The TBI unit is attached to the top of the intake manifold by several bolts. These fasteners can sometimes work loose from vibration and temperature changes during normal engine operation and cause a vacuum leak. If you suspect that a vacuum leak exists at the bottom of the throttle body, obtain a length of hose. Start the engine and place one end of the hose next to your ear as you probe around the base with the other end. You will hear a hissing sound if a leak exists, but be careful of hot or moving engine components. Remove the air cleaner assembly, tagging each hose to be disconnected with a piece of numbered tape to make reassembly easier. Locate the mounting bolts at the top of the throttle body. Decide what special tools or adapters will be necessary, if any, to tighten the fasteners. Tighten the bolts to the torque, but don't overtighten them, as the threads could strip. If, after the bolts are properly tightened, a vacuum leak still exists, the throttle body must be removed and a new gasket installed. After tightening the fasteners, reinstall the air cleaner and return all hoses to their original positions.