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Thread: IAC got stuck again.

  1. #1

    Default IAC got stuck again.

    I did a search and found this thread, but it was never answered:
    https://forums.holley.com/showthread...light=sticking

    I've had this exact problem two or three times. At random times I'll pull in my driveway to unlock my gate, put the truck in park and the RPM will be up around 1100 RPM. If I cover the IAC inlet it'll settle back down. It's sucking so hard that if I put some super strong duct tape over the hole (4"x4" large piece), within 30 seconds it'll have sucked the tape 1/2" down the hole. After shutting the truck down and restarting, everything is back to normal. There aren't any wires behind the Sniper to cause interference. The closest electronic device is my Dual-Sync distributor. As stated, this has only happened a few times and I'm hoping it's not beginning to fail with only 1500 miles on the truck.

    Another problem, I'm not sure if related to another failing part, is my Target AFR is rarely on point. It'll be in the ballpark most of the time, but can creep around quite a bit. Attached is a datalog and config file. Towards the end is the IAC event. I'm also linking to a video looking down the barrel of the Sniper upon restarting the truck immediately after said event. Seems like too much fuel at idle. I've ordered the Earls AN fitting and fuel pressure gauge.

    Truck specs: new Vortec 350 long block from Chevrolet, Weiand Street Warrior dual plane intake, 1" open spacer, MSD 6EFI, Dual-Sync distributor, MSD 8.5mm Super Conductor plug wires, MSD Blaster SS coil, NGK TR55 plugs at factory Vortec gap of .060", Hedman Black Maxx ceramic coated long tube truck headers, O2 bung welded a few inches past collector and 2.5" pipes to an H-pipe two feet after O2 sensor, then mufflers another couple feet after that. Fuel pump is a new ACDelco EP381 in tank. Supply and return lines are both -6AN (3/8").

    I've done a bunch of grounding:
    Ground from battery to frame, from same bolt on frame to engine block, from same bolt on frame to alternator body, from same bolt on frame to inner fender and radiator core support, from rear of engine block to second frame bolt, from second frame bolt to firewall, from base of Sniper to yet another bolt on frame, from base of sniper to body, from air filter base to body and frame through same cable as sniper base ground, from body and frame to inner ground buss under dash. Sniper and Dual-Sync harness triggered by relay pulling power direct from battery. MSD 6efi triggered from OEM pink coil wire. Sniper power leads direct to battery. 175 amp Alternator direct to battery and battery direct to firewall junction. Most of the Sniper harness is behind the glove box away from other wires. All wiring related to the Sniper, MSD 6efi and Dual-Sync individually wrapped with aluminum foil tape, then tape is grounded to body and frame.

    091019.zip Video down barrel: https://youtu.be/z3JKbYUv7Ek

  2. #2

    Default

    LOL. I know what you mean. I too have had this issue. I've searched and read the forums, but no one will answer the concern. Does your TPS always read zero at idle? Sometimes mine will be 1%. No, the linkage isn't sticking. I have to shut off the car wait a few seconds then restart it for it to read zero, for awhile, then back to 1% TPS. Rinse & repeat. I hope you find the answer. If you find the answer from some other forum or internet search, please update this thread. Thanks, Danford1

  3. #3

    Default

    I see a ton of grounds described but don't think I see one from block to battery? Maybe I just missed it. But the main thing I see is in the last 1/4 of your datalog where the TPS is 0% for a long period. I assume you are just letting the car sit there and idle?

    Your target RPM at this point is 750 but the actual RPM are almost 1,100. The IAC is at 0% so it can't lower RPM any more and the Idle Spark assist has pulled the timing down from the table value of 25° to an actual of 17°. That means the ECU is pulling all of the timing it can to try to lower the idle, but it can't get the idle down to the target.

