when you have 0% pedal position and are wanting engine braking to help slow you down? I noticed this in one of my log files. I let off the gas complely in 2nd at 2800rpm, but the TB stayed 25% open. Seems quite a lot?

  9. #9
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    Quote Originally Posted by jimxms View Post
    @cadillactech - I looked at someone elses stock file from a M7 C7 Z51 and my driver demand rows seem to be exactly the same. Were you maybe thinking of the A8 tables as I downloaded a tune from that too and those are very different.

    @Jason - I capped off everything behind the throttle body today to eliminate any chances of a vac leak. I also removed my BOV completely so the car wasn't making any boost, thus eliminated any kind of boost surge from the equation and the issue is still there.

    Is it normal for the tune to keep the throttle body 25% open when you have 0% pedal position and are wanting engine braking to help slow you down? I noticed this in one of my log files. I let off the gas complely in 2nd at 2800rpm, but the TB stayed 25% open. Seems quite a lot?
    Ya my bad, was glancing at another A8 file and didn't realize that was the one I compared yours too. Your driver demand table looks fine for the low throttle areas.

    It does the same thing in SD or MAF only mode? I noticed that while peak torque or some other torque tables have been touched, the virtual torque tables are stock. Just something to note, if you are keeping the throttle open usually you don't need to change them but sometimes you need to in order to correct other issues.

    I also see your peak torque is reading at 879lbft(Max reading in scanner) and I am not sure that is accurate. I know you want your peak torque table to match your actual engine torque pretty close. In the tune you have the peak torque table at 959lbft.

    Maybe try lowering it to something more realistic like 600lbft and see what happens? You could go back to stock, multiple the whole table by 1.5 and see where that gets you, it should be like a dyno graph curve similar to what the engine actually is making.

    What kind of power is this thing putting down?

  10. #10
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    Quote Originally Posted by cadillactech View Post
    Ya my bad, was glancing at another A8 file and didn't realize that was the one I compared yours too. Your driver demand table looks fine for the low throttle areas.

    It does the same thing in SD or MAF only mode? I noticed that while peak torque or some other torque tables have been touched, the virtual torque tables are stock. Just something to note, if you are keeping the throttle open usually you don't need to change them but sometimes you need to in order to correct other issues.

    I also see your peak torque is reading at 879lbft(Max reading in scanner) and I am not sure that is accurate. I know you want your peak torque table to match your actual engine torque pretty close. In the tune you have the peak torque table at 959lbft.

    Maybe try lowering it to something more realistic like 600lbft and see what happens? You could go back to stock, multiple the whole table by 1.5 and see where that gets you, it should be like a dyno graph curve similar to what the engine actually is making.

    What kind of power is this thing putting down?
    Thanks for all the pointers, I'll have a go at playing with the torque tables.

    I'm not exactly sure what the car is putting down as I haven't had it dyno'd yet, but I'd estimate somewhere in the 1100whp range. The procharger is pushing around 25psi, and thats with my 95mm tb, ported heads, sheetmetal long runner intake and long tube decat headers...etc

    I just noticed in my log that it seems to be pulling timing/spark right around the 1000-2000rpm range, but I cannot see the reason!
    Last edited by jimxms; 06-27-2021 at 03:51 PM.