How to Diagnose the Cooling System in a BMW 325i

Written by kevin mclain
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The cooling system of the BMW 325i involves the radiator, antifreeze, thermostat, cooling fan and the water pump. All of the cooling system components work together to keep the engine block at a safe and cool operating temperature. Once the thermostat opens, the water pump sucks the antifreeze out of the radiator and into the engine block. The antifreeze circulates throughout the engine block to keep it cool. The antifreeze circulates back through the radiator and repeats the process to to provide a constant cooling flow to the engine block at all times.

Skill level:

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Things you need

  • Flashlight
  • Flat head screwdriver

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  1. 1

    Park the BMW 325i on a level surface and open the bonnet. Wait an hour or two for the engine to completely cool down.

  2. 2

    Inspect the drive belt that operates the water pump for any cracks or fraying. Use a flashlight if needed. If the belt has excessive cracking or fraying, replace the belt immediately. Also check to make sure that the water pump belt is properly tensioned. The belt should have no more than a half-inch of slack in it to be properly tensioned. To check the tension of the belt, push inward on the belt with your hand. If the belt pushes in more than a half-inch, tighten the belt until it has around a half-inch of slack.

  3. 3

    Inspect the top and bottom radiator hoses for any leaks, holes or cracks. Also check the band clamps that tighten the radiator hoses with a flat head screwdriver to ensure that they are tight. Replace the radiator hoses if any leaks or holes are present.

  4. 4

    Unscrew and remove the radiator cap from the radiator. Crank the engine and wait for the engine to reach its operating temperature. Once the temperature stops increasing, generally this will mean that the operating temperature is stabilised.

  5. 5

    Inspect around the water pump for any signs of leaking. The water pump has a "weep hole" on the front of it that will leak when the water pump has quit working. Replace the water pump if any leaks are spotted around the sides of the water pump or from the weep hole.

  6. 6

    Inspect the antifreeze inside of the radiator to ensure that the antifreeze is circulating. The antifreeze should show a flowing motion inside of the radiator. This will ensure that the thermostat is open and that the antifreeze is circulating out of the radiator and into the engine block. If the antifreeze is not moving inside of the radiator, this will mean that the thermostat is not open. If the thermostat does not open after the engine has reached its operating temperature, replace the thermostat immediately.

  7. 7

    Inspect the radiator cooling fan to ensure that it is working. The cooling fan should engage around the same time that the engine reaches it operating temperature. The cooling fan keeps the antifreeze and the radiator cool. If the fan is not working, replace it immediately.

  8. 8

    Inspect the front, back and sides of the radiator for any leaks. Make sure that the vents on the front and the back of the radiator are not bent or pushed into the radiator. If a large percentage of the front and back radiator vents are belt or pushed inward, the antifreeze will not be able to flow throughout the radiator properly. Replace the radiator if any leaks, holes or a large percentage of the front and rear radiator vents are damaged.

  9. 9

    Inspect the temperature gauge inside of the BMW 325i to ensure that it is working. If the temperature gauge is not working, replace it immediately.

  10. 10

    Turn the engine off and inspect the radiator, water pump and the top and bottom radiator hoses for any leaks while the engine is off. Close the bonnet.

Tips and warnings

  • If any of the cooling components show any signs of leaking, replace the component immediately to keep the cooling system working properly.
  • Wait for the engine to completely cool off before working around the radiator. Skin contact with a hot radiator or a hot engine block can result in serious burns.
  • Never remove the radiator cap from a hot engine. This can also result in serious burns to the arms, hands and face.

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