How To

Keeping Your Cool

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by Butch Thomas  More from Author

How to Diagnose and Change a Thermostat

The cooling system of an engine consists of the water or coolant pump, radiator, hoses, pressure cap, fan, thermostat and the coolant itself. Though these components are all very reliable, there may be a time when a problem occurs – and when it does, it is usually found in the thermostat. By nature of its design it constantly opens and closes and eventually one day refuses to do either. The position it fails in will determine how it impacts your day. A thermostat that is stuck open will create an excessively long engine warm up period but will not strand you. In contrast, one that sticks closed will cause the engine to overheat very quickly and can possibly cause expensive damage such as a cracked head or blown head gasket.

As simple as a thermostat visually appears there is a good amount of engineering involved along with the proper diagnostic and service procedures. To showcase these we worked with Rob Ida for our photo shoot. He is an award winning custom car builder and owner of Rob Ida Concepts. He volunteered his 1971 Chevy Vega with a 327 V-8 as an example of a traditional American thermostat replacement job. The steps would apply to any domestic engine.

Understanding How It Works

When thinking about cooling system function, it needs to be remembered that the purpose is not so much to keep the engine cool but rather to prevent it from overheating. The first use of a thermostatically controlled cooling system was employed by the Cadillac Motor Car Company in 1914 on their V-8 engine. This breakthrough in cooling system componentry proved to be an important early step in the development and advancement of liquid cooled engines.

The basic purpose of a thermostat is to regulate the flow of coolant through the radiator in accordance with the cooling requirements of the engine. In a modern application the reasons for having a thermostat in the cooling system are to:

  • Reduce engine warm-up time
  • Maintain optimum running temperatures
  • Meet the requirements of the car interior heating system

The thermostat consists of a poppet control valve which is opened by a thermal expansion device or element and is closed by a return spring. Movement of the valve is therefore directly related to the temperature of the coolant surrounding its operating element. The thermostat is almost always installed between the coolant outlet from the cylinder head jacket and the inlet to the radiator header tank.

At normal operating temperatures the control valve is open and coolant circulates through both the engine and the radiator, but when cold, the valve is closed and circulation through the radiator is prevented, or at least restricted. The engine is allowed to warm-up much more quickly since a smaller quantity of coolant is being heated by it.

The control valve in the thermostat is energized by an element charged with a wax substance having a high coefficient of thermal expansion. The element consists of a cylindrical metal body containing the wax substance (often called a pill) which surrounds a rubber insert. This in turn embraces a central operating thrust (push) rod. As the coolant temperature rises, the wax melts and compresses the rubber insert; but since this rubber is constrained to act like an incompressible hydraulic fluid, it displaces the thrust rod. The control valve is opened against its separate return spring. For the modern cooling system the hydrostatic or wax thermostat has the important advantage that it is relatively insensitive to pressure variations.

Radiator Bypass Circuit

Rather than attempt to completely block the coolant flow from the engine during warm-up, either a permanent or variable radiator bypass circuit is used in conjunction with the thermostat. For this purpose a transfer connection must be provided between the underside of the thermostat and the inlet side of the coolant pump. This ensures that when the thermostat is closed to the radiator the pump can recirculate the coolant around the engine to avoid any local overheating during warm-up.

The simple permanent bypass transfer connection is no more than a passage since it must not be so large that it reduces the normal flow of coolant through the radiator when the thermostat is open.

Up until recently most Chevrolet engines used a constant bypass system that always had an open flow path and thus, eliminated some of the coolant from entering the radiator even under normal operating conditions. A bypass-style thermostat like those often found on the newer engines such as the LT1 and LS1 employs an additional leg on the thermostat that completely restricts the bypass when the coolant is at the specified operating temperature. Thus, all the coolant is forced through the radiator.

A Detailed Look at Thermostats

The direction of coolant flow in an engine is controlled by the thermostat and radiator pressure cap. The thermostat is used to maintain the coolant temperature within a certain range by modulating the flow of liquid. Thermostat control occurs just at the opening temperature and the thermostat will modulate the coolant flow up to a point when it fully opens and permits the maximum possible coolant through the radiator. During modulation or thermostat control, the coolant temperature across the radiator may vary from about 15 to 30 degrees F, depending on the coolant flow rate. When the thermostat is fully open, the coolant temperature across the radiator should remain constant with no more than a 10 to 20 degree F difference.

