BMW E46 cooling system components include:
- Radiator and coolant overflow tank.
- Belt driven coolant pump bolted to the front of the engine block.
- Electric cooling fan attached to rear of radiator. The cooling fan is controlled by the engine control module (ECM) via an output final stage.
- Automatic transmission models: Belt driven fan attached to front of coolant pump. Viscous clutch controls fan speed based on engine temperature and rpms.
- Electrically heated thermostat.
- 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.
Pressure testing your BMW cooling system is the best way to find leaks. However, not everyone has a pressure tester. I would suggest acquiring one if you plan to maintain your own vehicles; it can save quite a bit of time when locating a cooling system leak. You can also check for leaks without a pressure tester, but results are less accurate and the procedure is time consuming. I will cover pressure testing in this tech article, as well as some tips on checking for leaks without using one.
A cooling system pressure tester is used to pressurize a cooling system and hold it at a specified pressure while looking for leaks. Coolant leaks can be internal (i.e.: head gasket) or external (i.e.: radiator). It is important to take these things into consideration when looking for a coolant leak. If you are losing coolant, but not seeing any on the ground, this would likely be an internal coolant leak. Puddles of coolant found under your vehicle would be from an external leak. Remember to properly clean any coolant that gets on the ground and properly dispose of it. Place a drain pan under suspected areas of leaks to minimize coolant spills.
E46 models develop coolant leaks toward the front of the engine, in places like the radiator outlet sensor, water pump and thermostat. Start your testing with a cool engine. If you do not find the leak, warm the engine with the pressure tester installed. At times, leaks will surface as parts expand from engine heat. Look for leaks at and around all the components listed above.
Do not remove the expansion tank cap to install the pressure tester while the engine is hot. Coolant or hot steam may escape and will scald you. To do any work on the cooling system, wait until the engine has cooled off.
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 you're 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. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
When pressure testing a cooling system, be sure the vehicle is cool and lacks pressure before removing the coolant expansion tank cap (green arrow).
Working with a cold engine, install pressure tester to coolant expansion tank. Pressurize system to 1.5 bar (21.8 psi). Pressure should not drop more than 10% over a two-minute time period. If pressure drops right off, that is a sign of a leak. Look for coolant on the ground. If no coolant reaches ground, you could have an internal leak. If you suspect a head gasket leak, you need to test cylinder sealing via a leak-down test. If coolant leaks externally, use a flashlight and follow coolant to source.
Coolant leaking down the front of engine could be water pump or thermostat housing. These are the two most common areas. Use a small mirror, looking from the top of engine and try to locate source of leak. From the bottom, coolant will leak down the side of crankshaft pulley from either source (green arrows).
Coolant leaking down radiator could be from radiator or expansion tank. If on the driver side, suspect expansion tank (green arrow). If on passenger side, suspect radiator or coolant sensor in lower radiator hose (purple arrow).