Silverado cylinder 8 misfire no codes no fix

[WarpDrive42]

This one's got me scratching my head. 2012 Silverado 4.8 flex, cylinder 8 misfire started right after seafoaming the intake. No codes, no vacuum leaks, compression and leak-down tests check out. Swapped injectors, coils, plugs, wires, O2 sensor. Misfire stayed. Fuel pressure is fine, even replaced the computer. Tore the engine down, lifters, valves, pistons, cam, everything looks good. Still misfiring on 8. What the hell am I missing?

 

[JohnConstantine]

Read the title and I was like I got this, then I read what you did and was like, okay maybe I don't have this. Seems like you changed everything, have you tried a new engine? Just kidding.

Since it happened after seafoaming I'll skip those harness and ground recommendations, even though you might want to check them, so I'll ask you if you gave the valves a good look? Maybe the seafoaming disloged something that got stuck on the edge of the valve or the valve seat?

 

[Neuro Glyph 76]

That's truly like chasing a ghost, given all the work you've put into it, and @JohnConstantine brings up a good point about the valves and Seafoam. Sometimes, a hairline crack in the cylinder head or block that only acts up under certain conditions can be a silent killer, causing such a stubborn misfire without throwing codes. Anyway, have you managed to get to the bottom of this perplexing issue?

 

[5ierraHauler]

That sounds really frustrating, those pesky cracks or valve problems can be hard to spot. Have you found anything new or managed to fix it yet?

 

[pastaco0ker]

Ah, that's brutal. could there be carbon debris stuck in the injector seat or valve area causing intermittent sealing issues?

 

[MacNCheez]

That's truly like chasing a ghost, given all the work you've put into it, and @JohnConstantine brings up a good point about the valves and Seafoam. Sometimes, a hairline crack in the cylinder head or block that only acts up under certain conditions can be a silent killer, causing such a stubborn misfire without throwing codes. Anyway, have you managed to get to the bottom of this perplexing issue?

yeah, OP said valves look good, but what about the seat for the exhaust valve on #8? Could there be a microscopic crack in the seat or a very slight imperfection that only causes an issue when hot or under specific exhaust pressure from combustion? It's rare, but after all you've done

 

[mrclean]

This one's got me scratching my head. 2012 Silverado 4.8 flex, cylinder 8 misfire started right after seafoaming the intake. No codes, no vacuum leaks, compression and leak-down tests check out. Swapped injectors, coils, plugs, wires, O2 sensor. Misfire stayed. Fuel pressure is fine, even replaced the computer. Tore the engine down, lifters, valves, pistons, cam, everything looks good. Still misfiring on 8. What the hell am I missing?

Man, if it started right after Seafoam and everything else checks out, I'd be tempted to scope the exhaust valve at temp. Could be something subtle that only shows when things heat up.