Engine coolant is the working fluid for the cooling system, which controls the operational temperature of the engine. Each combustion event inside your engine creates a lot of heat that the engine needs to be at a certain temperature to operate efficiently. Engine coolant maintains about the same temperature year round, regardless of ambient temperature.
A properly maintained cooling system must have a few things in order: adequate supply of coolant, a radiator that acts as a heat exchanger with the outside air, a fan or air flow source, a water pump to keep the coolant circulating, and a thermostat to regulate the engine at its optimum operating temperature. The coolant must also have the correct mixture and chemical compounds to promote heat transfer, protect against freezing, and also inhibit corrosion. To keep your BMW operating correctly, it's important to check the level, strength, and overall condition of the coolant on a regular basis. You also need to change the coolant before it degrades to the point where it doesn't perform its job adequately.
One failure mode associated with dirty coolant is known as electrolysis. Electrolysis occurs when stray electrical current routes itself through the engine coolant. The electricity is attempting to find the shortest path. Impurities in the coolant often generate a path of least resistance that the electricity travels across. The source of this stray electricity is often from electrical engine accessories, which have not been properly grounded. A missing engine or transmission ground strap can also cause the coolant to become electrified. Sometimes the path of least resistance becomes a radiator, a heater hose, or even the heater core. These components are often well grounded, and offer a ground path from the engine to the chassis by means of the semi-conductive path of the coolant.
Electrolysis can destroy your engine quickly. Although it's semi-normal to have very small amounts of voltage potential in your coolant system, values greater than about a tenth of a volt can start reactions between the coolant and the metal in your engine. In particular, electrolysis affects primarily aluminum engine components, resulting in pitting and scaring of the aluminum surface. This eating away of the metal can cause coolant system leaks, and in particular, radiator leaks around aluminum welds. Cast-iron components are also vulnerable, but typically the aluminum metal parts fail first. On BMWs in particular, electrolysis can be easily seen attacking aluminum cylinder heads. Figure 1 shows a picture of the thermostat area of a cylinder head that has been partially damaged by electrolysis. Notice how the aluminum has been eaten away, and eroded by the chemical/electrical reactions.
The process works somewhat like electrical discharge machines (EDM). These machines work by passing a large electrical current through metal, literally zapping away bits of material until nothing remains. Unfortunately, the electrolysis process works in a similar way, zapping bits of metal in proportion to the amount of electrical current passing through the coolant. A poorly grounded starter can literally destroy a radiator or head within a matter of weeks, depending upon how often the car is started. A smaller current drain, like an electric cooling fan may slowly erode components over many months.
How can you test for electrolysis? Other than actually seeing visible signs of erosion, you can perform a current flow test. Connect the negative terminal of a voltmeter to the chassis ground. Test for adequate continuity by touching another point on the chassis - the resistance should be near to zero. With the engine cold and running, submerge the positive probe into the coolant tank; making sure that the probe does not touch any metal parts. The voltage should be less than .10 volts. If not, methodically turn off or unplug each electrical accessory until the reading reads below .10 volts. Have an assistant switch accessories (like the A/C compressor, heater blower, etc.) while you measure the voltage.
If an accessory doesn't have an on/off switch, test it by temporarily running a ground from the housing of the accessory to the chassis. Ground each component and check the voltmeter. If the wire restores a missing ground connection to the accessory, then you've found a component with a faulty ground.
During this test, be sure to check the starter. Not only will a poorly grounded starter struggle to turn over the engine, it will also zap away tremendous amounts of metal in your cooling system. Watch the meter carefully when starting the engine. Any voltage spike will indicate a faulty ground connection.
Your BMW will lose a little bit of coolant here and there over time due to evaporation from the reservoir. However, a significant loss of coolant over a very short period of time almost certainly signifies a leak in the system. Sometimes a leak can be seen when you park the car overnight. Often the coolant leaks out and then evaporates while you're driving, leaving no tell-tale mark of coolant on the pavement. If you suspect a coolant leak, visually inspect all of the hoses, the water pump, the reservoir, and the radiator for seepage or the 'weeping' of coolant out of seams and gaskets. Check the seal on the radiator cap. Check that the radiator cap is fastened securely - the way the BMW radiator cap is designed makes it easy to make the simple, yet deadly mistake of leaving the cap cocked - allowing coolant to leak out when the engine is running. If you suspect a leak that you cannot see, a pressure test can verify the integrity of your system. See our tech article on cooling system leak testing.
Draining and filling your cooling system is a maintenance requirement as well as being part of many repairs. Be careful when working with coolant. It is poisonous and especially dangerous for pets. Clean all spills immediately and rinse the area with water.
6-cylinder engines used in BMW E60 automatic transmission models hold about 10.6 liters of coolant. Manual transmission models hold 10 liters.
8-cylinder engines used in BMW E60 automatic transmission models hold about 14.2 liters of coolant. Manual transmission models hold 13.8 liters.
In this article, I'll go over the steps involved with replacing the engine coolant on the BMW E60 models. Be sure to work with a cool engine and confirm the cooling system lacks pressure before opening the cooling system.
Lift and support the front of the vehicle safely. See our tech article on lifting your BMW E60.
Remove the front engine splash shields. See our tech article on engine splash shields removing.
|Draining cooling system:|
Draining cooling system: Working in the engine compartment, remove the expansion tank cap. Be sure that the engine has cooled before removing the cap. Cover the cap with a rag when opening. If there is still system pressure, this will lessen the spillage.
Draining cooling system: Place a five-gallon bucket (yellow arrow) under the left side of the radiator. Remove the radiator drain plug and drain the coolant (green arrow). You can use a pair of pliers to grab the petcock (if you are gentle). Allow the coolant to drain into the bucket. Once the coolant has stopped dripping, reinstall the radiator drain plug.
Draining cooling system: Most engines will have a 13mm drain plug (green arrow) on the right side of the block. Remove the drain plug and drain the coolant into the pan.
|Filling and bleeding cooling system:|
Filling and bleeding cooling system: Using a 50% distilled water 50% engine coolant mix, slowly fill the expansion tank until the coolant level indicator reaches MAX. The level indicator diagram is located on the tank (green arrows). Reference this when filling to obtain the correct level. The red float (red arrows) may stick. Be sure it is free before filling. Check the level of the float with the top of the fill hole (inset-red arrow). Trapped air is common when filling a cooling system and can result in improper cooling. It is important to bleed your cooling system each time the cooling system is serviced. Install the expansion tank cap and be sure all bleeder screws are tight. Turn the ignition ON (do not start engine). Set the temperature controls in the vehicle interior to full warm and blower fan speed to Low. Start and run the engine at idle until it reaches operating temperature and check the cooling level, top up as needed. When done, check the cooling system for leaks. On late 6-cylinder models with electric coolant pumps: Install the expansion tank cap. Turn the ignition ON (do not start engine). Set the temperature controls in the vehicle interior to full warm and the blower fan speed to Low. Press the accelerator pedal to the floor and hold it down for ten seconds. Run the electric coolant pump for about 12 minutes to circulate coolant and bleed air from the cooling system. Once the pump has run the cycle, check the level of coolant in the expansion tank and adjust it as needed. Start and run the engine at idle until it reaches operating temperature and check the cooling system for leaks.
Filling and bleeding cooling system: When tightening the reservoir cap, tighten it all the way. Then check that the arrows (green arrows) are aligned. This ensures the cap is on correctly.