BMW Z3 cooling system components include:
- Radiator and coolant overflow tank.
- Belt driven coolant pump bolted to the front of the engine block.
- Belt driven fan attached to front of coolant pump. Viscous clutch controls fan speed based on engine temperature and rpms.
- Electric (auxiliary) cooling fan attached to front of radiator. The cooling fan is controlled by the engine control module (ECM) via an output final stage.
- Electrically heated thermostat.
- Automatic transmission cooler or heat exchanger (if equipped).
- 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 hoses 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 owning 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. Always start 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.
Do not remove the expansion tank cap or work on any other part of the cooling system 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.
To avoid marring the paint and trim, work with a plastic prying tool or wrap a screwdriver tip with masking tape before prying out body or interior items.
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.
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 (example: head gasket) or external (example: 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 is likely an internal coolant leak. Puddles of coolant found under your vehicle could 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.
Z3 models often 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.
When pressure testing a cooling system, be sure the vehicle is cool and lacks pressure before removing the coolant expansion tank cap. (green arrow)
Once the cap is removed, install pressure tester adapter to the 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 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 of leak. Coolant leaking down the right side of engine could be water pump or thermostat. These are the two most common areas. Looking up at the bottom of engine, coolant will appear to leak down from the cluster of cooling system components. Coolant leaking down the left side of engine can be from coolant hoses or fitting that are located under the intake manifold. There is a cooling hose on 4-cylinder models that attached to a plastic fitting. 6-cylinder models also have a plastic fitting with a hose. However this one is less likely to leak. On 4-cylinder models, there is a plastic coolant pipe at the back of the cylinder head. This would show up as coolant leaking down transmission bell housing area. Coolant leaking down radiator could be from radiator or a radiator hose. Inspect the side tanks on the radiator for signs of leaks or fresh coolant. Use a mirror to inspect the bottom of hoses and connections at the radiator.
This photo shows a radiator hose that has a small leak when under pressure. Coolant (green arrow) is sneaking out past the hose at the fitting. The fix here was a new hose and clamp.