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.
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