Hi
Haven't read a whole topic yet, but I just want to say that I have also similar problem. The same exhaust valve, gone twice.
Subaru Impreza GF6, 1994, 1.8 [ EJ18, SOHC, naturally aspirated ] AWD, European spec.
I bought car with ~205'000km, and after some 4 months driving exhaust valve was gone.
I changed whole head (from bit newer engine, it was said to be 1.8 USDM), and it run ok for some 14 months, around 15'000km. And then again - the same cylinder (#4), the same valve was gone.
What's the reason for those deposits on valve face?
both heads:
newest below, oldest above.
Removed pistons as well, because I thought that cause is oil that gets in cylinder. All seems fine:
As next I'm going to check ignition cables, spark clubs must be ok, since they were changed short after first engine head was gone.
For a pity I don't have equipment to check injectors, probably that's the fault.