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Thread: IAC issues with 400 Ponch & Comp Cams XR276HR.

  1. #1
    Join Date
    Feb 2017
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    St Chuck, MO, USA
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    10

    Default IAC issues with 400 Ponch & Comp Cams XR276HR.

    I've been able to pin down that one of my biggest issues is the IAC on my Holly Terminator. Specifically, this is on a 400 Poncho, three pedal Trans Am and when I let off the throttle in gear (TPS 0% like coasting to a stop or slower cruising) the IAC moves from parked to dead closed (IAC 0%) causing the engine to act lean and buck like a mule (very lean) until you apply a little throttle (TPS >0% and IAC resets). When I review the data logs, what I think is actually happening when the IAC closes is it's spiking MAP and the system is trying to compensate for what it reads as a rich condition. In reality, the spiked MAP increases the exhaust gases being pulled in through the slightly overlapped Exhaust/Intake openings, showing rich to the AFR.

    I've covered the IAC hole with tape, disabled the Idle Spark control and dialed in a hot idle with 20° advance of 875+/- 50 RPM, then reset TPS, enabled the Idle Spark & IAC. IAC at idle is 8-15%, spark jumps +/-2-3°, it holds idle and seems happy there.

    A great example of the issue is documented in the attached log starting at ~135 sec. I'm sitting at a traffic light, then throttle up and shift 1st to 3rd gear to the next light. Traffic starts slowing ahead approaching the light, so let off the throttle in gear at ~172 sec and at ~176 sec, the RPM drops through the IAC ramp window and the IAC goes to 0% causing the engine to start bucking wildly. I downshift to 2nd, the RPM rise and the process repeats as I continue to slow at ~194-205 sec only I'm holding the TPS just above 0% as best I can with traffic until I clutch it at 205 sec, then IAC resumes idle control.

    At ~216 sec, I made a Bonzai down the entrance ramp onto the highway and "let it learn" while varying the RPM and load until I get off the highway at ~550 sec. Here I downshift to 3rd and at 564 sec, when the RPM falls through the ramp window, the IAC repeats, the bucking starts, so I downshift to 2nd and hold slight throttle. As I continue to slow, at 571 sec I let off the throttle (TPS to 0), the buck starts again, so I clutch it and roll to stop.

    The rest of the log is me going light to light on an urban highway and you can see the process repeats exactly the same way until I shutdown in the garage at 911 sec.

    The setup is HP EFI V4, Comp Cams XR276HR Xtreme Energy 224°/230° Hydraulic Roller, Holley Dual Sync and I'm confident of the install (I've had it apart and back together a lot). If I set the "RPM above Idle to Start Ramp" at 500 RPM, it hangs at about 1500-1700ish. When I set it at 1000 RPM above, it of course starts ramping the IAC (making the lean condition) while you are cruising at say neighborhood/parking lot/slow traffic speed, it goes bad lean in seconds and starts to buck. The cam is not crazy big, but the whole bucking around is undesirable. Thoughts?
    Attached Files Attached Files
    "New" 1979 Trans Am, 400 Pontiac, Super T-10, Chassisworks front end, DSE Quadlink rear end, Wilwood 13in discs with Hydratech hyd brake booster, HP TBI EFI, HP Dual Sync, Comp Cams Hydraulic roller, MSD 6AL (6420) & Blaster 2, dual Spall 13" Elec fans, 150amp Alt, Compustar/DroneMobile alarm...and a lot of other new stuff I'm trying to get put together...

  2. #2
    Join Date
    Feb 2017
    Location
    St Chuck, MO, USA
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    10

    Cool IAC issue SOLVED!

    The instructions and all the forum posts I've read here and elsewhere indicated that the IAC HOLD Position should be in the 20%-55% range. As noted above, this causes what I understand to be a lean condition as the RPM falls through the IAC Ramp Down range. This is counter intuitive as the IAC being closed should do the opposite...except there's that valve overlap issue above. It also causes an air leak past the throttle plates all the time (possible source of inconsistent mixture).

    Out of frustration, I tried setting the IAC HOLD to 10% and the condition still existed, though lessened. I then dialed in IAC HOLD to 0% and the bucking condition went away! I've read nowhere that it was advisable or practical to run the IAC that low, but the proof of function is sitting right in my garage.

    Now I can downshift and let the engine coast down almost to idle while in gear, then throttle back up again with no bucking at all (think entering and exiting a turn or slow traffic). The throttle response on acceleration was cleaned up as well. The IAC HOLD number is the only thing changed from the GF and datalog attached above and the difference is literally night and day in driving performance.

    The lesson learned: The IAC closing as it attempts to bring the engine to idle while it is still 'coasting' (TPS 0%) in gear created the bucking (indicative of over advanced or very lean condition). By closing the IAC HOLD position to 0%, a consistent mixture is achieved during ramp down. My gut feeling is that the changing AFR and MAP caused by the IAC movement as it ramped down was too much for the CL Comp to keep up with. I believe this same condition would exist in an automatic, you just wouldn't notice it as much and it is likely just dismissed by most.

