This post is information on a situation I ran into installing an LS3/525hp motor in my 56.

Upon installation and driving 10 to 15 miles, the engine MIL fault light came on. Reading the code (needing a Tech 2 compatible reader), it turned out to be a ‘high idle’ fault code. It turned out that the idle checked out to be at the specified 975 rpm (high for most motors, but I was told this motor with the ASA cam was programmed to that rpm).

The tuner indicated he’d run into this before, and the problem was it took too long to return to the idle rpm with no throttle. He changed a parameter to reset the idle time constant (more critically damped rather than over damped, for the engineer types). The engine does return to idle noticeably quicker.

i also had this code blocked from being able to set the engine MIL light, so it doesn’t potentially mask a problem I’d care about.