General Motors advises inspection for piston damage when the MIL is on and the engine misfires and displays a ticking/knocking noise. This may be caused by an aftermarket ECM calibration/component and detonation that leads to melted pistons.
Applicable vehicles include Buick, Cadillac, Chevy, GMC and Hummer vehicles equipped with any of the L-series engines.
If normal diagnosis isolates no compression on the misfiring cylinder(s) with 100% leakage into the crankcase, using a lighted borescope, inspect the top of the related piston(s) through the spark plug hole to inspect for damage.
Inspect for aftermarket components such as exhaust, ignition, high performance air filter, air intake system, etc.
Determine if factory ECM calibration numbers are present.