In 2000, the Century was manufactured with a 3.1-liter V-6 engine that was assembled with an aluminum upper and lower intake manifold set. General Motors split the intake manifold to allow ease of access to the fuel injectors located between the two manifolds. Under normal circumstances, the lower intake is only removed when the cylinder heads require removal or the lower manifold gaskets develop oil, coolant, or vacuum leaks. General Motors recommends to replace the lower intake bolts with new ones coated with a thread locking compound. An alternative to replacing the existing bolts, would be to clean them and apply G.M. thread-locker part number "12345382" to the threads.
Upper and lower intake manifold torque specifications
On the first pass, tighten the four lower middle bolts of the lower manifold to 62 inch-pounds, then the outside bolts to 62 inch-pounds. On the second pass, tighten the middle bolts to 115 foot-pounds, then the outside bolts to 18 foot pounds. Tighten the upper manifold bolts to 18 foot-pounds, starting with the center bolts then working outward.