How to bleed cooling system BMW 525i

According to BMW, you should replace your coolant every 2 to 4 years depending on the type. However, when you drain all the coolant from your radiator there is still some left in block. To remove this coolant you will need to identify the drain plug and remove the remaining coolant.

That is not all. Once you have drained the coolant from your car system you will need to refill it while avoiding air from being trapped in the tubes. A car cooling system is quite complicated with its extra hoses, castings, and preformed coolant tubes. The chances of air getting trapped here is very high. To help in this, BMW engines come with bleed screws.

How to drain the coolant

The first thing you need is to locate the drain plug that is found at the base of the radiator. This will allow you to drain the coolant to a drain bowl.

The next step is to locate the bleeding screws. You can find this on the thermostat housing. Open this screw but do not remove it all the way. This will apply for six cylinder engines. For the eight cylinder engines you open the bleed screw next to the filler neck. This is on the expansion tank that is located on the radiator right side.

Now it is time to add your coolant. You do this via the cooling system expansion tank that is located on the right side of the radiator. Do this slowly while being careful not to spill it.

Continue filing up with the coolant until the liquid comes out of the bleed screws. Once done close the bleed screw but do not over tighten it.

For the eight cylinder engines you will need to add the coolant until it comes through the expansion tank. Once done close the screws.

Lastly, start the car engine and turn the heater to maximum heat. The car’s fan should also be set at high. Let this continue until the lower radiator hose starts getting hot and the thermostat opens. Let your engine cool off.

When filling and bleeding the cooling system, you will need to fill it with 50% distilled water and 50% engine coolant. Mix the solution together and fill it into the expansion tank until the coolant level is at maximum.

How does the coolant work?

A car’s engine does get very hot with all the combustion that takes place. For the car’s engine to operate optimally it requires a certain amount of temperature. The coolant is there to cool off the car’s engine and runs through its cooling system. The coolant will remain the same temperature in spite of outside temperature.

To aid the coolant do its work it will work in conjunction with other cooling system components like the radiator, which acts as a medium of exchange for heat that is coming from the engine block. You also have a fan to supply airflow to the radiator, the thermostat to regulate the engine temperature and the water pump to ensure the coolant keeps flowing.

The coolant is made from components that prevent freezing and inhibit corrosion. Since the coolant is so important to the normal functioning of your engine it is imperative that you constantly check its overall condition and you replace it at the recommended time intervals. If you do not check up on it, it will deteriorate to the point it becomes useless for heat dissipation.

Effect of Electrolysis on the Coolant

A dirty coolant will attract a process called electrolysis. In this scenario you have stray electrical currents that route themselves on the engine coolant. The impurities in the coolant create for the electricity a path of least resistance as it tries to find the shortest path. This electric currents often come from the car’s electrical engine accessories that have not being grounded properly. While electrolysis is normal in the engine if it reaches a certain level it can destroy your engine. This is because high levels will institute a reactions between the engine and coolant. Some of the components to first be eaten away will include the any aluminum engine components.

When the problem becomes more serious you will start noticing that you have radiator leaks around aluminum welds. For a BMW car you will notice that the case is more severe for those cars with aluminum cylinder heads.

You can test for electrolysis by performing a voltage test. To do this connect the negative part of your voltmeter to chassis ground and the positive into the coolant tank. You should do this when the engine is cold and running. While at it ensure that your positive probe does not touch any metallic parts. If all is well you should get a voltage reading that is less than 10. Higher amounts than this are dangerous.

It is common to find out that your BMW is losing some little coolant. This is normal but when the amounts keep rising then know that you have a leak somewhere. You can check for leaks whenever you park your BMW for the night. In the morning if you have a significant leak you will notice some colored liquid beneath the bonnet. This is a clear indication of a leak. You can look for the source of the leak by physically inspecting the water pump, hoses, and radiator.

Also, ensure that the radiator cap is well fastened in place as it tends to be forgotten during service. If you have checked the entire coolant system and still cannot find the source of your leaks it is time to perform a pressure check. This will clearly sort you out.


The coolant is an important part of your engine cooling system. However, after sometime the coolant becomes contaminated and is need of replacement. The process of bleeding your cooling system ensures that there is no air left in your cooling system that can hinder your coolant from operating efficiently. When done well your engine will run efficiently.

Jurgen Shmidt works as an automotive engineer, his duties revolve around vehicle design, development manufacture, and testing. Shmidt is also involved in marketing, sales, and the after-sale care of cars, apart from his 8 hours job, he interacts with enthusiasts who have concerns or wish to learn about the various models of the BMW luxury cars. Shmidt believes that the demand for detailed information surpasses its availability and that is why he dedicates most of his free time, researching and compiling reliable information. The information compiled on bmwexpert has been derived from hands-on experience and not some snazzy press release, thus we believe you will find answers to your troubling questions about the BMW.

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