The temperature stamped or marked on the thermostat is the “start to open” rating. This is often referred to as when the thermostat just “cracks open” and allows some coolant to flow. This is in contrast to the popular belief that the temperature specification identifies when the thermostat is fully open. Most new vehicles have a thermostat that is designed to “crack” at about 195 to 205 degrees F. If the thermostat opening is set too low or too high, engine damage can result. High coolant temperatures can cause detonation, loss of power, damage to bearings and other moving parts, etc. Low engine operating temperatures lead to excessive fuel consumption, dilution of lubricating oil by the addition of unburned fuel, the formation of sludge from the condensation of water in the crankcase along with a host of other concerns. Most thermostats are designed to be fully open 10 to 20 degrees F after cracking. For example, a 160 degree F thermostat would start to flow at that temperature and be fully open between 170 and 180 degrees F.

The thermostat can be located on either the suction or pressure side of the water pump. Older theory placed the thermostat on the pressure side as was common with most domestic engines. Later designs had the thermostat on the suction side of the coolant pump such as seen in the modern Chevrolet LS1 and some Ford engines.

Many schools of thought are used to defend thermostat placement by the engineers who designed the system being studied. Some believed that suction side placement is not appropriate for use with a centrifugal coolant pump since the end result is the coolant is drawn through a restriction. A general rule though, is the placement of the thermostat will impact the temperature calibration for the unit to crack open. It is accepted that a 180 degree F thermostat placed on the pressure side of the cooling system would need to be calibrated for 160 degrees F when placed on the suction side to create identical engine operating temperatures. This is important for the readers with newer engines such as the LS1 who are used to the older pressure side location.

Other elements of thermostat design impact the level of flow restriction they impose on the cooling system when fully open – the ideal being little or no flow penalty. Most less expensive and early design poppet-style thermostats created not only a small flow orifice when fully open but limited the travel height of the flow plate. The Robertshaw Company markets a balanced sleeve design that is a superior choice for a replacement thermostat in almost any poppet-style application. The only issue with the Robertshaw balanced sleeve is that it requires more room so there is not one for every application. Engines such as the Buick V-6 and others have too small a thermostat housing to fit a balanced sleeve style. Also, this design is more costly than a traditional poppet thermostat. All too often both the enthusiast and engineer design a cooling system for high flow rates only to diminish this effort with a poor flowing thermostat.

The thermostat valve is activated by a temperature sensitive power element (wax pill) that can develop hydraulic pressures of several thousand pounds per square inch. As the coolant warms up, the power element expands and allows the valve to lift off its seat at a preselected temperature that is within plus/minus two degrees of the rating. The power element contracts during the cooling cycle and the valve mechanically returns to its seat by the spring. Over time the wax can leak out, or the spring becomes weak. A leaky wax pellet will cause the thermostat to fail closed while a bad spring will usually cause it to stick open.
A thermostat can either have a jiggle pin or calibrated bleed hole to permit deairation during service fill and engine operation. Again, some designs of less quality offer no bleed holes.


A thermostat cannot be seen when installed so the diagnostic procedure is usually through elimination of other parts. If your engine suddenly overheats and there are no leaking hoses or the seal from the water pump is not dripping, then the cause is most likely a stuck closed thermostat. This can occur while the vehicle is being driven well after warm-up. If the capsule holding the wax pellet ruptures, then the spring will cause the thermostat to close and stop the flow of coolant to the radiator. A thermostat that fails to open will produce an extended warm-up period, poor heater performance and if operated in a cold climate, no engine heat at all. Thus, if there are no other visual signs of component failure the thermostat needs to be removed and examined.

