If it's the AOS, it will be very hard or nearly impossible to remove the oil cap while the engine is running. Do you have any blue smoke coming out of the exhaust. When I replaced mine I had just a very light smoke, hardly noticeable. I could not remove the oil filler cap while engine was running. When I replaced it, the boot on the bottom of the AOS was dramatically pinched in and could have been causing my problem. The AOS comes complete and runs around $120.00. Any vacuum leak will cause problems with engines running performance. They are the hardest to find.