Cap should be firmly tightened.
Get the CEL code read (AutoZone and most places like will pull it for free w/ their OBDII). Most likely it'll be code P1128.
Sounds like you might have a bad cap again. More than likely though it's a cracked oil filler tube which is spreading open when you firmly tighten the cap...
It is DIY-able w/ plenty of instructions here. Search on oil filler tube if that is indeed the case.
Start w/ getting the CEL code read first. Post it here for further diagnosis.
Good luck
