Could also be a vacuum leak. I had the same issue when I attempted to change my cam follower (I had 2 metal pipes so failed) - there's a vacuum pipe to the right of the HPFP but down a little which can easily break loose.
Check your PCV by having the engine running and taking the oil cap off to see if there's any noticable change. If it runs like a bag of crap but gets better when you put it back on it could be something else.
