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Here is a Technical Service Bulletin From GM on this vehicle.
SUBJECT:SERVICE MANUAL UPDATE, SECTION 6B, WATER PUMP REPLACEMENT
MODELS AFFECTED: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.
Water pump pulley bolts to 13 N-m (116 lb.in.).
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.).
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.
I don't know what engine it really has but here are the instructions for both engines.
Disconnect the negative battery cable, then drain the coolant.
Remove accessory drive belts. Remove the radiator and heater hoses from the water pump.
Remove the water pump attaching bolts.
Remove the engine support strut.
Place a floor jack under the front crossmember of the cradle and raise the jack until the jack just starts to raise the car.
Disconnect the front two body mount bolts with the lower cushions and retainers.
Remove the cushions from the bolts.
Thread the body mount bolts with retainers a minimum of three (3) turns into the cage nuts so that the bolts restrain cradle movement.
Release the floor jack slowly until the crossmember contacts the body mount bolts retainers. As the jack is being lowered watch and correct any interference with hoses, lines, pipes and cables.
Do not lower the cradle without its being restrained as possible damage can occur to the body and underhood items.
Remove water pump from engine.
To install, reverse the removal procedure. Tighten the water pump bolts to 25 ft. lbs. (34 Nm).
Connect negative battery cable.
Fill with coolant, start the engine. Top off the coolant system and check for leaks.
Disconnect the negative battery cable.
Drain the cooling system.
Remove the serpentine drive belt.
Disconnect the coolant hose at the water pump.
Unsecure the water pump pulley bolts. The long bolt should be removed through the access hole provided in the body side rail. Remove the pulley.
Remove the water pump attaching bolts, then remove the water pump.
Install the water pump with the attaching bolts. Tighten the bolts to 22 ft. lbs. (30 Nm).
Install the pulley. Install the water pump pulley bolts. The long bolt should be installed through the access hole provided in the body side rail.
Install the coolant hose at the water pump.
Install the serpentine drive belt.
Connect the negative battery cable.
Fill cooling system and check for leaks. Start the engine and allow to come to normal operating temperature. Recheck for leaks.