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Mercedes W204 - The coolant level rises when the cap is removed

diabeel 9096 7
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Treść została przetłumaczona polish » english Zobacz oryginalną wersję tematu
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  • #1 17047929
    diabeel
    Level 17  
    I noticed such a strange phenomenon ... they added some coolant at the ASO, because about 100ml was missing.
    After a long ride, I lifted the hood and with the traffic jam closed, it seemed to me that there was not enough coolant.
    I unscrewed the cap, heard pssst and the fluid level rose to normal.
    Is it normal for the cap to hear a short hiss and the fluid level to return to normal?
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  • #2 17047986
    DriverMSG
    Admin of Computers group  
    diabeel wrote:
    Is it normal for the cap to hear a short hiss and the fluid level to return to normal?
    On a warm or cold engine? While the engine is running or not?
    This effect may occur in a system with air pollution.
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  • #3 17047997
    diabeel
    Level 17  
    Engine stopped - warm or cold. Sometimes there is only a hiss, and sometimes after unscrewing the fluid level rises to the correct level.
    The car normally heats up to 90C and maintains this temperature
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  • #4 17048391
    mario 77
    Level 23  
    Check the fluid level on a cold engine, e.g. after a night.
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  • #5 17048429
    diabeel
    Level 17  
    In the morning it is the same, i.e. if the car warms up, then I put it away and wait until the morning, then when I unscrew the filler cap there is a slight hiss and the amount of fluid increases to the correct level.
    I would like to add that the engine is at the correct temperature, the hoses are not hard and the heating in the cabin is lux, there is no smell from the expansion tank.
  • #6 17048474
    DriverMSG
    Admin of Computers group  
    diabeel wrote:
    and an increase in the amount of fluid to the correct level
    How much are we talking about? 1 centimeter, 3, 10 - level in the reservoir?
  • #7 17048738
    diabeel
    Level 17  
    About 1-2cm maximum
  • #8 17048783
    DriverMSG
    Admin of Computers group  
    This is not cause for panic. Rather normal symptom. The pressure in the system increases with the temperature of the fluid (liquid expansion), the hoses swell, etc. and the air pressure collects in the reservoir itself - in other words, by opening the cap you release compressed air, so the fluid level in the reservoir changes.

    For peace of mind, you can replace the reservoir cap itself. It seems there should be a valve in it, and I don't know how to check if it works properly.
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Topic summary

The discussion revolves around a phenomenon observed in a Mercedes W204 where the coolant level appears to rise upon removing the cap after a drive. Users inquire whether this is normal, noting that the engine temperature remains stable at 90°C and there are no signs of issues such as hard hoses or odors from the expansion tank. Responses indicate that this behavior can be attributed to normal pressure changes in the cooling system due to thermal expansion of the coolant and air pressure accumulation. It is suggested that replacing the reservoir cap, which contains a valve, may provide additional reassurance regarding system integrity.
Summary generated by the language model.
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