- Check vacuum lines. On these vans the vacuum lines have a habit of cracking around the corners by the manifold, causing loss of vacuum pressure.
- Check and replace the vacuum reservoir. This part will also deteriorate under the hood's heat and become weak, unable to maintain vacuum pressure.
- Check the actuator on the plenum. With a helper watch under the hood as you turn the temperature knob from cold to hot while the van is idling, see if the door is physically sliding open and shut.