BMW X3 models utilize an electrically-heated engine thermostat. Other cooling system components consist of:
Radiator and coolant overflow tank.
M54 and 6-cylinder engines: Belt driven coolant pump bolted to the front of the engine block.
N52 engines: Electric coolant pump bolted to the right front of the engine block.
Electric cooling fan attached to rear of radiator. The cooling fan is controlled by the engine control module (DME) via an output final stage.
Automatic transmission cooler (heat exchanger).
Heater valve and heater core (for climate control).
Coolant level sensor inside expansion tank.
Coolant temperature sensor at cylinder head.
Radiator outlet temperature sensor.
Coolant hose and lines.
The ECM controls and monitors operation of the thermostat. Controlling the thermostat function according to a map allows the engine management system (DME) to raise engine operating temperature quickly and precisely to the optimal range and to maintain it there for maximum efficiency and minimum emissions.
If a fault occurs in the thermostat, a fault code is stored in the ECM, usually with a description of "Map cooling circuit". A fault code can be present yet the vehicle will lack any cooling system issues, such as overheating. This is because the thermostat has a fail-safe mechanical function as well. If you have this fault code, replace your thermostat and bleed your cooling system. Other symptoms of a faulty thermostat are engine overheating, slow to warm up and lack of heat.
In this article I will describe how to replace the thermostat in your M54 6-cylinder engine. Be sure to work with a cool engine and confirm the cooling system lacks pressure before opening the cooling system. N52 engines will be covered in a separate article.
Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.
Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Do not work on your vehicle if you feel the task is beyond your ability.
Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
Drain the cooling system. See our tech article on cooling system draining and filling.
Remove the engine cooling fan. See our tech article on radiator cooling fan replacing.
When replacing the thermostat (green arrow), I suggest replacing both coolant hoses (yellow arrows) that connect to it. Many times I have replaced a thermostat, filled and bled the cooling system, only to find an O-ring on one of the coolant hoses has failed and leaks. If you are on a tight budget, be sure to thoroughly clean the hoses and O-rings before reinstalling them to the thermostat.
Start by removing the four T25 Torx fasteners (red arrows) for the fresh air intake duct. Next you will unclip the five air filter housing retaining clips. Lift them up and unhook from the lid of the intake air housing. Start with the two near the mass air flow sensor. Then release the front clips.
Working at both radiator hoses, use a flathead screwdriver to lever out the coolant hose retaining clips (green arrows). Be sure to pull the retaining clips out until they rest at the stop (yellow arrow).
Next, remove the coolant hoses from the thermostat housing. The green arrow indicates the left hose being pulled away from the thermostat housing. This can be tricky. The hoses have been attached to the thermostat housing for quite a while and may not come off easily. You will want to pull the hose off, while rocking it back and forth. Be careful not to damage the hose if you plan to reuse it. Once the hose moves off the thermostat housing slightly, you can use a flathead screwdriver to gently lever the hose off while pulling. Be very careful with this technique as everything you are prying on is made of plastic and can break.
Working at the top of thermostat housing, locate the thermostat electrical connector. Press the thermostat electrical connector wire (green arrow) and remove the electrical connector from the thermostat housing by pulling it straight off.
Using a plastic scraper and a Scotch-Brite pad, clean the thermostat sealing surface. It is important not to use a metal scraper or razor blade here. You could damage the sealing surface and the thermostat seal (green arrow) will not seat properly, resulting in a leak. Once clean, confirm the sealing surface isn't pitted. It if is, it may not seal correctly. You may have to add some epoxy and sand it down to get a smooth and even sealing surface. I see this happen more on older BMWs. Install the new thermostat and evenly tighten all the fasteners. Install the engine lifting eye nut and tighten it. Install the coolant hoses. Listen for an audible click to confirm the retaining clips have engaged. A small amount of new coolant can be used to lubricate the coolant hose O-rings. This will make installing the hose easier. Connect the thermostat electrical connector. Install the cooling fan and fill and bleed the cooling system. Remember to check the cooling system for leaks and top up the coolant when complete.