My 93 B230FT was runnning great until 2 days ago. It suddenly started a very rough idle. It still runs fair at speed, but not nearly as well as before. It's getting spark but a pressure test revealed 155 PSI in #1 and #4 but only 70 PSI in #2 and #3. I suspect burned valves and am prepared to pull the head. My questions are: Why would valves on 2 cylinders burn at once? It seems odd that both went at once, same day. I have noticed no other problems (synthetic oil level is fine, coolant fine etc.) Also, could there be any other problem besides valves causing this such as a head gasket etc? What should I look for when I pull the head? Will burned valves be obvious? Or can they be burned and not obvious? If valves are burned I know to take it to a machine shop but what parts should I plan to replace? Only valves and head gasket? Which seals? Do these parts come as a kit package from volvo or elsewhere?

Thanks for the help. I've been driving volvos since 1973 (9 total including my wifes and daughters) and have never had to pull a head or oil pan.