V6 LS3 (2 dead holes).

[416velle]

I've built a lot of LS engines, I've diagnosed a lot of em, nothing has thrown me for a loop more than this. Backfiring at idle under 1300 RPM, cylinders 6 and 8 are dead.

416 LS3, ported 823's, ported LS3 intake, 13:1 compression, K1 crank/rods, wiseco pistons, milled heads, [email protected] cam, running off Terminator X Max EFI, sippin on E85 with dual 255lhp pumps and 65lb Deatschworks injectors, built by yours truly.
FRESH engine, just fired it up 5 days ago as of now, knew there were problems almost right away but wanted to get a heat cycle or 2 in it.
Over 1300RPM it sounds like a normal engine would on 6 cylinders; anything less than that is backfires HORRENDOUSLY through the exhaust, sounds almost like theyre firing on intake strokes but when I take out the plugs of 6 and 8 and theyre drenched in fuel, with zero evidence of wanting to fire. The headers are also cold to touch even after running a full heat cycle.

Heres everything I checked:

If I unplug either the injectors or the plug wires of 6 and 8, it doesn't backfire at 1000rpm idle. If I leave injectors plugged in with plug wires off, it doesn't backfire.
TR6 spark plugs gapped at .035. 8:1 AFR according to WB. 60PSI fuel pressure on the dot. Coils have power, coils have ground. Injectors have power/ground and aren't stuck, took the intake out/unplugged injector harness but left fuel lines connected, primed the pump and not a drop of fuel out the injectors. No vacuum leaks. No bent valves, good compression test; 3% deviation between worst and best cylinders. 6-8 have spark (shocking, literally) and spark looks comparable to the good cylinders. The wires are good, swapped em around. Coils are good, swapped around. 6 and 8 are NOT FLIPFLOPPED, plug wires were always snapped in all the way and likewise on coil side with both audible clicks.

Things I haven't checked:
injectors (havent switched em around yet though I don't see why this would prevent a cylinder from firing all together), coil harnesses though I did ohm out bank 2 and it was OK with no shorts/split wires/etc. Haven't looked at the injector harness (not sure why I would but at this point I might be game). Haven't pinned out the entire engine harness from coil connectors to ECU connector.



What else am I missing here? I'm at a loss. Spent my entire 3 day weekend trying to fix this to no avail.