How to Replace a 1986 GMC Truck 5.7L Thermostat

The thermostat on your 1986 GMC 5.7-liter engine is a cooling system device that controls the flow of engine coolant entering the engine as the temperature increases above normal. It's important to replace the device at the first signs of engine temperature irregularities or overheating. When the engine temperature rises, the thermostat will automatically open and allow the flow of coolant from the radiator to enter the engine block. Replacing this inexpensive part is simple, and doing so can prevent the engine from overheating, causing expensive engine damage.

Instructions

1

Drive the truck up onto the vehicle ramps slowly and set the parking brake. Chock the rear tires using the wheel chocks to prevent it from rolling, as an added safety measure.

2

Allow the engine to completely cool before performing the thermostat replacement. Put on your safety glasses and remove the radiator cap to relieve the pressure from the radiator, hose and cooling system.

3

Disconnect both the negative and positive battery cables. Place the clean 5-gallon bucket underneath the radiator drain plug to capture the drained fluid.

4

Reach underneath the front end and loosen the drain plug (bolt) using a socket wrench. Drain out only enough anti-freeze to empty out the upper radiator hose that attaches to the thermostat housing cover. Re-tighten the drain plug.

5

Remove the carburetor cleaner and filter by unscrewing the center wing nut using your hand. Disconnect the upper radiator hose where it attaches to the thermostat housing cover. It's held in place by a squeeze-type clamp. Keep the opposite end that connects to the radiator attached.

6

Remove the mounting bolts that attach the housing cover to the manifold using the socket wrench. Gently tap the housing cover with a hammer to break the seal or pry it off with a flat-head screwdriver. Remove the cover and set it aside.

7

Scrape off the old gasket, debris and adhesive from the housing cover and thermostat opening using a gasket scraper. Be careful not to allow any debris to enter into the engine opening.

8

Test the new thermostat before installing it to be certain that it's operating properly. Fill a 4-inch-deep bowl with very hot water. Grab the thermostat upright using your pliers and hold it in the hot water, being sure to cover the entire unit. The thermostat should open immediately, indicating that it is functioning properly.

9

Apply a thin bead of gasket adhesive to the replacement gasket. Place the gasket directly onto the thermostat opening on the manifold. Be sure to properly align the mounting holes so that the thermostat will seat properly and to avoid leaking.

10

Insert the replacement thermostat with the spring side facing down into the engine. Carefully place the housing cover over the thermostat and secure it in place with the mounting bolts, being careful not to shift the gasket.

11

Reconnect the radiator hose and secure it in place using the squeeze clamp. Replace the carburetor cleaner and filter unit.

12

Re-connect both battery cables, start the engine and then pour the drained anti-freeze back into the radiator. Allow the engine to run for several minutes while you check for leaks. Top it off with additional antifreeze as necessary and replace the fill cap.

13

Remove the wheel chocks and carefully drive the truck off of the vehicle ramps.