Case Studies

BMW Heated Thermostats – The Myth, The Mystery and The Magic

General and BMW Heated Thermostats – Myth, Mystery and Magic
When your BMW’s check engine light illuminates, it’s your car’s way of communicating that something isn’t quite right under the hood. One of the many trouble codes that can leave BMW owners scratching their heads is the coolant system MAP faults.

In this blog post, we’re diving deep into the world of BMW diagnostic trouble code, shedding light on the mysteries surrounding it, and specifically addressing its association with thermostat issues.

As any BMW enthusiast or owner knows, these machines are precision-engineered marvels, but they’re not without their quirks and intricacies. That’s why it’s crucial to decipher these trouble codes quickly and accurately. In the case of MAP fault, the culprit often lies within the thermostat, a seemingly small yet vital component of your BMW’s cooling system.

Throughout this guide, we’ll explore what the MAP fault code signifies, delve into the role of the thermostat in your BMW’s performance, guide you through the diagnostic process, and equip you with the knowledge to troubleshoot and potentially resolve this issue on your own. Whether you’re a seasoned DIY mechanic or simply a BMW owner looking to better understand your vehicle, this post will empower you to tackle MAP fault and ensure your BMW continues to run smoothly.

How Old School Works
Many legacy engine components have been brought under electronic control, primarily to improve performance and reduce emissions.
The humble engine mechanical thermostat is a recent example.

Thermostats have been important in automotive internal combustion engines for almost a century. Thermostats were originally incorporated to speed engine warm-up and reduce piston ring wear. Today, thermostats are important in increasing engine combustion efficiency and reducing emissions. To accomplish that task, thermostat functions are controlled by the engine’s electronic control unit (ECU), ensuring precise regulation of temperature based on the engine’s loads.

The combustion process in a passenger car engine runs optimally at an operating temperature of approximately 230°F. However, the engine temperature was kept below this ideal temperature level in older engines to prevent component damage. Since an engine requires a certain power reserve, especially when operating at full load, conventional thermostats start to open at an engine temperature of approximately 110°F by opening the coolant circuit.

The most basic type of thermostat is a bypass valve thermostat. These have a sensing element containing a wax and aluminum mixture that expands when heated. When the engine is cold, the wax is solid; as it expands, the sensing element slides, opening a valve and allowing coolant to flow to the radiator. A tension spring presses against the sensing element and closes the valve when the operating temperature falls below the opening level of the thermostat. This process may occur multiple times a day, especially in colder climates.

This technology—which still serves reliably to this day—has been in use for decades, but the open temperature setting of the thermostat can be adjusted only slightly by changing the wax compound in the sensing element.

Mapping Optimum Performance
New technologies are pushing engine efficiency and combustion quality ever closer to optimum operating conditions. As we push the engine closer to the desired 230° range to improve both emissions and fuel economy, we need a more advanced thermostat technology.

An electrically assisted (also called map-controlled) thermostat provides broader and faster operation than traditional thermostats, In addition to the mechanical function of the wax sensing element, electrically assisted thermostats incorporate an electric heater within the sensing element. This heater is controlled by the vehicle’s ECU, which receives information on the speed and load conditions of the engine. It uses this information to regulate the temperature of the coolant. A data set, or “map”, is stored within the ECU to govern when and how heat is added to ensure optimum engine performance.

Consequently, the thermostat can influence the temperature considerably more quickly, allowing the engine to operate under various loads and operating conditions within the corresponding optimum range.

This level of temperature regulation yields several benefits:

  • optimum combustion due to increased cylinder wall and component temperatures;
  • reduced fuel consumption due to reduced viscosity of the engine oil and, consequently, reduced friction loss;
  • lower pollution emissions due to improved combustion;
  • improved power output at full load due to reduced coolant temperature;
  • increased comfort due to higher coolant temperatures and, as a result, an improved interior heating performance.

How It Works – All Map Equipped Cars
In standard operation, an electrically assisted thermostat functions in the same manner as a conventional thermostat—only at a higher engine temperature. Coolant flows around the wax of the thermal expansion sensing element. As the temperature rises, the expansion material melts, increasing in size and moving a piston, which in turn increases the flow volume of the coolant. If the temperature drops, a spring pushes the piston back to its starting position, reducing the flow rate of the coolant or closing the coolant circuit altogether.

Under partial load conditions (city driving), the thermostat stabilizes the engine at a higher temperature by staying closed longer to obtain such benefits as good power response, lower emissions, and reduced friction (with a corresponding reduction in fuel consumption).

