Installing a Thermostat in a Car

Introduction

Installing a thermostat is one of the easier car repairs, if you can get to the thermostat easily. In most cars, the thermostat sits on top of the engine, at the other end of the top radiator hose. In a few cars, you will need to remove at least one thing (depending on the vehicle, it may be the intake, the air compressor, the alternator or any other part) to get to the thermostat. If you are replacing a water pump or radiator, and the thermostat has not been replaced for at least two years, you should also replace the thermostat.

Removing the Old Thermostat

Remove the top radiator hose from the thermostat housing. Bend the hose so it is out of your way. In most cases, you do not need to remove it from the radiator, unless you are also replacing the radiator. Remove the two bolts holding the thermostat housing on to the engine block. Lift the housing off of the block. You may have to pry it a bit, but be careful when using any tool to pry, as it could chip the housing. Remove the thermostat, making note of how it is placed in its cavity. The new one will need to be installed in the same direction. Remove the old gasket material from the housing and the block. Use a scraper to get the gasket off, if needed. If there is a rubber o-ring, make sure the o-ring has been removed. A new o-ring should come with the thermostat. If you leave the o-ring on and try to install the new o-ring over it, the housing will leak.

Installing the New Thermostat

Put a dab of red RTV (waterproof gasket maker) on your finger and wipe it along the clean thermostat housing. You want just enough RTV on the housing to make the gasket stick to it---you do not need a bead, as you are not making a new gasket out of the RTV. Line up the gasket on the thermostat housing, making sure it has a perfect fit. Press the gasket into the RTV. Let it set for a minute or two, so that the RTV starts to set and the gasket won't move when you are matching the housing up to the engine block. Set the new thermostat in its cavity in the engine block. Place the housing over the thermostat and insert the two housing bolts. Do not overtighten the bolts, as you will crack the housing. They need to be snug enough not to come out. If you are using air tools, a 3/8-inch air ratchet can put too much torque on the bolts. Put them in by hand or use a -inch air ratchet. Replace the top radiator hose and its clamp on the thermostat housing. Check the water and antifreeze in the radiator. Top it off, if needed. Start the vehicle and let it get to operating temperature. Between 185 and 195 degrees Fahrenheit, the thermostat should open and the fan should come on.