after replacing the valve cover gasket the car started to smoke. we were told to change the pvc valve. we did and the smoke stopped but the check engine light came on so when they read the codes it said there was a misfire. we changed the tubes on the coil packs and the check engine light went out but it started smoking again. it stops smoking after running it for a bit but will restart once it has set for a few hours. any idea what is causing this????