Here is a Technical Service Bulletin From GM on this vehicle.
SERVICE MANUAL UPDATE, SECTION 6B, WATER PUMP REPLACEMENT
1991-93 CENTURY MODELS
The following procedure should be used to remove and install the water pum on a 3.3L engine used in the 1991-93 Century. This procedure also revise the fastener tightening specification chart located at the end of the section.
Water Pump Replacement
Tool Required: J 36660 Torque Angle Meter
Remove or Disconnect:
1. Negative battery cable. 2. Engine coolant partially. Refer to "Draining" under "Draining, Flushing and Filling Cooling System" in Section 6B of the Service Manual. 3. Serpentine belt. 4. Belt idler pulley bolt and pulley. Pulley bolt has left-hand threads. 5. Water pump pulley bolts and pulley. 6. Water pump attaching bolts. 7. Water pump.
Water pump mating surfaces.
Install or Connect:
1. Gasket and water pump. 2. Attaching bolts.
Water pump to block bolts to 30 N-m (22 lb.ft.). Water pump to front cover bolts to 15 N-m (11 lb.ft.) plus 80 degrees using tool J 36660.
3. Water pump pulley and bolts.
Water pump pulley bolts to 13 N-m (116 lb.in.).
4. Idler pulley and bolt. Pulley bolt has left-hand threads.
Hold idler pulley stationary so that when tightening bolt a false reading is not read by applying pressure against the idler spring tension.
Belt idler pulley bolt to 75 N-m (55 lb.ft.).
5. Serpentine belt.
6. Top off radiator
with 50/50 ethylene glycol/water coolant mixture. Refer to "Filling" under "Draining, Flushing and Filling the Cooling System
" in Section 6B of the Service Manual.
7. Negative battery cables.
For coolant leaks.
FASTENER TIGHTENING SPECIFICATIONS
*Belt idler pulley bolt to 75 N-m (55 lb.ft.). Water pump -toblock bolts to 30 N-m (22 lb.ft.) Water pump -tofront cover bolts to 15 N-m (11 lb.ft.) plus 80 degrees. Water pump pulley bolts to 13 N-m (116 lb.in.).
*Belt idler pulley bolt has left-hand threads.