The 2003 Pontiac Grand Prix has a set of two transmission cooling lines that run between the transmission and the radiator. Transmission fluid runs into the car's transmission to lubricate the gears but heats up as it circulates. The fluid is returned to the radiator, where coolant lowers the temperature of the fluid before it is returned to the transmission. These two lines must be replaced if a leak occurs because it could lead to low transmission fluid levels and grinding gears.
Place a collection pan directly under the transmission drain pan, which is located in the center of the vehicle. With the 18 mm socket wrench, remove all but one of the bolts holding the drain pan in place. Let the transmission fluid drain from the pan until the flow slows to a trickle. Then remove the final bolt and pour out remaining fluid into the collection pan.
Reinstall the transmission drain pan and the bolts that you just removed. Locate the two transmission lines, which are attached to the side of the transmission and run to the bottom of the radiator.
Slide a flare nut wrench onto the center of one of the lines and run it down to the nut on the transmission. Twist the nut to loosen it and disconnect the line from the transmission. Go to the other end of the line, remove the nut on the radiator and disconnect the line from the radiator. Repeat this procedure to remove the other transmission line.
Connect the new transmission lines to the transmission and radiator. Secure the lines using the old nuts and tighten with the flare nut wrench. Don't over-tighten or you could strip the threads.
Lift the hood of your 2003 Grand Prix and prop it open with the hood support rod. Locate the yellow transmission dipstick above the drain pan. Pull out the dipstick. Place a funnel in the opening and pour in three quarts of transmission fluid.
Put the transmission dipstick back in and press firmly to ensure that it is tightly seated. Close the hood.
