A Practical Guide to the Engine Coolant Thermostat for the 2005-2011 BMW M6
Keep your S85 V10 engine running at the correct temperature by understanding thermostat failure and choosing the right replacement part.
- The most common failure is sticking open, which causes the engine to run too cool and triggers a P0128 fault code.
- For the S85 V10 engine, it is strongly recommended to use an OEM (Mahle-Behr) thermostat for reliability.
- Replacement is moderately difficult as it requires removing the intake plenums for access.
- Always replace the associated gaskets and O-rings to prevent leaks after the repair.
Signs of a Failing Thermostat in Your M6
A thermostat can fail in two ways: stuck open or stuck closed. For the S85 engine, failure in the 'stuck open' position is more common and is a built-in safety design to prevent catastrophic overheating.
- Engine Takes Too Long to Warm Up: If your temperature gauge never seems to reach the middle, or it drops towards cool when you're cruising on the highway, the thermostat is likely stuck open.
- Check Engine Light: A thermostat that is stuck open will almost always trigger a Check Engine Light. The most common fault code is P0128 (Coolant Temperature Below Thermostat Regulating Temperature) or the BMW-specific code 2B59.
- Engine Overheating: This is the most dangerous symptom and happens if the thermostat fails in the 'stuck closed' position. If you see the temperature gauge climb into the red, steam from the engine, or get an overheating warning, pull over immediately and shut off the engine to prevent severe damage.
- Poor Fuel Economy: An engine that runs too cool is inefficient and will use more fuel than necessary.
OEM vs. Aftermarket: A Word of Caution for the S85 Engine
The cooling system is not an area to cut corners on a high-performance engine like the S85 V10. While aftermarket options are available, owner and specialist experiences strongly favor using the original equipment (OEM) part for reliability.
Tier 1: Recommended
- Genuine BMW / Mahle-Behr: Mahle-Behr is the original manufacturer for the BMW thermostat. Choosing either the Genuine BMW part or the Mahle-Behr equivalent will give you the best fit and reliability. Given the critical role of this part, this is the highly recommended path.
Tier 2: Use with Caution
- Calorstat / Gates: These are established aftermarket brands. While they may perform adequately, the potential risk of failure on an S85 engine often outweighs the small cost savings compared to using the OEM supplier part.
Tier 3: Not Recommended
- Motorad / Beck Arnley: These are generally considered budget-tier options. Forum discussions often contain reports of premature failure with these brands. Beck Arnley is also known to re-box parts from various manufacturers, so you may receive a lower-quality part in their packaging.
Pro Tip: Replace as a Set
The thermostat itself is an insert within a housing. When replacing it, it is highly recommended to also replace the thermostat housing gasket and any related O-rings to prevent leaks. Many owners choose to perform a larger cooling system refresh at the same time, including the water pump and hoses, as these components share a similar service life.
Estimated Replacement Costs
| Part / Service | Estimated Cost |
|---|---|
| New OEM (Mahle-Behr) Thermostat | $45 - $75 |
| New Aftermarket Thermostat | $38 - $118 |
| Shop Labor | $300 - $500 |
| Total Professional Replacement | $350 - $575 |
Note: Labor costs can vary significantly based on shop rates and location. The job requires removing the intake plenums for access, increasing the labor time compared to other vehicles.
Frequently Asked Questions
Can I replace the M6 thermostat myself?
This is possible for an experienced DIY mechanic. The job requires removing the large intake plenums to access the thermostat housing, which can be intimidating. You will need to drain the coolant and have the correct tools. If you are not comfortable with this level of work, it is best to leave it to a professional BMW specialist.
Are there any recalls for the M6 thermostat?
No. There are no active recalls or technical service bulletins (TSBs) from BMW or the NHTSA specifically for the engine coolant thermostat on the 2005-2011 BMW M6. Failures are considered normal wear and tear.
Should I use a lower-temperature thermostat?
Lower-temperature thermostats (e.g., 71°C) are available for track-focused cars. However, for a street-driven M6, it is best to stick with the stock 79°C thermostat. Using a lower-temp unit can cause the engine to run too cool, leading to poor fuel economy, increased emissions, and potentially triggering a P0128 check engine light.
What other parts should I replace at the same time?
At a minimum, you must replace the thermostat housing gasket and O-rings to ensure a good seal. Because of the labor involved in accessing the thermostat, it is a good time to consider replacing other cooling system components like the water pump, coolant hoses, and coolant temperature sensor if they are original or high-mileage.
Technical Specifications
OEM Part Numbers: 11537836155
Opening Temperature: 79° C (174° F)
We Have This Part in Stock
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.
- BMW M6: