• Welcome to SVTPerformance!

Odd idle issues, ISC duty cycle

Discussion in 'Engine/Tuning' started by xtreme_exploder, Dec 8, 2019.

  1. xtreme_exploder

    xtreme_exploder Member Established Member

    Messages:
    207
    Joined:
    Jul 17, 2013
    Location:
    OKC
    I have an idle problem that is very difficult for me to figure out. Car is an '03 Cobra with a Whipple Crusher, 80lb injectors/return system, and ported heads w/ Comp "stage 2" blower cams. I have the idle set at 750 rpm, and I'm assuming that isn't too low. While the car is idling, it has a weird issue where the idle will drop to around 600 rpm every 20 seconds or so. My data logs show the ISC duty cycle instantly dropping by at least 10% when that happens. Then it'll slowly pick itself back up, before doing the same thing. I've tried adjusting the throttle blade position with the IAC and TPS unplugged, only seems to make it worse. I can't ever get the duty cycle to the 30% or so that everyone says is ideal. My integrator will slowly wander up and down, which i don't understand. The car often has trouble starting or will stall after firing up. Also, the spark source is always at minimum clip while idling, not sure why that is. My datalog is linked.
     
  2. xtreme_exploder

    xtreme_exploder Member Established Member

    Messages:
    207
    Joined:
    Jul 17, 2013
    Location:
    OKC
    Looking over the log some more, the only thing I notice is that the sudden drop in IAC duty cycle happens when the engine finally gets close to the desired idle RPM. You'd figure the duty cycle would just chill there. Weird.
     
  3. BLOWN9646

    BLOWN9646 E85 COBRA Established Member

    Messages:
    2,089
    Joined:
    Apr 12, 2006
    Location:
    N IL
    E85 tune? Might need COP's, maybe someone else can confirm
     
  4. slo984now

    slo984now AKA 01yellerCobra Established Member

    Messages:
    15,411
    Joined:
    Oct 5, 2005
    Location:
    San Diego
    When you set the idle with the IAC unplugged what RPM did you aim for? I usually set it for about 50 RPM under what I have it set to in the tune. It gives the computer a little wiggle room if needed.

    Also make sure everything mechanical is good. No leaks, intake is tight, etc. Tuning won't fix a mechanical issue.
     
  5. xtreme_exploder

    xtreme_exploder Member Established Member

    Messages:
    207
    Joined:
    Jul 17, 2013
    Location:
    OKC
    bent over backwards looking for vacuum leaks and such. I just set the car to idle as low as it would go without stalling while the IAC was unplugged. I'll try opening it up some more to see what it does.
     
  6. slo984now

    slo984now AKA 01yellerCobra Established Member

    Messages:
    15,411
    Joined:
    Oct 5, 2005
    Location:
    San Diego
    I wouldn't do that. There's another way of locking the IAC through the tune then adjusting the throttle stop to hit the target RPM. But I could never find the right settings. The last time I messed with it I actually opened up the throttle a little. It would rev a little then settle back down as the IAC corrected.

    Don't forget to adjust the TPS after messing with the throttle stop.
     

Share This Page