I’m having a prblem with my engine. When i first start the engine, it seems to run fine for about 1 min then it starts running very rough and I get multiple cylinder,(clyinders 2, 4, and 7) misfire codes. I have replaced the pcv valve because I had a loud whining noise and once i remove the old valve, about 1/4th of the o-ring was melted/destroyed. I’m thinking there must still be a vacuum leak or sensor problem since the engine is running perfectly fine for that first minute. Any help or suggestions about where to look would be greatly appreciated.