A Guide to the Engine Coolant Thermostat for the 2008-2014 BMW M3 (E9X)
Keep your S65 V8 engine's temperature in check by understanding thermostat failure and choosing the right replacement part.
- A failing thermostat usually gets stuck open, causing the engine to warm up very slowly and triggering a '2B59' fault code.
- Choose an OEM-equivalent brand like Mahle-Behr or Calorstat for reliability; the significant labor cost makes using cheaper, less reliable brands risky.
- Replacement is a complex DIY job, requiring removal of the intake plenum and taking 3-5 hours.
- While replacing the thermostat, it's wise to also replace the coolant temperature sensor and associated o-rings to prevent future issues.
Understanding Thermostat Failure in Your E9X M3
The thermostat in the S65 engine is designed to fail in the "stuck open" position. This is a safety measure to prevent the engine from overheating, which could cause catastrophic damage. When the thermostat is stuck open, coolant continuously flows through the radiator. This makes it take a very long time for the engine to reach its proper operating temperature, especially in cold weather or during highway driving. 🎬 Watch: See what a failing thermostat looks like from the driver's seat. While this is safer than failing closed, running the engine too cold for extended periods can lead to poor fuel economy, increased engine wear, and higher emissions.
Common Fault Codes
A failing thermostat will almost always trigger a Check Engine Light. The most common fault code associated with this issue is 002B59 (Coolant Thermostat, Monitoring) or simply 2B59. This code means the engine control unit (DME) has detected that the engine is not warming up within the expected time frame. Another related code could be 2AF9, which points to the coolant temperature sensor, a part that works closely with the thermostat.
Choosing the Right Aftermarket Thermostat
When replacing a critical component like the thermostat, quality is key. For the E9X M3, the consensus among experienced owners and mechanics is to prioritize OEM or OEM-equivalent parts.
- OEM-Equivalent Brands: Brands like Mahle-Behr and Calorstat by Vernet are often the original equipment manufacturers for BMW. Choosing one of these brands typically gets you the exact same quality as a part from the dealer, but without the dealership price tag. These are highly recommended.
- Quality Aftermarket Brands: Gates is a well-known brand in the cooling system world. While some users have good experiences, others prefer to stick with the OEM supplier for a part this critical and difficult to access.
- Standard Aftermarket/Reboxed Brands: Brands like Beck Arnley and Motorad often source their parts from various manufacturers, meaning quality can be inconsistent. Some users report premature failures with these brands. Given the significant labor involved in a thermostat replacement, saving a small amount on the part itself is often not worth the risk of having to do the job twice.
Pro Tip: Replace the Sensor and O-Rings
While you are replacing the thermostat, it is highly recommended to also replace the coolant temperature sensor and the various o-rings and gaskets. BMW even issued a Technical Service Bulletin (TSB SI B12 07 12) for a faulty coolant temperature sensor on the S65 engine, which can cause similar issues. Replacing these small, inexpensive items at the same time can prevent future leaks and electrical issues.
🎬 Watch: How to replace the thermostat and coolant temperature switch together.Cost of Replacing Your M3's Thermostat
The cost can vary significantly depending on whether you do the work yourself or take it to a shop. The part itself is moderately priced, but the labor is intensive due to the location of the thermostat underneath the intake manifold.
| Item | Estimated Cost |
|---|---|
| New OEM (Dealer) | $150 - $200 |
| New Aftermarket (OEM-Equivalent) | $75 - $120 |
| New Aftermarket (Standard) | $45 - $80 |
| Shop Labor | $450 - $750 |
| Total Professional Replacement | $525 - $950 |
Frequently Asked Questions
Is it safe to drive with a bad thermostat?
If the thermostat is stuck open (the most common failure), you can drive the car for short periods, but it's not recommended for long. The engine will run too cool, leading to poor fuel economy and increased wear. If the thermostat were to fail in the much rarer stuck-closed position, you should not drive the car at all, as it will overheat very quickly and cause severe engine damage.
Is replacing the M3 thermostat a DIY job?
This is an advanced DIY job. It requires removing the intake plenum, which can be intimidating. For a mechanically experienced person, it can take 3-5 hours. If you are not comfortable with extensive work in a tight engine bay, it is best to leave this job to a professional mechanic.
🎬 See this comprehensive DIY walkthrough for replacing the water pump and thermostat.Should I replace the water pump at the same time?
Unlike many other BMW models with electric water pumps that fail frequently, the mechanically-driven water pump on the S65 engine is quite robust. While some owners choose to replace it as a preventative measure, it is not considered a mandatory "while you're in there" part like it is on other engines. The thermostat can be replaced without removing the water pump.
Are there any recalls for the thermostat on the 2008-2014 M3?
No, there have been no specific recalls issued by the NHTSA for the engine coolant thermostat on the E9X M3. There was a Service Action (code 546) for a small number of M3s produced in 2009 related to improperly seated retaining springs on the thermostat housing, but this was not a full recall.
Helpful Videos
Technical Specifications
OEM Part Numbers: 11537836155
Electronically controlled map thermostat with an opening temperature of 79°C (174°F).
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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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