Skip to Content

What Is A Thermostat?

Car Thermostat

Thermostats are also known as stats, do a very simple job but without them, our homes, cars, and lives would be very uncomfortable.

So, what is a thermostat? A thermostat senses the temperature of a system and then performs a physical action or sends a signal to another component to change or maintain a predetermined temperature setting. It’s a crucial component used in many different applications. Thermostats are commonly found in :

  • Car engine cooling systems
  • Home heating systems
  • A/c systems
  • Laundry machines

By the end of this guide, you’ll understand what a car thermostat does and how it does it. You’ll also learn how to test its operation.

Car Thermostats

Thermostats usually live in a pretty hostile environment, with lots of heat or cold conditions, so it’s not surprising that they fail regularly.

There are many different types of thermostats; in this guide, we’ll look at how the thermostat is used in a car cooling system.

Most cars will have at least two thermostats, one for the a/c system and another for the cooling system; they operate in independent systems and aren’t connected.

Mostly when people refer to car thermostats, they are referring to the cooling system thermostat (stat).

engine coolant system

It’s located in the cooling system, submerged in the coolant. The top hose at the engine under the thermostat cap is the more usual place to find them.

What Does Thermostat Do?

A car engine produces a lot of heat as it runs; water (coolant) is circulated around the engine to help keep it cool.

When you start your car for the first time in the morning, the engine is obviously cold, and so is the coolant. The problem is engines aren’t efficient until they’re warm, and the cabin heater offers no heat.

The solution is simple, temporally prevent most of the cold radiator coolant from circulating around the engine, instead circulate just the coolant that’s around the engine (water jacket coolant).

The thermostat (stat) stays closed, blocking the flow of cold rad coolant to the engine. When the coolant around the engine heats up, usually around 195°F, the stat opens and allows the cold coolant from the radiator to flow to the engine, this causes the stat to close again.

The cycle of opening and closing of the stat continues until all the coolant in the radiator and other parts of the system is heated above 195°F.

The engine and cooling system are now up to operating temperature, and the stat is now fully open and will remain so until the engine is shut off and the coolant temperature drops below 195°F.

So how does it work?

A common stat has a body, valve, return spring, bleed valve, piston, and cylinder filled with wax.

The wax in the cylinder is melted by the heated engine coolant, this causes the cylinder to move which opens the valve.

The valve is a simple rubber sealing ring that is attached to the end of the cylinder. The return springs help close the valve when the coolant cools.

The jiggle valve is a small bleed valve that helps bleed trapped air from the engine side of the thermostat. It’s important, especially in vertically mounted stats, as trapped air around the stat will prevent it from opening, which can cause overheating and all kinds of hurt.

Symptoms Of A Bad Thermostat?

Car thermostat

The symptoms of a bad stat vary; it depends if it’s stuck in an open or closed position. Most stats will fail in the closed position. As you know, the stat should open as the coolant around the engine (water jacket) heats.

Stuck closed thermostat

A stuck closed stat will cause the engine to overheat even on the highway; your temperature gauge will get very high. Your rad fan may come on and run constantly or much longer than usual or may not come on at all, depending on the fan sensor location.

Your car’s gas mileage will suffer, too; the engine isn’t running at the optimum temperature for efficiency.

If the failed stat is ignored, it can cause the head gasket to blow or, worse, damage to the cylinder head or engine block. All of these repairs are major and will cost a packet; crazy to think a stat that only costs a few bucks can cause so much damage.

Head gasket failure chart

Some latest model cars have electronic stats that are commanded to open by the engine control unit. If you have this type of stat, your ECU monitors all sensors and will set the engine fault light if it sees a stat fault.

Stuck open thermostat

A stat that fails to close is much less likely; it won’t cause damage; it’s more of an irritant. The symptoms of a stat that won’t close include a much longer warm-up time. You may notice that the engine choke stays on longer than usual, or the car heater just won’t heat up on shorter journeys.

Your gas mileage will be bad too.

Can You Run A Car Without A Thermostat?

Yes, you could run an older car without the thermostat, but doing so will cause your engine to use more gas, your interior heating will take longer to work, will affect emissions, and could cause your car to fail a smog test.

Running a more modern car without the stat can cause a check engine light to come on. The engine control unit will try to get the car to operating temperature quickly. It does this by over-fuelling until the engine is warm; removing the stat will cause the ECU to over-fuel for longer, which can cause damage to the Catalytic converter.

Blocked car rad

My old classic Mercedes had a heating problem; I’d turn the heater on in traffic to help cool her. I removed the thermostat as a quick fix, and it helped drop the operating temperature in summer traffic. However, I knew the real problem was a stuffed-up radiator.

I backflushed the system, and it removed about 30 years of crap from the core; the improvement surprised even me. Your local shop will use a specialized machine to run detergent through the complete cooling system.

You’ll notice the car will run better, will be easier on gas, and the heater will be hotter and will warm up faster.

What Causes A Thermostat To Go Bad?

Coolant testing

When coolant gets old, it turns acidic, and the acid attacks all the components of the system, rubber seals, metal gaskets, and the metal in the engine itself. Most manufacturers will recommend changing your coolant every three years.

Testing coolant

Refractometer, a Hydrometer, or a dip strip kit. You can also check the acidity of your coolant with a voltmeter. Any reading above .3 of a volt means it’s harmful to the engine and needs to be changed.

fresh coolant

The thermostat, as you know, is a simple mechanical component; general wear and tear will cause the piston, seals, and return springs to fail.

Old acidic coolant, overheating, and corrosion within your cooling system will currently shorten the life of a stat.

I like to change the thermostat when I change the coolant; they’re inexpensive compared to a cylinder head.

How Do I Test My Thermostat?

Rad hose

The easiest way to test your thermostat is to warm the engine and check and compare the heat of the top and bottom radiator hose. If they’re heating at about the same rate, it suggests there’s no thermostat fitted, or it’s stuck open.

An infrared thermometer is a pro way to test it, but your hand will work too; obviously, turn the motor off before testing.

If the top hose from the radiator slowly starts to warm, but the lower hose remains cold, then all is normal, and your thermostat is doing its job.

If the top hose isn’t getting warm, but the engine temperature is at or above normal temperature, then suspect a stuck closed thermostat.

Thermostat testing

If you’re removing the thermostat, you can easily check its operation by placing it in a bowl of plus 195°F water. If the stat promptly opens, then it’s all good.

You can place it in cold water or let it cool naturally to check it closes again without binding.

Not all stats are the same; they have different heat ratings. Older cars will be more forgiving if the stat isn’t as per the factory but newer ECU-controlled cars are less so.

Modern cars are quite fussy about having the correct rated stat; your parts store will guide you.

Stat rating

Related Questions

Will a stuck thermostat cause no cabin heat? Yes, if the thermostat is stuck open, it will cause poor cabin heat, but if the thermostat is stuck closed it will cause the engine to overheat.

New thermostat not opening? If a new thermostat is not opening, it may be installed back-ways. The longest part of the stat body goes towards the engine. The other possibilities include wrong thermostat and a faulty new stat.