How Do You Know if Your Thermostat Is Stuck Open

Thermostat: how it works, symptoms, bug, testing

Updated: July 19, 2021

A thermostat is the central component of the engine cooling system. The cooling organization keeps the engine from overheating. The system is filled with liquid coolant (antifreeze) and is connected into a loop with a radiator, see the diagram.

The coolant flows through the engine where it absorbs the heat and into the top of the radiator. A radiator is built of many flat aluminum tubes surrounded by fins. The air passing through the radiator cools down the coolant as it flows down the radiator.

I or two electrical fans fastened to the back of the radiator turn on when needed to force more air through the radiator. A water pump pulls the coolant from the radiator and moves information technology dorsum into the engine.

The normal operating temperature of a modernistic engine is between 194°F (90°C) and 221°F (105°C). The job of a thermostat is to assist the engine warm up faster and maintain the engine temperature above the minimum operating temperature (194°F or ninety°C).

A conventional thermostat is a simple temperature-controlled two-way valve that opens at a specified temperature.

In about cars, a thermostat is installed on the engine and is connected to the upper or lower radiator hose. Meet a few examples: thermostat in a Ford ii.3L EcoBoost engine. This photograph shows the thermostat housing in the Mazda 2.3L turbo DISI engine. This is the photo of a thermostat housing in the Contrivance Charger SRT Hemi engine.

How a thermostat works:

Thermostat within the housing. Ford engine cutaway. Click for a larger photo

When the engine is started cold, the thermostat (the main valve) is closed; there is no flow through the radiator. The smaller by-pass valve is open and the coolant circulates just inside the engine and through the vehicle heating organization.

As the engine warms upwardly closer to the operating temperature, the thermostat gradually opens up, allowing coolant flow through the radiator. In cold weather, if the engine temperature drops close to the lower limit of the operating range (194°F or ninety°C), the thermostat closes once more.

Symptoms of a bad thermostat

Symptoms of a bad thermostat can be divided in two types:
Thermostat stuck open: When the thermostat is stuck open, the engine temperature drops beneath normal when driving, particularly on the highway in cold weather. A stuck-open thermostat tin also cause lack of heat from the heating organization. The Check Engine low-cal may come on likewise.

Thermostat stuck closed: When the thermostat is stuck airtight, the engine might overheat. Besides these two cases, the thermostat may also demand to be replaced if the thermostat housing is leaking coolant.

The engine takes very long to warm upwards, can information technology be caused past a thermostat?

Yep, a thermostat that is stuck open is one of possible reasons why it may take as well long for the engine to warm up. A low coolant level can also cause the same trouble. In many mod cars, however, it's actually normal.

Many late-model cars take smaller fuel-efficient engines. This ways they fire less fuel and equally a result, produce less rut. In add-on, newer technologies that reduce friction within the engine also reduce the amount of oestrus generated by friction.

In many newer cars it takes upwards to xx-25 minutes of driving before yous can become decent heat going in winter. A heater may also be not as strong every bit in older cars with a larger engine. It's a trade-off for ameliorate gas mileage. As nosotros mentioned, ane sign that the thermostat is stuck open (or missing) is when the engine temperature drops beneath normal when driving on the highway in cold weather condition.

How the thermostat is tested

When dealing with any cooling organisation problem, the coolant level is checked first. Read more: how to cheque the coolant level.
In old days, testing of the thermostat involved removing and heating it while submerged in h2o. At the temperature marked on the thermostat, it would starting time opening. In modern cars, the operation of the thermostat is monitored past the engine figurer (PCM). In most cases, if a thermostat is stuck closed or open, the Check Engine light would come up on. Most mutual Check Engine light codes related to a thermostat are P0128 and P0126.

Mechanic measures radiator hose temperature with an infrared thermometer

Tin can you exam a thermostat without removing information technology? Mechanics examination the thermostat by measuring the temperature of the upper and lower radiator hoses with an infrared thermometer, while monitoring the engine temperature with a scan tool. See the chart below. In this chart, the xanthous line is the temperature of the engine, the cerise line is the temperature of the upper radiator hose and the bluish line is the temperature of the lower radiator hose.

When the engine is started common cold, the upper hose warms upwards with the engine, but since there is no flow through the radiator, the lower radiator hose remains cold.

The engine warms up at idle. The yellow line is the engine temperature. The thermostat opens at 185°F (85°C) and the temperature of the lower radiator hose (blue line) becomes almost the same as the temperature of the upper radiator hose (ruby-red line).

In this Honda in the nautical chart, the thermostat starts opening at virtually 185°F (85°C). As the thermostat opens and coolant starts flowing through the radiator, the temperature of the lower radiator rises quickly close to the temperature of the upper hose. At about 204°F (96°C), the radiator fan comes on and the temperature drops a few degrees. The thermostat in this car works properly.

If a thermostat was stuck open, the lower radiator hose would start warming up equally shortly as the engine is started.
If a thermostat was stuck closed, in that location would be no flow even later on the engine reached the operating temperature, and the lower radiator hose would remain cool.

Of course, lack of flow through the radiator can exist caused by many other reasons as well a stuck-closed thermostat. If a thermostat is stuck closed, the engine might overheat, which can issue in expensive repairs. If there is a suspicion that the thermostat is stuck airtight, it's commonly replaced.

Thermostat replacement

Replacing a thermostat

In an average vehicle replacing a thermostat may price $150-370 parts and labor. In some cars the access to the thermostat is hard, requiring more labor. A new thermostat normally comes with a new gasket. When the thermostat is replaced, the engine will need to be refilled with fresh coolant (antifreeze). The cooling arrangement will need to exist properly bled, to remove all air pockets.

Later on servicing the cooling organization, it'south always important to verify that the fans come up on when the engine is fully warmed up and the heating organisation within the car can provide a skillful heat at idle. Lack of heat at idle can be caused by air pockets within the cooling system.

Should the thermostat be replaced every bit a office of regular maintenance? Information technology's not necessary, however, it might be a good idea to replace a thermostat when the engine is rebuilt or replaced.
A thermostat is also often replaced together with a water pump, especially if they are located in the aforementioned area.

Some manufacturers recommend updating the engine computer software when there is a Check Engine code related to the thermostat, after it has been replaced. Nosotros posted several links where you lot can become a subscription fee based access to a factory repair manual in this article.

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Source: https://www.samarins.com/glossary/thermostat.html

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