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Thread: Hanging RPM's

  1. #1
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    Hanging RPM's

    I'm tuning a 1999 Corvette M6. Light mods on it. When the AC is off it drives perfect, turn it on and the RPM's hang when you go to shift gears. IAC counts go up when the AC is on only, barely do it with AC off. I have looked for a table that controls this but found nothing.

    Anyone have any suggestions?
    Four wheels & a motor.

  2. #2
    Advanced Tuner
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    idle speed w/ac on under idle tab speed maybe.. see what it is set to.. then there is a setting somewhere else for how fast it drops to idle when in gear and out of gear as well.. then you have the infamous possible vac leak

  3. #3
    Senior Tuner mowton's Avatar
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    Post tune and a log of the event. Include Dynamic Air Pid and MAF Airflow in you log.

    Does the AC on/off effect the stationary idle? If AC off is ok then take a look at the airflow with and without AC on. Hanging idle (otherwise known as cruise control) points to excessive air commanded. Good place to start looking is either idle air flow correction tables or RAF.

    Ed M
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  4. #4
    Advanced Tuner blownbluez06's Avatar
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    considering your "light mods", I might guess you have a vacuum leak. Log LTIT and STIT's and if they're high negative, you probably do have a vacuum leak. It's important to pressure or smoke check your intake and vacuum system before tuning the airflow tables to ensure you're not compensating for any leaks.
    Hsquared racing engines RHS 427, Procharger F2, Moran Billet Atomizer injectors, Alky Control,Mast LS7 heads, Nitrous outlet kit,Tilton quad disc clutch, DSS shaft, RKT56 ZR1 trans, RPM Quaife diff. Built and tuned by yours truly.

  5. #5
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    It has a FAST Intake, aftermarket Throttle Body, and Long Tube Headers with full 3" Dual Exhaust. Next time I'm around the car I'll have to log data. Dynamic Air is pretty much onpoint with the RAF Table. Idle is perfect it just hangs only with AC on. The IAC counts go up and then come down slowly, this is the only time it moves while driving pretty much. I turned off Throttle Cracker (set it all to 0).

    It's something to do with the AC when it's on.
    Four wheels & a motor.

  6. #6
    Advanced Tuner blownbluez06's Avatar
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    Quote Originally Posted by the_merv View Post
    It has a FAST Intake, aftermarket Throttle Body, and Long Tube Headers with full 3" Dual Exhaust. Next time I'm around the car I'll have to log data. Dynamic Air is pretty much onpoint with the RAF Table. Idle is perfect it just hangs only with AC on. The IAC counts go up and then come down slowly, this is the only time it moves while driving pretty much. I turned off Throttle Cracker (set it all to 0).

    It's something to do with the AC when it's on.
    I would stop looking at IAC counts on your car. I have never had to zero a cracker table to get a car to act right and tuned many C5's. If the problem is only with the AC on, reduce the compensation for the AC. You can cap or limit it in the Idle airflow tab or you can adjust it based on airflow (torque in this case) vs A/C system pressure (in the form of voltage) in the torque management section. Disable the LTIT's in your idle airflow table, log STIT's in your scanner until you get them stable with AC on and off and then bring LTIT's back in.
    Hsquared racing engines RHS 427, Procharger F2, Moran Billet Atomizer injectors, Alky Control,Mast LS7 heads, Nitrous outlet kit,Tilton quad disc clutch, DSS shaft, RKT56 ZR1 trans, RPM Quaife diff. Built and tuned by yours truly.