I have a 2000 GS300. The check engine light went on, and the engine began to run very rough, like one cylinder wasn't firing. The dealer said that the seal on a casting plug leaked coolant into one of the cylinders and shorted-out a spark plug. The dealer said that it was not a head gasket leak, but a plug used to fill one of the holes used to remove sand in the original casting process. He also said that he had never seen this happen before. Anyone out there heard of this one?