Gen 3 – Rebuilt cylinder head, now number 3 is black, misfiring

I’ve owned 6 or so Subarus and worked on them over the years. My tool box is limited – no sophisticated scanners.

On my 2008 Legacy 2.5 I bought a used passenger cylinder head because mine had cracked when I overtightened the oil pressure sending unit (I think that’s what it’s called).

I had the head compression tested and resurfaced at a reputable shop. I installed it and the car ran bad, misfires at idle and the short term fuel trim was running -29 at idle and you could smell gas. I checked compression on the cylinders, checked for vacuum leaks, tested the coil pack with a meter, tested the spark plug wires with a meter and watched the engine for spark to ground in the dark. I also watched the spark on each plug with a spark tester. I replaced the fuel injectors. I even drained the gas down to probably 2 gallons and added 5 gallons of 93 octane as this car had not seen new gas in almost a year. No luck – still got misfire codes. First P0301 and P0302 then all 4 cylinders.

I pulled the passenger cylinder head and was surprised to see that the head was blackened on cylinder 3 but not on cylinder 1. What is this telling me? Is it poor spark/incomplete combustion on #3? I try not to just replace parts but not sure my next move.

Any ideas are much appreciated.

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