I replaced the intake gasket when i installed the ls1. i sprayed around the intake to see if i was getting a leak, nothing happened. maybe the inner gasket slipped or something. i am gonna pull the header on that side to make sure it is seated correctly. o2 sensor shows active on that side all though it is the original {214k miles} . i am kinda lost at this point to know what to check for. i guess i will pull the intake to see if maybe that gasket slipped like i said but other than that what should i check? vac ports or anything else i should be looking for? thanks