Return of a Tuning Tale. Chasing classic knock

[Neat]

The lean condition you are seeing may be cylinder misfire. I'm not sure about the narrowband O2 sensors, but I have seen the wideband on my car show lean when a cylinder drops out.

It was explained to me that the O2 sensor is just that, an oxygen sensor. It is not an air/fuel meter, even the widebands that are sold as air/fuel ratio meters use an oxygen sensor - which, not surprisingly, only senses oxygen. When the spark drops out for some reason, there is no burn to consume the oxygen or the fuel. The unburned oxygen and unburned fuel make their way into the exhaust, but the oxygen sensor only senses the oxygen and reports a lean condition; even though raw fuel may be coming out the tail pipes.

The momentary lean condition you describe sounds like it might be just that. There may be an intermittent loss of spark (which jives with your carbon traced coil) which causes a non-fire on one or more cylinders, the O2 reports lean because of the excess oxygen in the exhaust. I don't know how fast the ECM reacts, or if it even does closed loop correction during your knock / misfire event, but my LS powered car with a Holley stand alone will make a closed loop correction for the false lean condition in about 1/25th of a second. The sudden addition of fuel (again, on my LS car, not sure on LT5 stuff) causes a dopey rich effect, and one or two combustion cycles later I'll get multiple flame fronts from the excessively rich mixture hiding out in the ring pack / ring land area. The ECM then reports knock. The problem is exacerbated if the dodgey coil comes back online after only one or two misfires and then drops out again.

The scenario above only happens at light throttle. At WOT the extra fuel from the closed loop correction of the false lean is consumed enough to prevent extra fuel from hiding from the combustion event.

That's all real world stuff I've experienced with my LS car and the Holley EFI. I don't know how much, if any, of that is applicable or helpful to the LT5 / stock ECM; but maybe something in there will be helpful. Please keep us posted!