Your Car’s Thermostat Housing – What You Need to Know

Every car has a thermostat, and most cars also have thermostat housings. Though most drivers don’t pay much, if any, attention to this automotive part, it plays a crucial role in keeping the vehicle running smoothly. Read on to find out everything drivers need to know about their cars’ thermostat housings.

What Is a Thermostat Housing?

Thermostat housings act as coolant outlets in most internal combustion engines. They’re typically located on either the cylinder head or the engine block itself to keep coolant running smoothly through ports found in both these essential parts. The thermostat housing sits between the engine block and cylinder head and the car’s radiator, regulating the flow of this vital automotive fluid.

What Does the Thermostat Housing Do?

The thermostat regulates the flow of the engine coolant between the radiator and the engine block and cylinder head. When the engine’s heat is below the optimal operating temperature, the thermostat housing remains closed, allowing the coolant to circulate through the engine normally without transferring it to the radiator. When the engine’s temperature approaches the high end of its operating range, the thermostat housing opens so the radiator can cool the fluid before it moves back to the engine block and cylinder head.

Different Types of Thermostat Housings

There are two main types of car thermostats: stand-alone and integrated models. Both perform the same crucial job, delaying the circulation of coolant to the radiator until the engine has warmed up to the upper range of its operating temperature. 

The primary difference between integrated and stand-alone car thermostats is that the former are physically attached to the thermostat housing, while the latter are not, allowing them to be replaced independently from the housing. Most mechanics recommend integrated thermostat housings because they are much easier to replace should the thermostat stop functioning as usual. They’re almost ubiquitous in today’s cars.

How Does the Thermostat Housing Work?

The thermostat housing contains the car’s thermostat, which controls the flow of coolant to the radiator by routing it out of the engine when it reaches the high end of its operating temperature. To determine when the coolant must be routed to the radiator, the thermostat contains a small, wax-filled cylinder. When the engine reaches the programmed operating temperature, usually around 180oF, the wax begins to expand. 

The expansion of the wax pushes the end of a small rod, or piston, which is connected to a valve on the engine. The movement of the rod opens said valve, which allows the flow of coolant to the radiator. The valve will remain open until the engine’s temperature returns to below the optimal operating temp programmed into the thermostat, at which point, it will close. Generally, the wax won’t contract to close the valve until the vehicle has been turned off.

What Parts Are in the Thermostat?

There are several different types of car thermostats, but all of them have the same basic components. In addition to the wax, piston, and valve assembly described above, each thermostat contains:

  • A flange
  • A spring
  • A frame

Some car thermostats also feature disks at their bases that close bypass circuits inside the engines when they open radiator circuits. These bypass circuits allow the coolant to continue circulating inside the engine even when the radiator circuit is closed to prevent hot spots from forming.

Symptoms of a Broken Thermostat Housing

Drivers don’t really need to know exactly how their cars’ thermostat housings work to keep their vehicles on the road. What’s more important is learning how to recognize the symptoms of a broken thermostat housing. When drivers say their thermostat housing is broken, they usually mean that the valve is stuck open or shut. 

A valve that’s stuck shut will fail to allow the coolant out of the engine and into the radiator once the engine reaches operating temperatures, causing it to overheat. A valve that’s stuck open will allow coolant to flow into the radiator constantly, making it difficult for the engine to reach optimal operating temperatures. Thankfully, there’s no need to open up the hood to determine whether the thermostat housing is failing. Look for these symptoms instead:

High Engine Temperature Readings

The most alarming symptom of a failing thermostat housing is a temperature gauge that reads high into the red almost immediately upon starting up the car. If the car has only been running for around 15 minutes and the temperature gauge is already pinned at the top, stop driving it immediately. Chances are, the thermostat housing’s valve is stuck shut and the coolant is not circulating properly. The engine will overheat if the situation is not resolved.

Erratic Changes in Temperature

Erratic fluctuations in engine temperature can also indicate a failing thermostat housing. In this case, the valve may be only partially stuck. It’s not opening and closing exactly when it should be, which leads to dramatic spikes and drops in operating temperature. Don’t ignore this symptom, as it will lead to poor engine performance and, eventually, engine failure.

Coolant Leaks

A vehicle’s coolant can leak from any part of its two circuits. However, if the coolant leak is most obvious around the thermostat housing, there’s a good chance the valve is stuck shut and is not allowing the coolant to flow properly. The leak may also be originating from a broken seal or a crack in the housing. Either way, it will only get worse over time and should be addressed as soon as possible.

Testing for Thermostat Issues

There are a few specific tests drivers can perform at home to see if the vehicle’s thermostat housing is to blame for engine overheating problems. Start by identifying the thermostat housing while the engine is cold. Next, start it up and place a hand on the radiator or top hose, being careful to keep those hands clear of the fan. 

If the car’s thermostat is functioning as intended, the hose or radiator will remain cool for several minutes then warm up quickly. If the valve is stuck open, the hose will begin warming gradually from the moment the engine is turned on. If the hose doesn’t warm up noticeably at all, but the engine starts overheating fast, the valve is stuck shut.

How to Fix Thermostat Housings

In most cases, mechanics don’t fix thermostat housings. They simply replace them when something goes wrong because the parts are relatively affordable, while the labor required to fix a broken thermostat housing is more substantial. That said, there are some circumstances where it makes more sense to fix the vehicle’s existing thermostat housing.

If the thermostat housing is leaking as a result of a broken seal between the engine and the housing itself, replacing the seal may be an option. The mechanic will first inspect the part to make sure there is no warping. If the thermostat housing is not warped, he or she can replace the runner seal and reinstall the same housing.

Problems with cracked thermostat housings or valve assemblies that are no longer functional are best resolved by replacing the unit. To do that, a mechanic waits until the engine has cooled, then simply unscrews the fasteners that hold the thermostat housing in place and removes them. In some cases, he or she must also disconnect the lower radiator hose and drain out any coolant in it. Before installing a new housing, the gasket and/or sealant material must be cleaned from the mounting surface.

Once the new thermostat is in place, the mechanic will start the engine and bring it back up to temperature. He or she will then remove any air in the coolant lines using a specialized vacuum tool, then replace any coolant that has been lost as a result of damage or during the replacement process. 

Some drivers may be wondering how hard it is to repair or replace a thermostat housing at home instead of bringing the car to the mechanic. While it’s true that the part itself is easy to remove and replace, failures in following the guidelines for bleeding the coolant system of air can lead to overheating and serious engine damage. Unless drivers know how to follow proper replacement and bleeding procedures safely, it’s best to take the vehicle to a trained mechanic.

Thermostat Housing Replacement Cost

The cost of replacing a thermostat housing varies by the mechanic. Typically, parts are priced at an average of $124, and labor costs anywhere from $228 to $161. Keep in mind, however, that those are just estimates. Vehicles with very specific parts requirements may be more expensive to fix. If the seal, rather than the thermostat housing itself, is to blame for a leak, the repairs may be much cheaper. Even labor costs vary substantially between different shops. It’s always best to request quotes from several reputable mechanics in the area and choose one that offers both competitive pricing and excellent workmanship.

Final Thoughts

A car’s thermostat housing plays an essential role in keeping it on the road. If the thermostat doesn’t work correctly, the engine may not work at optimal efficiency, and ignoring the problem could lead it to overheat and get destroyed. Having a thermostat housing replaced as soon as it starts failing is a much more affordable option than replacing an entire engine, so it’s always best to take the car to a mechanic as soon as there are any signs of trouble.