I've had a lingering P0507 code after doing my wifes UIM/LIM replacement on her '98 Bonneville(high idle). Not enough time to really get at it until tonight.
After checking for vacuum leaks, I tried Will'* suggestion of pulling the oil fill cap with the engine running. Hey, won't come off! Then I tried really hard--finally came off with huge suck.
Then went to check PCV valve as he suggested. Not only no bad seals, but no PCV valve! Did same thing as Bigfoot (by the way Bigfoot, I wear a size 15 boot). I never put the valve in either
All our cars and trucks have the PCV valve in a different place, is my lame excuse.
Anyway, found the old one and cleaned it up and stuck it back in until I can get a fresh GM on. Yep, idle dropped back to normal (1300 down to 800 or 900 or so). Can't remember if the engine light will reset or maybe I'll have to do it manually.
Anyway, I relate this to encourage fellow newbies to bring problems to the forum no matter how dumb you think you might look. I'm a retired Old Fart, so I don't even care any more, but no one will razz you anyway(unless you're a newbie AND an old fart, in which case willwren will inevitably think up something(I imagine even before midnight