REMOVAL & INSTALLATION
To operate at peak efficiency, an engine must maintain its internal temperatures within certain upper and lower limits. The cooling system circulates fluid around the combustion cylinders and conducts this heated fluid to the radiator, where the heat is exchanged into the airflow created by the fan and the motion of the vehicle.
While most people realize that an engine running too hot (overheated) is a sign of trouble, few know that an engine can run too cool as well. If the proper internal temperatures are not achieved, fuel is not burned efficiently and the lubricating oil does not reach its best working temperature. While a too cold condition is rarely disabling, it can cause a variety of problems which can be mistaken for tune-up or electrical causes.
The thermostat controls the flow of coolant within the system. It reacts to the heat of the coolant and allows more fluid (or less) to circulate. Depending on the amount of fluid being circulated, more or less heat is drawn away from the inside of the engine. While we are beyond the days of having to install different thermostats for summer and winter driving, it is wise to check the function of the thermostat periodically. Special use of the vehicle such as trailer towing or carrying heavy loads may require the installation of a thermostat with different temperature characteristics.
The thermostat requires replacement if the engine runs too cold, as indicated by low operating temperature on the gauge, low heater output, poor gas mileage and a slight lack of performance. If the engine is overheating, there are a number of different possible causes, including not only low coolant due to leakage, but improper engine tuning and/or a dirty cooling system or partially plugged radiator.
If the engine is overheating, it's best to test the thermostat before getting into more complicated diagnosis. You should also test the thermostat if the water temperature gauge is reading low and you're not sure whether it's a thermostat or a gauge problem. Any time the engine has suffered an overheating period, the thermostat should be changed; high temperature can damage it.
- Disconnect the negative battery cable. Remove the air cleaner (if necessary to gain access) and then drain the cooling system below the level of the tubes in the top tank of the radiator.
- You can usually remove the thermostat housing, located at the intake manifold end of the top radiator hose, without disconnecting the hose. It may be necessary to remove other small hoses and/or wiring connectors from the housing. Remove the two mounting bolts and gently lift the housing off the manifold. Once the housing is removed, you may wish to remove it from the end of the hose, making reinstallation easier.
- If the thermostat is to be tested, suspend it well off the bottom of a pan of water on the stove. Immerse a thermometer that reads up to about 250°F (121°C) in the water. Heat the water. Note the temperature at which the thermostat valve begins to open and continue heating until the water boils. The valve should begin opening around 190°F (88°C), the exact temperature should be stamped on the thermostat, and be wide open as the water boils. The valve must open at least 0.31 in. (8mm) and should start opening within just a few degrees of the specified temperature; otherwise, replace it.
- Scrape both gasket surfaces thoroughly, including the indentation in the intake manifold. Install the thermostat with the wax pellet and spring downward. Some models have a ribbed section inside the manifold to keep you from installing the unit upside down. Make sure the unit seats in the indentation so there is a flush surface for the gasket to seal against.
- Coat a new gasket with sealer on both sides and install it to the manifold with bolt holes matching up. Note that the gasket must be installed above the thermostat.
- Install the housing and two bolts, turning them gently and going back and forth until they are just snug. You are dealing with cast aluminum which is very brittle-DO NOT overtighten these bolts!
- Connect the hoses and wiring as necessary and install the air cleaner if it was removed.
- Refill the system with coolant. Operate the engine just above idle until the thermostat opens. Refill the system as necessary. Check for leaks. If any leak is found around the thermostat housing, do not attempt to tighten the bolts. After the engine cools the housing must be removed and scraped, then reinstalled with a fresh gasket and fresh sealer.
Diesel Engine See Figure 7
The thermostat for the diesel engine is located on the water intake side of the cooling system, between the lower radiator hose and the block. The thermostat is equipped with a bypass valve.
- Disconnect the negative battery cable. Drain the coolant to below the level of the thermostat.
- Remove the lower radiator hose from the water inlet fitting.
- Remove the water inlet fitting and remove the thermostat.
- When reinstalling, make certain the thermostat flange is correctly seated against the dimpled area of the inlet fitting.
- Apply sealant to both sides of a new inlet fitting gasket and install the gasket against the water pump.
- Install the inlet fitting and carefully tighten the two bolts.
- Install the lower radiator hose and secure the clamp.
- Refill the coolant.
- Start the engine, checking carefully for leaks. Allow the engine to warm up fully. Watch the temperature gauge for any indication of improper warm-up (underheating or overheating).