It's a while to get the bottom of the problem with my customer's Corvette C4 but I finally cracked it on Friday. It's had an issue with the idle being too high, the owner bought a new Idle Air Control valve for one of the throttle bodies but on replacement there was no difference, in fact it was a bit worse. At the same time the brake booster gave up the ghost, with the booster replaced I went through the vacuum system but could find no faults. There's a plug under the dash on which you can jumper 2 wires and put the ECU into test mode, with this done and the ignition turned on it should flash any error codes through the check engine light, there were none. It also cycles the IAC valves so I removed them but kept them plugged in and one of them worked fine but the other didn't move. With the valves swapped between the wiring plugs the fault stayed with the plug so I stripped off the insulating tape and found the wiring had been messed about with and 2 of the wires were crossed. Once corrected and reassembled the engine fired up and settled into a much better idle, 700rpm instead of 1200.
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