When under a sudden, heavy load, an additional heat source comes into play with the map-controlled thermostat. Once the conditions of the stored operating map have been fulfilled, a heating element integrated into the expansion material is enabled by the engine management system. This additional heat source allows the wax to expand quicker, opening the thermostat fully regardless of actual coolant temperature, so the coolant flow is increased, immediately allowing the engine to operate within the optimum temperature range without danger of overheating.

Since the electric thermostat is controlled by the engine computer and mapped to driving conditions, when the sudden heavy demand is removed, the current is shut off to the electric heating unit and the thermostat again acts like the traditional wax unit with a fully open temperature of approximately 230°F. These actions can happen many times a day, especially if the vehicle is driven in the mountains where the engine and cooling system would experience heavy demand going up a grade, then may cool as much as 30° to 50°F going down the other side. The beauty of this concept and design is that it operates completely unnoticed by the driver and continues over the life of the thermostat with no required service.

BMW Heated Thermostat

How It Works – In Your BMW
The thermostat in your BMW is a small yet crucial component located within the cooling system. Its primary function is to regulate the engine’s temperature. It achieves this by controlling the flow of coolant through the engine.

Here’s How it Works

  • Temperature Sensing — The thermostat contains a temperature-sensitive element. When you start your BMW, the thermostat remains closed, preventing the flow of coolant to the radiator.
  • Warming Up — As your engine warms up, it reaches the thermostat’s set opening temperature (usually around 195°F or 90°C). At this point, the thermostat begins to open gradually.
  • Optimal Operating Temperature — Once fully open, the thermostat allows coolant to circulate through the radiator, dissipating excess heat and maintaining the engine at its optimal operating temperature.
  • Fuel Efficiency and Performance — Proper temperature control is crucial for fuel efficiency and engine performance. An engine that’s too cold can consume more fuel and produce higher emissions, while one that’s too hot can suffer from reduced efficiency and potential damage.

Impact of a Faulty Thermostat – All Cars
When your BMW’s thermostat malfunctions, it can have several negative consequences:

  • Engine Overheating — A thermostat stuck in the closed position can prevent coolant flow, leading to engine overheating. This not only damages engine components but can also cause severe engine damage if not addressed promptly.
  • Reduced Fuel Efficiency — An engine that’s constantly too cold due to a stuck-open thermostat can consume more fuel than necessary, leading to reduced fuel efficiency and increased emissions.
  • Poor Performance — Temperature control is essential for engine performance. An improperly functioning thermostat can result in sluggish acceleration and reduced power output.
  • Excessive Wear and Tear — Constant temperature fluctuations due to a malfunctioning thermostat can lead to increased wear and tear on engine components, potentially reducing the overall lifespan of your BMW.

In summary, the thermostat might be small, but its role in maintaining your BMW’s performance, fuel efficiency, and longevity is substantial. When this critical component encounters issues, such as those indicated by the fault code, it’s essential to address them promptly to ensure your BMW continues to run smoothly and efficiently.

Service Considerations
As is the case with conventional thermostats, electrically assisted thermostats are not subjected to material wear; they’re maintenance-free and designed to last for the engine’s entire service life. However, external factors such as the use of low-grade coolant and failure to regularly service the coolant can lead to material failure. Other possible causes of failure include previous damage caused by thermal overloading or contamination due to work carried out on the cooling system—for example, when replacing the coolant or water pump, the radiator, the coolant hose, the timing belt, or a V-belt.

Technicians need to remember that the MAP-controlled thermostat is only part of a more complex cooling system consisting of coolant passages in the engine, a coolant mixing chamber around the thermostat, coolant hoses, a radiator, electric fans, and the coolant itself. When replacing faulty parts in the cooling system, thermostats and/or integral thermostatic housings should also always be replaced at the same time, because any loss of functionality or even complete failure can have severe consequences—including engine damage.

An Ounce Of Prevention
If you’re an Atlantic Motorcar customer, we’re going to be keeping an eye on this for you, and will advise of you of the proper test and repair plans.
If you’re not, make sure it gets done, kind of like that old bromide about voting, “early and often”. 😉

Our Goal For You and Your Car
Our goal is to save you money, not spend it. This is why each and every car that we service gets a free Courtesy Maintenance Inspection during its first visit.

Our goal is to let you know about the small problems before they become big ones. Right now we have a number of customer cars with well over 200,000 miles, and several approaching 300,000! And these cars are not just limping along – most look and drive pretty much the way they came out of the showroom.

Proper maintenance is an investment in the life of your vehicle.
Knowing, not just doing, but actually knowing and understanding the difference, makes all the difference.
Be sure your car is properly loved, our professionals will attend to both you and your car’s needs.

Atlantic Motorcar…Extraordinary Service for Extraordinary Cars, just a phone call away, (207) 882-9969.


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