Well I would suspect a cracked and or warped head,it is not a good idea to do one head though since U will have 2 have the damaged head surfaced this will cause a raise in compression on that bank ,so pull the head first to confirm damage and when U have the head off fill the bad c-lender with a good cup of oil it should take at least overnight for the oil to leak down ,also inspect the c-lender walls to see if it has damage .Chico(pops)