How does a thermostat work ?
Question posed to Stant:
(Stant is a well-known manufacturer of thermostats and related automotive parts.)
Does an automobile thermostat in a 1975 Chevrolet with a 350 V8 engine traveling at highway speed regulate temp by opening fully then closing fully, repeating this cycle over and over, or does the thermostat regulate temp by partially closing as the coolant temp varies?
The opening and closing of a thermostat is based on the temperature of the coolant running against the "heat motor". The heat motor is operated by a special wax that melts at a given temperature. As the wax melts, it expands and hydraulically pushes a pin that opens the valve allowing coolant to flow. I hope this helps. See the attached article for more details.
The attached article reads as follows:
The thermostat has two important jobs to perform; to accelerate engine warm-up and to regulate the engine's operating temperature. A quality thermostat ensures excellent fuel economy, reduces engine wear, diminishes emissions and blow-by, improves cold weather drivability, provides adequate heater output, and detours overheating. This is accomplished by blocking the circulation of coolant between the engine and radiator until the engine has reached its predetermined temperature. The thermostat then opens as required in response to changes in coolant temperature to keep the engine's temperature within the desired operating range.
Thermostats have a “rated” temperature such as 180F or 195F. This is the temperature the thermostat will start to open, give or take 3 degrees.
Usually located within a metal or plastic housing where the upper radiator hose connects to the engine, most of today’s thermostats utilize the "reverse poppet" design, which opens against the flow of the coolant. Thermostats have a wax filled copper housing or cup called a "heat motor" that pushes the thermostat open against spring pressure.
As the engine's coolant warms up, the increase in heat causes the wax to melt and expand. The wax pushes against a piston inside a rubber boot. This forces the piston outward to open the thermostat. Within 3 or 4 degrees F. of the thermostat preset/rated temperature which is usually marked on the thermostat, the thermostat begins to unseat so coolant can start to circulate between the engine and radiator. It continues to open until engine cooling requirements are satisfied. It is fully open about 15-20 degrees above its rated temperature. If the temperature of the circulating coolant begins to drop, the wax element contracts, allowing spring tension to close the thermostat, thus decreasing coolant flow through the radiator.
On some applications, the thermostat performs an additional function. It closes off a bypass circuit inside the engine when it opens the radiator circuit. The bypass circuit circulates coolant inside the engine so that hot spots can’t form when the radiator circuit is closed.
Many thermostats have a “jiggle pin” that allows trapped air in the cooling system to pass through the thermostat and be removed from the system. If a Stant thermostat does not have a jiggle pin, it will have a "bleed notch” or other method of removing air from the system.
There is no such thing as a thermostat that will fail in a “safe” position. All thermostats will fail in either a closed or open position. One brand claims it fails in a safe position, but it simply locks itself open when it is a full stroke open position. It will not spring open if it fails in a closed position.
A thermostat fails “open” if the return spring breaks or debris prevents the thermostat valve from fully seating or closing. In this instance the thermostat allows continuous coolant flow to the radiator; therefore, the engine will be overcooled. The tangible effects are poor warm up and heater performance, increased engine emissions and reduced fuel economy. For these reasons, an engine should never be operated without a thermostat in place, even in extreme temperatures.
A thermostat will fail “closed” if the wax element has been damaged by overheating (from loss of coolant, a defective electric cooling fan or fan clutch) or corrosion (from not changing the anti-freeze often enough). This failure prevents the flow of coolant to the radiator; therefore, the engine will be overheated. The tangible effects are a boilover, the inability to operate the vehicle, and the likelihood of severe engine damage. For these reasons alone, when an engine overheats, it’s a good idea to replace the thermostat whether it caused the problem or not.
The temperature rating of a replacement thermostat must be the correct one for the application because of the adverse affects the wrong thermostat can have on drivability, engine performance and emissions.
The temperature rating specified by the car manufacturer is especially important in many 1981 and newer vehicles because the onboard computer monitors coolant temperature through a coolant sensor to control fuel enrichment, spark timing and operation of the EGR valve. Even on vehicles without computers, thermal vacuum switches that react to a specific coolant temperature are often used to open and close various vacuum circuits that regulate fuel enrichment, timing and EGR. If a colder thermostat is installed, the coolant may never get hot enough to trigger the appropriate control functions or to allow a computer system to go into “closed loop”. Too hot a thermostat can also interfere with the proper operation of engine controls, and increase the engine’s operating temperature to the point where it may experience detonation (spark knock).
Stant offers Superstat premium thermostats. These thermostats have a heavier frame and spring, larger heat motor, and a patented Weir valve. The unique valve provides a higher flow of coolant to the radiator than any other thermostat on the market. The early flow of coolant minimizes the “cycling” of temperature allowing the engine to run a steady temperature for higher efficiency.
One way to determine if the thermostat is doing its job is to feel the upper radiator hose after starting a cold engine. The hose should not feel hot until the engine has warmed up. If the hose starts to feel hot after only a couple of minutes, the thermostat may be stuck open or not closing completely. Once the engine is warm, the hose should feel hot as coolant circulates between the engine and radiator. If the hose does not feel hot, the thermostat may be stuck shut, blocking the flow of coolant.
A thermostat can be tested by suspending it, using a string through the valve, in a bucket of boiling 50/50 coolant and water. If the thermostat is working it will fall off the string as it starts to open after being in the hot/boiling coolant for a few minutes. When removed and allowed to cool, the thermostat should close.
- Don't overlook the water outlet covering the thermostat. Check for cracks, broken flanges, internal pitting and corrosion, and erosion at the hose neck (a real problem with most aluminum housings). The gasket surface must be flat and free from warping or deep scratches.
- Scrape the mating surfaces on the thermostat housing and engine to remove all traces of old gasket material. Use care on aluminum because the soft metal can be easily scratched.
- Temporarily stuffing a clean rag into the thermostat opening on the engine while the housing is removed helps keep debris out of the cooling system.
- Install the new thermostat so the copper heat sensing element is toward the engine. If installed upside down, it won’t open.
- Stant's Stat-Lok® thermostat gaskets are adhesive backed to make installation easier.
- Torque the thermostat housing bolts evenly and to the manufacturer's recommendations.
- To insure air has been removed from the cooling system after replacing a thermostat, be sure to run the engine a few minutes, let it cool, and refill the antifreeze as needed.