GM originally suggested the use of a slack-tube manometer to adjust this system. A traditional analog manometer can be used in its place and will avoid the hassle of filling the slack tube with distilled water and dye. A regular vacuum gauge cannot be used, since it measures inches of mercury, while a manometer reads in inches of water. Water is lighter than mercury, which makes the manometer more sensitive. It will first be connected to the rear throttle body at the venturi vacuum source, which is not where the air cleaner was connected. The port next to it is the venturi vacuum and is capped from GM.