The thermostat is an important part of the cooling system in that it regulates when coolant is allowed to flow through the radiator and engine. If the thermostat is not working correctly, the engine will either run hot or very cool. The 3.1-liter engine in the 1998 Chevy Malibu has the thermostat located on the front of the intake manifold, under the water inlet tube. A new thermostat for your Malibu is available through the Chevy dealer or any auto parts store.
Disconnect the negative battery cable from the negative battery terminal with a wrench. Isolate the cable end from the battery so that the engine cannot be starter while you have your hands in the engine compartment.
Open the radiator cap so that you can see into the radiator, helping you verify the level of the coolant as you drain it. Position a large drain pan or bucket below the radiator drain petcock then drain the coolant until it is below the level of the thermostat housing on the engine.
Locate the two bolts where the water inlet tube meets the engine intake manifold. Remove the bolts by turning them counterclockwise with a socket and ratchet. Move the water outlet aside. If the upper radiator hose is too stiff you may need to remove it, otherwise just bend the rubber hose and move the water outlet and hose as one unit.
Remove the thermostat from the intake manifold and discard it. Remove the gasket and using a gasket scraper, remove any remains that stick to the manifold or water inlet tube. Install the new thermostat into the manifold, making sure it is oriented properly. If it is backward, it will not work.
Install a new gasket on the mounting surface of the manifold and install the water inlet tube, threaded the two mounting bolts into by hand to start them. Tighten the two mounting bolts evenly until the water inlet tube is secure.
Fill the radiator with coolant. You can reuse the coolant you drained from the system if it is clean and you used a clean pan to catch it.
Install the negative battery cable on the negative battery terminal and tighten the retaining bolt with a wrench. Start the engine with the radiator cap off the filler neck and continue adding coolant until the system will not take anymore.
Allow the engine to run until it reaches normal operating temperature so the new thermostat opens. Turn the heat on all the way in the car and top up the coolant until it is full. Install the radiator cap and shut off the engine.
Visually check the water inlet tube where it meets the intake manifold to ensure there are no leaks in the system.
