Engine Coolant Thermostat Guide for the 2005-2012 Mitsubishi Galant
Keep your Galant's engine temperature in check by understanding, diagnosing, and replacing a faulty thermostat.
- A thermostat stuck closed can quickly cause catastrophic engine overheating; pull over immediately.
- A thermostat stuck open will cause a P0128 code, poor fuel economy, and weak cabin heat.
- For reliable performance, choose an OEM-temperature thermostat from a trusted brand like Gates or stick with the Mitsubishi OEM part.
- Avoid 'racing' thermostats with lower temperature ratings for a daily-driven Galant as they can negatively impact performance and comfort.
Is Your Galant's Thermostat Failing?
A thermostat is a simple part, but when it fails, it can cause noticeable problems. Thermostats typically fail in one of two ways: stuck open or stuck closed. Both situations can cause issues for your Galant's engine.
A thermostat that is stuck closed is the more serious problem. It traps coolant inside the engine, preventing it from reaching the radiator to cool down. This will cause the engine to overheat very quickly, which can lead to severe and expensive engine damage like a warped cylinder head or blown head gasket. If you suspect your thermostat is stuck closed, you should stop driving immediately.
A thermostat that is stuck open is more common and less immediately dangerous, but it still needs to be fixed. It allows coolant to flow to the radiator constantly. This makes it very difficult for your engine to reach its proper operating temperature, especially in cooler weather. This can lead to poor fuel economy, reduced heater performance, and a Check Engine Light.
Choosing the Right Aftermarket Thermostat
When replacing the thermostat on your Galant, you have a choice between OEM (Original Equipment Manufacturer) and various aftermarket brands. While OEM parts guarantee a perfect fit and function, quality aftermarket parts can offer good performance at a lower cost.
Based on owner feedback and mechanic consensus, aftermarket brands can be grouped into general tiers of quality and reputation.
Pro Tip: What About Temperature Ratings?
You will see thermostats listed with a temperature, like 180°F (82°C) or 190°F (87°C). It is critical to use the temperature specified for your Galant's engine (2.4L or 3.8L V6). This is called the "OE Temperature." Using a thermostat with a lower temperature, like a "racing" thermostat from Mishimoto, will cause the engine to run cooler. While this might sound good, it can decrease fuel efficiency and make your cabin heater less effective in the winter. For a daily-driven vehicle, always stick with the OE-specified temperature.
Brand Reputation Tiers
- Tier 1: OEM-Quality & Trusted Aftermarket: Brands like Gates are often considered reliable aftermarket choices. Aisin is a known OEM supplier for many Japanese vehicles and is a trustworthy option if available for the Galant. Stant is another long-standing brand with a generally good reputation. These brands are a safe bet for a balance of quality and price.
- Tier 2: Mixed Reviews: Beck Arnley claims to provide OE-matching parts, but customer reviews are mixed, with some reporting premature failures. Motorad also receives frequent negative feedback in forums for failing early, sometimes within months. While some products from these brands may work fine, the inconsistency makes them a riskier choice.
- Specialty: Mishimoto offers "racing" thermostats that open at a lower temperature (e.g., 145°F or 154°F). These are intended for highly modified or track-focused cars and are not recommended for a standard Galant, as they can prevent the engine from reaching its efficient operating temperature.
Estimated Costs for Thermostat Replacement
The cost to replace a thermostat can vary depending on whether you do it yourself or take it to a shop. The part itself is relatively inexpensive, but labor can add to the cost.
| Item | Estimated Cost |
|---|---|
| New Aftermarket Thermostat | $17 - $71 |
| New OEM Thermostat | $40 - $65 |
| Shop Labor | $125 - $184 |
| Total Professional Replacement | $414 - $484 (parts & labor) |
General Replacement Process
Replacing the thermostat on a 2005-2012 Galant is a feasible DIY project for those with some mechanical experience. The exact location of the thermostat housing will differ slightly between the 2.4L 4-cylinder and 3.8L V6 engines, but the general steps are the same.
- Let the Engine Cool: Never work on a hot cooling system. Let the car sit for several hours until completely cool.
- Drain Coolant: Place a drain pan under the radiator and open the drain plug (petcock) at the bottom. You only need to drain enough coolant so the level is below the thermostat housing.
- Locate and Remove Housing: Follow the upper radiator hose to where it connects to the engine. This is the thermostat housing. Remove the bolts holding the housing in place.
- Swap Thermostats: Remove the old thermostat and gasket. Note its orientation. Clean the mating surfaces on the engine and the housing. Install the new thermostat in the same orientation, ensuring the jiggle pin (if present) is at the top.
- Reassemble and Refill: Re-install the housing with a new gasket. Refill the cooling system with the correct type of coolant for your Mitsubishi.
- Bleed the System: With the radiator cap off, start the engine and let it run until it reaches operating temperature. You should see the coolant level drop as the thermostat opens and air escapes. Top off the coolant as needed and replace the cap. Check for any leaks.
Common Mistakes to Avoid
- Incorrect Orientation: Installing the thermostat upside down will block coolant flow and cause overheating.
- Skipping Gasket/Seal: Always use a new gasket or seal. Reusing the old one will almost certainly cause a leak.
- Wrong Coolant: Use a coolant that is compatible with Mitsubishi vehicles to prevent corrosion and damage to cooling system components.
Frequently Asked Questions
Are there any recalls for the 2005-2012 Galant thermostat?
No, there are no specific recalls or technical service bulletins (TSBs) from the NHTSA for the engine coolant thermostat on the 2005-2012 Mitsubishi Galant.
What is the code P0128?
The trouble code P0128 means the engine's coolant temperature is below the expected regulating temperature. This code is most commonly caused by a thermostat that is stuck open or opening too soon, preventing the engine from warming up properly.
My heat isn't working well. Could it be the thermostat?
Yes. If your thermostat is stuck open, the engine coolant may never get hot enough to provide sufficient heat to the heater core. This results in lukewarm or cool air blowing from your vents, especially in cold weather.
Should I replace the thermostat housing at the same time?
It is not always necessary, but you should inspect the housing for cracks, corrosion, or warping when you replace the thermostat. On older vehicles, the plastic or aluminum housing can become brittle. If it shows any signs of damage, it is best to replace it to prevent future leaks.
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Technical Specifications
OEM Part Numbers: 1305A278 MD350415 MD315301
Standard opening temperatures are typically 180°F (82°C) or 190°F (87°C). It is crucial to use the correct temperature for your specific engine (2.4L or 3.8L V6). Always verify the correct part using your vehicle's VIN.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
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- Is Your Galant's Thermostat Failing?
- Choosing the Right Aftermarket Thermostat
- Pro Tip: What About Temperature Ratings?
- Brand Reputation Tiers
- Estimated Costs for Thermostat Replacement
- General Replacement Process
- Common Mistakes to Avoid
- Frequently Asked Questions
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