This is my friend’s car and I have never looked under a Subaru hood before. She is getting an intermittent check engine light with codes for cylinder misfires on one and three.
Today, I inspected the plugs and all four looked identical, a nice uniform tan color and good condition. No sign of deposits or oil. Did a compression test and found the driver’s side bank at 155/150 and the passenger side bank at 185/190.
My question at this point is which side is cylinders 1 and 3? The front-most cylinder is on the passenger side. Is this the number 1 and 3? If so, then the low compression on the other bank cannot account for the misfire.
1 and 3 are on passenger side
Start thinking about the possibility of a breached head gasket on the bank with low compression. Have you checked the oil for evidence of coolant contamination?
If that head gasket needs to be replaced, be sure to replace both of them.
Compression (350 rpm and fully open throttle):
Standard: 1,079 - 1,275 kPa (11.0 - 13.0 kg/cm2, 156 - 185 psi)
Limit: 883 kPa (9.0 kg/cm2, 128 psi)
Difference between cylinders: 196 kPa (2.0 kg/cm2, 28 psi), or less
No evidence of coolant in the oil.
Looking at the compression numbers I tend to think there may be a valve timing issue. The driver’s side may be one or two teeth off. This may explain the misfire codes also.