Years back many shop manuals suggested you test the thermostat by placing the suspect unit in a pot of water on the stove with a thermometer. Then as the water heats, you can watch the thermometer and see if the thermostat starts to open and how many degrees it takes for it to fully open. Though a nice science experiment, it is impractical to perform this test. It is more cost and time effective to just purchase a new thermostat and install that instead of chasing what is wrong with the old one. Often, upon removal from the engine the failure can be seen such as a broken spring, stuck open valve or a ruptured wax capsule.

Not every thermostat fails a sudden death. As the thermostat begins to fail, it can cause the engine temperature to swing high and low. If the vehicle is equipped with a temperature gauge, you will easily notice this along with an abrupt change in heater performance. A thermostat with this behavior needs to be replaced since it is ready to fail at any time.

The best approach is to recognize that the thermostat is a consumable and should be replaced as a service item every few years. It is best not to wait for it to fail.

With a modified or collector car many enthusiasts like to run a cooler thermostat. This does not mean that the engine will always run cooler but that flow through the radiator will occur sooner. Thermostats are sold in 160, 180 and 190+ degree F ratings. If your collector car is only used in the summer, then a 160 degree F thermostat can be a good upgrade for no additional cost. If the car is driven in the winter, some engines will offer poor heater performance with a 160 degree F thermostat. For that reason a 180 degree F rated design is best for year round use. The choice is yours.

An infrared heat gun is an excellent method to check thermostat performance. Point it right at the thermostat housing. With the radiator cap off you will be able to determine the crack open temperature by seeing the flow begin.

A temperature gauge that has extreme swings can indicate a thermostat that is failing. If the cooling system is air bound the gauge will act in the same manner.

Over time the coolant’s freeze point will not be altered but the anti-corrosion inhibitors will be depleted. The coolant should be refreshed every two to three years for a vehicle with normal use.

To change the thermostat, begin by draining the old coolant into a clean vessel. This would be a good time to change the coolant if it is old.

This is a poppet-style thermostat. It provides a very small flow path when fully open but is the industry standard replacement part.

The wax pellet expands as the engine warms and works against a push rod that opens the flow valve. When the engine cools the spring closes the valve.

A thermostat is always marked with its crack open temperature. This example will begin to open at 160 degrees F.

A good practice with a poppet thermostat is to drill between one and four 1/8 inch bypass hole(s) before installation. This will increase the flow rate and make it easier to bleed the air out of the cooling system.

The bypass hole(s) need to be located by the inner ring so that it is exposed to the coolant.

Not all of the coolant needs to be drained if you are not changing the antifreeze. It needs to be below the thermostat housing. Begin by removing the top radiator hose clamp.

With the clamp loose, twist the hose and work it back and forth to remove it from the neck of the housing. In many cases it does not need to be removed from the radiator.

Carefully remove the bolts holding the housing. If they are rusty you may want to thread it up a few turns and apply penetrating oil. If they break off you will need to drill the intake manifold.

With the bolts removed gently tap the housing or pry against it with a small screwdriver. The gasket will act like glue and stick to the manifold.

Clean any corrosion from the housing surface, hose connection and intake manifold. Place a rag over the manifold hole so no foreign material gets into the coolant passage.

Wire wheel the bolt threads and cover with anti-seize compound.

The thermostat is always installed with the spring toward the engine. If it is installed backwards the engine will overheat.

Place the new gasket on the housing. Gasket cement or silicone is not required but can be used if desired. Install the housing and the radiator hose.

The easiest method to fill the radiator and bleed the air from the system is with a fill kit. The one we used is from Lisle and has a fitting for most necks.

The funnel then attaches to the adapter cap.

Fill the radiator with a 50/50 mix of coolant and distilled water. Bring the level to about half the funnel capacity. Start the engine and put the heater on to bleed the air out. As the engine warms the thermostat will open. Keep the funnel half full. When there are no more air bubbles the system is purged. The engine can be shut off and the radiator cap installed. Be patient, on some vehicles it may take 20 minutes to bleed out the air. If no fill kit is used, run the engine with the cap off, topping the coolant until there are no bubbles. Keep a pan under the car since it will spit out coolant as the air exists and the engine warms.

IDA Automotive Inc.
600 Texas Road
Morganville, NJ 07751


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