I've recently been having problems getting my car to start. The engine turns, but won't start all the way up. I took it to the shop and they claim it was the pc valve which they replaced with a new one. However, it's been acting up again. I've changed the spark plugs, wire set, and the core pack. I have noticed when I take the pc valve off, so the engine get breath in fresh air for a minute or two, put it back on, the car seems to start right up for a couple of days and then repeats itself. I'm not understanding this. Any suggestions?