The Isuzu pickup trucks have a thermostat in the engine block that regulates the flow of the coolant through the engine. The thermostat is a temperature-sensitive component that opens once that the engine reaches its operating temperature. When the engine cools, the thermostat closes until the temperature begins to rise again. The temperature-sensitive spring on the bottom of the thermostat allows the thermostat to open and close at the proper times. If the thermostat gets stuck, the engine will overheat. Replace the thermostat immediately.
Park the Isuzu pickup on a flat surface and set the parking brake. Open the bonnet and shut the engine off. Wait a couple of hours for the engine to completely cool down.
Slowly unscrew the radiator cap with a clean rag after the engine has completely cooled down. Set the cap in a safe area.
Slide under the front of the truck and locate the drain valve on the bottom of the radiator. Position the dripping pan under the drain valve. Turn the valve counterclockwise with the pliers to open. Drain coolant out of the radiator and then retighten the drain valve with the pliers.
Move to the top side of the engine and locate the upper radiator hose. Follow the upper radiator hose from the top of the radiator to where it connects to the thermostat housing port. Using a flathead screwdriver, loosen the hose clamp where the hose is attached to the thermostat housing port. Pull the clamp back onto the hose about 6 or 8 inches. Twist the hose back and forth and pull the hose at the same time until the hose releases from the thermostat housing tube.
Loosen counterclockwise and remove the two thermostat housing mounting bolts with a ratchet and a socket. Pull the bolts out and placed them in a safe area. Pull the thermostat housing off the engine block and place it on top of the radiator. Pull the thermostat straight up out of the engine block and discard it into the dripping pan. Insert the new thermostat, with the spring on the bottom of the thermostat, into the engine block.
Scrape away any excess gasket from the engine block and from the base of the thermostat housing with a flat metal scraper. Use a clean rag to wipe away any excess gasket and debris from the housing and the engine block.
Apply a thin bead of the red high-temperature silicone around the base of the thermostat housing. Stick the thermostat housing gasket to the silicone with the holes of the gasket matching up with the holes of the housing. Lower the housing over the new thermostat and onto the engine block. Screw the two mounting bolts back through the housing and the engine block. Tighten the mounting bolts with the ratchet and socket until both bolts are tight.
Push the top radiator hose back onto the thermostat housing port. Position the hose clamp in place over the hose and the port. Tighten the clamp with the screwdriver until the hose is completely tightened to the port.
Fill the radiator with antifreeze and water until the fluid level reaches the radiator fill neck. Add the antifreeze and water to the radiator according to the antifreeze to water ratio listed on the antifreeze container.
Crank the engine and wait five to 10 minutes for the engine to reach its operating temperature. Once the engine reaches its operating temperature, the thermostat will open and the fluid level inside the radiator will drop. Continue adding antifreeze and water to the radiator, with the engine running, until the fluid level once again reaches the fill neck. Screw the radiator cap back onto the fill neck.
Inspect the top radiator hose and the thermostat housing for leaks with the engine running. Turn the engine off and remove the dripping pan from under the truck.
The red high-temperature silicone is designed for components that are around high temperatures. The silicone will also act as a secondary seal to the thermostat gasket. This silicone can be found at most auto parts stores.
Never work on an engine until it has completely cooled down and never open a hot radiator cap. This can result in serious burns to the eyes and the skin. Dispose of used coolant properly.