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Draining and Filling Cooling System

Pelican Technical Article:

Draining and Filling Cooling System

Nick Czerula


1 hours1 hrs






Flathead and Phillips screwdrivers, 13mm socket

Applicable Models:

BMW X3 Sport Utility (2004-06)

Parts Required:

Engine coolant

Hot Tip:

Work with a cool engine.

Performance Gain:

Cooling system parts lifespan increased.

Complementary Modification:

Replace hoses

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 and 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, and 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 slowing 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 volt meter. 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 from can verify the integrity of your system. See our tech article on cooling system leak testing.

Remember your car may have been serviced before and parts 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. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve as they grow older. 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.

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 area with water.

M54 equipped 6-cylinder engines used in BMW E83 X3 automatic transmission models hold about 10.6 liters of coolant.

Lift and support front of vehicle safely. See our tech article on lifting your BMW X3.

Remove front engine splash shields.

Working in engine compartment, remove expansion tank cap (red arrow).
Figure 1

Working in engine compartment, remove expansion tank cap (red arrow). Be sure engine has cooled before removing cap. Cover cap with rag when opening, if there is still system pressure, this will lessen the spillage.

Place a 5 gallon bucket under left side of radiator.
Figure 2

Place a 5 gallon bucket under left side of radiator. Remove radiator drain plug (blue arrow), coolant reservoir drain plug (red arrow) and drain coolant. Use a large flathead screwdriver to remove drain plugs.

Allow coolant to drain into bucket (red arrow).
Figure 3

Allow coolant to drain into bucket (red arrow).

Once coolant has stopped dripping, reinstall drain plug (red arrows).
Figure 4

Once coolant has stopped dripping, reinstall drain plug (red arrows).

M54 engines will have a 13mm drain plug (green arrow) on the right side of the block.
Figure 5

M54 engines will have a 13mm drain plug (red arrow) on the right side of the block. Remove the drain plug and drain coolant into pan.

Filling and bleeding cooling system: Open the reservoir vent (red arrow).
Figure 6

Filling and bleeding cooling system: Open the reservoir vent (red arrow). Using a 50% distilled water 50% engine coolant mix, slowly fill expansion tank until coolant level indicator reaches MAX. The level indicator diagram is located on tank (blue arrows), reference this when filling to obtain the correct level.

Filling and bleeding cooling system: The red float may stick, be sure it is free before filling.
Figure 7

Filling and bleeding cooling system: The red float may stick, be sure it is free before filling. Check level of the float with the top of the fill hole. Blue arrow points to MAX level. Red arrow points to MIN level. 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 expansion tank cap and be sure all bleeder screws are tight. Turn ignition ON (do not start engine), set temperature controls in vehicle interior to full warm and blower fan speed to Low. Start and run engine at idle until it reaches operating temperature and check cooling level, top up as needed. When done, check cooling system for leaks.

Comments and Suggestions:
ben Comments: hi, is it necessary to remove the under panel of the car
January 11, 2018
  Followup from the Pelican Staff: Yes. - Nick at Pelican Parts  
A Comments: Hello, the last step says to bleed the system but doesn't really say how to do that...suggestions?
August 29, 2017
  Followup from the Pelican Staff: As noted, fill system until float is at right level. Close bleeders, install cap and run vehicle. Then recheck level. - Nick at Pelican Parts  
X3 Comments: Hello, step five says green arrow, but there is no green arrow in figure five. Is it the red one?
June 24, 2017
  Followup from the Pelican Staff: yes, thanks for catching that. Will fix the error. - Nick at Pelican Parts  
X3 Comments: Hello, step five says there is a green arrow pointing to the drain plug, but figure 5 shows only red and blue arrows. Which one is it?
June 24, 2017
  Followup from the Pelican Staff: yes, thanks for catching that. Will fix the error. - Nick at Pelican Parts  
Jonny G Comments: Hello, What's the coolant capacity for a 2009 X3 E83 with N52 engine? I would like to do the coolant service, but concern that I can't get to a 50/50 ratio without knowing the coolant capacity. Thanks!
May 30, 2017
  Followup from the Pelican Staff: Should be about the same as the M54. about 10.6 liters of coolant - Nick at Pelican Parts  

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Page last updated: Thu 3/22/2018 03:21:12 AM