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Thread: Idle Issue

  1. #1

    Default Idle Issue

    Good afternoon. I've done a search and could not find any threads with a similar issue. My dad is chasing a rough idle condition with his '74 Corvette with a ZZ502, manual transmission, and spark is controlled with a HyperSpark. About six months ago the IAC failed. The engine idle would fall to the point where it would die. I troubleshot the unit and tracked it down to the IAC. The ECM would slowly ramp the IAC up to 100% attempting to increase idle, however, the IAC was not responsive. We borrowed the IAC out of my Chevelle, and problem solved.

    He replaced the IAC and it was fine until recently where the idle became rough and unsteady. Today I worked on it again and ECM showed the IAC spiking from roughly 0% to 40%. I could hear air passing the IAC circuit with the spikes. The IAC would periodically spike to the 60% range and the idle would climb to about 1500 RPM before ramping down. We confirmed the wiring is away from plug wires, and I do not see anything in the tune that would cause this issue. Any ideas? We are lost. Thanks.
    Corvette Base
    Sniper Log 20190721-094135

  2. #2
    Join Date
    Nov 2014
    Ont. Canada


    Look at your IAC trace in your datalog. You have some EMI happening. This has been discussed many times. Do a search and you should find something on it. Also, your alternator is not charging very well, could be where the EMI is coming from.
    Last edited by 81 TransAm; 07-21-2019 at 09:07 PM.

  3. #3
    Join Date
    Dec 2009


    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.

  4. #4


    I'll have him try to track it down. I also noticed the low voltage when he was here, but figured it was because he had just started the engine and the cooling fans were running. I experienced some interference after I installed the Sniper in my Chevelle, but it looked nothing like this. Thank you both for the help.

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