    All that tells me you need to adjust the throttle blades. The Sniper is trying to lower the RPM to hit the target idle, but the butterflies must be open too far so it can't. Or you could have a vacuum leak. Get the car up to operating temperature, put it in park and turn the idle screw until the IAC is between 2-10%. If you can't turn the screw enough to get there you have a leak for sure. I'd also recommend you disable the Idle Spark at least while you are troubleshooting the mechanical idle screw.
    Last edited by gregs69rs; 09-11-2019 at 06:59 PM.
    69 Camaro
    400 SBC, ProCharger D1SC

  4. #4
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,539

    Default

    What makes you say too much fuel at idle? It's a TBI now, not a carburetor.
    How about a video of the engine at 1100 RPM with the IAC at 0%?
    And can you make videos with your phone sideways, since my PC screen is 16 x 9, not 9 x 16? Gary
    Regards, Gary

  5. #5

    Default

    Greg: I’ll shoot a picture of my ground situation off the battery to engine.

    The throttle blades are adjusted to achieve 5% IAC value at hot idle. This has only happened three times in 1500 miles where I’ll get home and the IAC will be stuck wide open, sucking like crazy. If I shut the truck off and immediately start it again everything is back to normal with zero malfunction for hundreds of miles. That’s why I think it gets “stuck”.

    Gary: If/when it ever happens again, I’ll get a video down the bores.
    Additionally, my fuel pressure gauge and AN fittings came in today. So I’ll be checking fuel pressure tomorrow as well.

  6. #6
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,539

    Default

    Sounds like everything you've done is correct. And the IAC is "stuck" from the position it would've been in as the engine was decelerating. The ECU is commanding it to 0%, but the valve is not following. Possibly poor connection at the IAC or faulty IAC. Gary
    Regards, Gary

  7. #7

    Default

    I went ahead and installed the fuel pressure gauge just now. It’s dark out when I’m at home, so you’ll have to forgive the video. It’s uploading to YouTube as I’m typing, but with my rural connection it’ll take a while. The pressure primed and idles at a rock solid 58 psi according to the gauge. If I ease into the throttle it dips ever so slightly. However, if I blip the throttle rapidly, the gauge can fall as low as 50 psi.

    When I initially plumbed the new fuel lines, I ran nearly a quart of fuel through via repeated key on priming cycles before hooking the AN to the back of the Sniper. All fuel equipment is new including the tank, so I wasn’t expecting to see anything come out and I didn’t. I have a new fuel pressure regulator that I bought as a backup. Should I install it just to check and clean the screen as well?

    Anyhow, check back for a link to the videos later. There’s one of the fuel pressure responding to throttle, one detailing my wiring practices and a bumpy country road (again forgive me) one of me shifting from park to drive, creeping around a few yards and back to park again while looking at the handheld.

  8. #8

    Default

    Video of fuel pressure: https://youtu.be/fCjIvVelCx4
    Video of some handheld metrics: https://youtu.be/JA2ZZs_upxk
    This video shows what AFR as well as a few other metrics do going from park to drive, driving forward a couple yards then into park again.
    Last edited by HeavyChevy; 09-12-2019 at 06:48 AM.

  9. #9

    Default

    I think you need more fuel pump or bigger lines. Your dropping about 8 psi, which IMO is a lot. It will probably be more going down the road when the engine is under load and needs more fuel. On my 1250 Super Sniper my fuel pressure never varies like that until I get into Boost when it goes up 1:1. But I run a Aeromotive Dual Phantom pump and have -10AN feed and return lines running through a Aeromotive regulator. I know this doesn't help your IAC problem, but it'd be one more thing I'd address when you can.
    Last edited by djh61187; 09-12-2019 at 07:15 AM.

  10. #10

    Default

    Everything in the video of the handheld looked pretty normal to me. Are you saying there was a problem there? If so, what are your concerns?

    Also, in the datalog you posted, do you think the IAC was stuck wide open at some point in the datalog?
    69 Camaro
    400 SBC, ProCharger D1SC

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