The fast 2.0L/2.5L engine in your 1999 Mercury Cougar needs the fuel pump to keep the appropriate fuel flow to maintain the needed pressure in the system. This is one of the reasons the motor is able to develop its power on the road. However, a worn-out pump may leak fuel, lower the pressure and become noisy. Fortunately, you can access the fuel pump through the floor under the back seat without having to drop the tank to change the unit.
Disconnect the ground, or black, cable from the battery using a wrench.
Loosen the fuel filler cap to release pressure in the fuel tank.
Open the bonnet and remove the air cleaner assembly to clear the space on the fuel rail. Use a ratchet and socket.
Find the fuel pressure relief valve on the fuel rail. This valve looks like the air valve on your Cougar tires.
Wrap a shop rag around the pressure valve and, using a small flathead screwdriver, depress the valve to relieve fuel system pressure. Catch the squirt of fuel that will come out from the valve with the shop rag. Do not let any fuel splash on the engine or any other component.
Replace the air cleaner assembly using the ratchet and socket.
Lift the rear seat cushion and remove it from the vehicle.
Pull the plastic grommet on the floor by hand. Use a flathead screwdriver to pry out the grommet, if necessary. This hole will give you access to the fuel sending unit/pump assembly, mounted on top of the fuel tank.
Unplug the fuel pump electrical connector from the top of the fuel sending unit.
Detach the fuel lines from the fuel sending unit/pump assembly. Depress the side tabs on the line fittings and pull the lines off the sending unit assembly.
Disengage the fuel sending unit/pump assembly from the tank by turning counterclockwise the locking ring with a drift punch and a hammer. The is a method suggested by Eric Michael Mihalyi in "Ford Contour, Mystique Cougar 1995-1999."
Remove the locking ring and carefully lift the sending unit/pump assembly off the fuel tank. Remove the O-ring seal from the tank opening and discard it.
Cover the fuel tank opening with a clean shop rag to prevent dust from going into the tank.
Lay the sending unit/pump assembly on a workbench and remove the pump from the assembly using a flathead screwdriver and a pair of needle-nose pliers.
Install the new fuel pump in the sending unit/pump assembly using the flathead screw driver and the pair of needle-nose pliers.
Remove the shop rag from the tank opening.
Apply a light coat of new engine oil to the new O-ring seal and place the seal on the groove around the fuel tank opening.
Insert carefully the sending unit/pump assembly into the tank and make sure the O-ring remains in position.
Place the locking ring on the sending unit/pump assembly and turn the ring clockwise until it hits the stops. Use the drift punch and hammer to lock the assembly in place.
Attach the fuel lines to the fuel sending unit/pump assembly by pushing the line plastic fittings against the nipples on the sending unit/pump assembly.
Plug the fuel pump electrical connector to the top of the fuel sending unit.
Replace the plastic grommet to close the access hole on the floor.
Reinstall the rear seat cushion.
Tighten the fuel filler cap.
Connect the ground, or black, cable to the battery using the wrench.