Detonation under no load

[1985olds]

I have an l76 with tsp cnc ported ls3 heads, Tsp 229/244 .615/.629 cam and holley terminator x max. Heads were not milled, but blokes was decked slightly. It's a fresh rebuild and has about 5-10 mins of run time
I heard a noise (rattle) and pulled off the heads. It has signs of detonation on cylinder 8. The engine only has idle/free rev time, no driving, so no load on the engine. The noise started when the holley went into learn mode. What would cause detonation with bo load?

[1985olds]

Using sunoco 93

[1985olds]

Thinking bad injector or coil

[btmatt]

Oil contamination in the cylinder?

[1985olds]

Oil contamination in the cylinder?

I dont think so, there was no evidence of oil in the cylinder.

[btmatt]

Hmm. What did you see in the #8 hole? I know the early 6.0 did not offset the piston pin and they slap when cold. It could be mistaken as detonation...

[1985olds]

Hmm. What did you see in the #8 hole? I know the early 6.0 did not offset the piston pin and they slap when cold. It could be mistaken as detonation...

You can see the detonation on the top ring land. I took the piston to my engine builder and he confirmed.

[badazz81z28]

You can see the detonation on the top ring land. I took the piston to my engine builder and he confirmed.

Could be a controller issue, a bad injector, vacuum
leak? I know people have blamed the steam pipes for some failures, but I don’t know.

[1985olds]

I sent out the injectors and had then cleaned. They came back good with no issues. Waiting to get the engine back together to check for vacuum issues. I'm running ngk tr5gp plugs, I'm thinking a colder plug might be the next. Could the plugs be too hot for this mild setup?