I have a Mk4 Astra LS with a Z16XEP twinport engine. It is idleing irratically, and on inspection there seems to be a build up of negative pressure in the engine. As you lift out the dipstick or take of the filler cap the revs pick up and there is a lot of air being sucked in via either opening. Air is being sucked out of the engine through the breather pipe that runs from the rocker cover to the throttle body. I presume this is not normal.

Does anyone have any suggestions as to what could be causing this, or a diagram of the throttle body, so i can see if there is some sort of regulator or valve, that may be damaged.