Thermostat Housing: Leak Causes, Replacement Cost, Symptoms

Your vehicle’s cooling system ensures that your engine maintains optimal operating temperatures while you’re on the road. But over time, parts of this system may wear out and require replacement. This article will discuss the importance of your thermostat and what may happen if its housing leaks coolant.

What is a Thermostat?

The thermostat regulates the flow of coolant between the engine and radiator. The thermostat remains open to allow coolant to flow into your engine and maintain optimal operating temperature.

What Causes Thermostat Housing Leaks?

Your vehicle can either have a stand-alone or an integrated thermostat housing made of high-quality plastic or metal. While these materials are designed to last for a long time, exposure to extreme temperatures and movement in the coolant rubber hose may cause your thermostat to eventually spring a leak.

Coolant will also leak out of the system once the housing is cracked, warped, or damaged in any way. A failed or worn seal may also cause a similar issue. Your engine may overheat if your thermostat housing keeps on leaking, so it’s best to address this issue right away.

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Pro Tip: In addition, there are typically plastic fittings here and there on modern vehicles that carry hot coolant, not to mention the plastic-and-aluminum radiators, so the thermostat housing is only one potential leak source. But plastic coolant-carrying parts tend to be leak prone as they age. Coolant surge tanks also leak and may need replacing.

Is it Safe to Drive with a Bad Thermostat Housing?

Although it’s possible to drive with a damaged thermostat housing, it’s highly recommended that you don’t. Ignoring a coolant leak is flirting with disaster, and cooking your engine can ruin your whole day. It’s best to enlist the help of a mechanic to check your thermostat and replace the faulty parts as needed. You don’t want to wait until your vehicle overheats, potentially causing catastrophic internal engine damage.

Signs That Coolant Is Leaking from the Thermostat Housing

Ideally, your temperature gauge should have a low-temperature reading when your car has been parked for a while. As you continue to drive, the needle is expected to reach the midway point of the temperature gauge. This is an indication that your engine is at its optimal temperature.

• OBD Codes: A healthy engine will run at just above 200 degrees, which is right near the center of the gauge. A cold running engine will throw P0128 or P0125 codes and can cause the engine to develop engine-destroying sludge over time.
• Thermostat valve stuck closed – If your thermostat’s valve is stuck closed, you may notice a high-temperature reading. You’d be risking damaging your engine components if you continue to drive your vehicle.
• Thermostat valve stuck open – Note that a stuck thermostat valve doesn’t often happen. More likely, it’ll open too soon or malfunction in the open position. If the housing is leaking to the point that the coolant gets low, it doesn’t take long to cause some serious damage.
• Low coolant warning light – The low coolant warning light on your dashboard may switch on if your coolant level drops below the normal range due to a leaking housing. The check engine light could also illuminate once your engine overheats or if there are thermostat performance issue trouble codes logged. You may also notice fluid leaking from underneath your vehicle.
• Inoperative heater – Your car’s heater relies on hot engine coolant. A leaking thermostat housing can also lead to a low coolant level, resulting in an inoperative heater.

Fixing a Thermostat Housing Leak

Coolant leaks may come from a crack in the housing or from the seal between the housing and engine. In either scenario, the faulty component will need to be replaced.

If you’re not experienced in DIY auto repair, it’s best to take your vehicle to a professional. A mechanic will know how to diagnose the issue properly and rule out any other problem that may cause similar symptoms.

However, if you also plan to replace the thermostat, make sure the thermostat plate is in the recessed area before tightening the housing. This is even more critical if the thermostat uses a paper gasket and is mounted vertically. Sometimes the recess for the thermostat will be in the thermostat housing and sometimes it’ll be in the manifold where the thermostat mounts. Use glue and the gasket to hold the thermostat in place, but don’t use too much glue. A little dab should work for these.

How Much Does a Thermostat Housing Replacement Cost?

A replacement thermostat housing will cost you around $50 to $250 on parts alone. Labor costs can range anywhere between $130 and $170. Keep in mind that you may need to spend more on other related repairs. Some plastic thermostat housings may be replaced with cast aluminum aftermarket units that don’t cost any more than the plastic original and will last a lot longer.

Thermostat Housing Configurations

You can typically find the thermostat in the housing where the upper radiator hose connects to the intake manifold cooling passage. Some light truck diesel engines have two thermostats.

Older thermostat housings used gaskets but almost all new thermostats use o-rings or seals on the thermostat itself, and most thermostats will have a small bleed valve in the plate to prevent air from being trapped under the thermostat.

If the thermostat is vertically mounted, make sure the little bleed valve is mounted in the 12 o’clock position. If the thermostat is mounted horizontally, it doesn’t matter, but make sure you always put the thermostat in with the wax element away from the radiator and toward the engine. The temperature stamped on the wax element is the temperature at which the thermostat begins to open.

Ford Explorer 4.0L SOHC engines have a plastic thermostat housing front and center and it almost always cracks and begins to leak somewhere between 75,000 and 120,000 miles.

Thermostat Locations

Notable exceptions to the upper radiator hose rule would be the 2.7L V6 Dodge/Chrysler engine (Dodge Stratus), which has the thermostat mounted low on the front side of the engine block behind the A/C compressor.

And 2.2L Toyota Camry engines from the 1990s have the thermostat mounted where the bottom radiator hose connects to the engine. The slant 4 engines in VW Rabbits were set up the same way. These engines are easy to fill with coolant because no air is trapped in the cooling system by the thermostat.

Even as far back as the 1980s, Ford 2.8L V6 Mustang engines had the thermostat mounted where the lower radiator hose connects.