General Motors pickup trucks use reciprocating cylinder technology that requires a liquid cooling system, and the primary radiator for this system can break down and require replacement. The average backyard mechanic can replace a radiator in about two hours.
Drain the coolant into a drain pan. Most models of GMC radiator will have a drain plug on the lower left or right of the radiator body. This plug can be loosened by turning it counterclockwise until coolant comes out. It may not be necessary to totally remove the plug, just loosen it. Uncap the radiator and release the pressure on the radiator to drain it completely.
Disconnect the coolant hoses from the radiator. There are two hoses, upper and lower, that will come off once their hose clamps are loosened with a screwdriver or vice grips. The hoses might have become stuck due to age, and may need to be cut and replaced with new hoses. Some coolant might drip out when the lower hose is uncoupled.
Remove the fan shroud and fan. There are four screws on the back, and once freed the shroud should slide out towards the top of the radiator. The wiring harness will disconnect at the adapter plug at the fan. If the fan is mechanical, then it must be unbolted at the fan clutch and slid off towards the front.
Disconnect the oil cooling lines, if so equipped. There will be an upper and lower line, and they unbolt with line nuts in a counter-clockwise direction. Some oil might drip out, so have the drain pan handy.
Remove the old radiator. Unbolt the top radiator plate, if necessary, and slide the radiator out towards the top of the engine compartment.
Replace the old radiator with a new one. Slide the new radiator into place, and attach the top radiator mount plate. Reattach the fan and fan shroud, and replace the oil lines and hoses.
Fill the new radiator with 50 percent water and 50 percent coolant. Once the radiator is full, crank the truck and continue to pour until the radiator can take in no more coolant.