    I've applied this whole thought process to why having the IAC HOLD value at anything else but held closed at 0% until the engine actually reaches the target idle RPM would be beneficial - I can't see why having this open at all until idle RPM is reached is helpful. I invite explanation on this because I'm sure there is a reason...it just wasn't working for me and some enlightenment would be appreciated.
    "New" 1979 Trans Am, 400 Pontiac, Super T-10, Chassisworks front end, DSE Quadlink rear end, Wilwood 13in discs with Hydratech hyd brake booster, HP TBI EFI, HP Dual Sync, Comp Cams Hydraulic roller, MSD 6AL (6420) & Blaster 2, dual Spall 13" Elec fans, 150amp Alt, Compustar/DroneMobile alarm...and a lot of other new stuff I'm trying to get put together...

  3. #3

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    Have you fiddled with the RPM Above Idle To Start Ramp and the RPM Above Idle To Re-enable Idle Control values? That last one especially can be fickle. Andrew
    IG @projectgattago

  4. #4
    Join Date
    Feb 2017
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    St Chuck, MO, USA
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    Default

    I did tonight, it's now set at 350 RPM Above Idle and it's still flashing back to idle without any hangs - hot or cold. It could likely go lower, but that's far enough down in the operating range that having it control the spark shouldn't be an issue either. I haven't changed the 100 Above To Re-enable, that seems reasonable. With the IAC HOLD set at 20%+, it wouldn't idle down. It would hang at 1500-2500 RPM, depending on how far open the IAC was held, go figure.

    I'm super curious to understand this IAC setup. As I see it, its supposed to add air when the engine's RPM falls below target idle. If it's above the target, why engage it? Activating this in an operating range that's significantly used for driving seems counter productive. I can attest, it has terrible effects on driveability. Since setting the IAC HOLD to 0%, it drives like it has a well tuned carb on it. Drove like crap until I made that one tweak, then, instantly, it's behaving like a prized thoroughbred.
    "New" 1979 Trans Am, 400 Pontiac, Super T-10, Chassisworks front end, DSE Quadlink rear end, Wilwood 13in discs with Hydratech hyd brake booster, HP TBI EFI, HP Dual Sync, Comp Cams Hydraulic roller, MSD 6AL (6420) & Blaster 2, dual Spall 13" Elec fans, 150amp Alt, Compustar/DroneMobile alarm...and a lot of other new stuff I'm trying to get put together...

  5. #5
    Join Date
    Dec 2009
    Location
    Connecticut
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    22,691

    Default

    This is tuning. My IAC Hold Position is at 20% and it's fine.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  6. #6
    Join Date
    Feb 2014
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    Northern Ohio
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    25

    Default

    Quote Originally Posted by Dan Morrison View Post
    I did tonight, it's now set at 350 RPM Above Idle and it's still flashing back to idle without any hangs - hot or cold. It could likely go lower, but that's far enough down in the operating range that having it control the spark shouldn't be an issue either. I haven't changed the 100 Above To Re-enable, that seems reasonable. With the IAC HOLD set at 20%+, it wouldn't idle down. It would hang at 1500-2500 RPM, depending on how far open the IAC was held, go figure.

    I'm super curious to understand this IAC setup. As I see it, its supposed to add air when the engine's RPM falls below target idle. If it's above the target, why engage it? Activating this in an operating range that's significantly used for driving seems counter productive. I can attest, it has terrible effects on driveability. Since setting the IAC HOLD to 0%, it drives like it has a well tuned carb on it. Drove like crap until I made that one tweak, then, instantly, it's behaving like a prized thoroughbred.
    I've read your posts about what you were dealing with concerning the IAC, I'm trying to understand how this works as well. I'm truly a novice with this even though I installed this system back in July of 2014. When I go to the screen that shows IAC Hold Position is 30%, IAC Ramp decay is 4.0 sec, IAC Ramp Start (RPM Above Idle) is 1000, and IAC Kick is 50%. So with that being said, my issues are what you were dealing with when coming to a stop and around town driving. I found the Idle Spark enabled, which I don't use the ECU for timing, so disabled it, that helped with some of my low speed/idle issues, but it still needs some tuning. I don't understand how these values interact with one another, so I haven't changed anything. These are the values the initial program installed when I uploaded it this past week. Any help in understanding how these things work would greatly be appreciated. I've never done any tuning or know where to start. I've fought these issues since installing and have come close to putting a carb back on, but I refuse to let this beat me. Thanks for any advice and help.

  7. #7
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    Quote Originally Posted by Chetter'sZ View Post
    I don't understand how these values interact with one another, so I haven't changed anything. Any help in understanding how these things work would greatly be appreciated.
    EFI Software Help Information/Instructions:
    ‒ On the top Toolbar, click "Help" & "Contents". This opens all Help topics.
    ‒ When navigating the software, click "Help ?", drag it to any parameter and click again.
    ..This automatically opens the definitions for that specific parameter.
    ‒ Tuning information can be read by clicking the F1 key, when you're viewing any screen.

    http://forums.holley.com/showthread....neral-IAC-Info (General IAC Information - Read "IAC NOTES")
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  8. #8
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    Thanks, I'll try this.

  9. #9
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    You're welcome.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  10. #10
    Join Date
    Feb 2014
    Location
    Northern Ohio
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    One more question, I opened and read the link concerning IAC note and info, my question is under the IAC Speed it shows 4BBBL TBI. I see that you scroll down in that box to show other options including Slow. Do I use Slow as indicated in what I read about IAC notes where you have "Advanced Idle Control in the Typical IAC Control/Ramp Down settings"? I appreciate your responses and it really helps as I continue to read & learn. Car is running so much better now as I do baby steps on some changes writing down what I changed and the order I changed things.

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