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Greetings Bimmerland... wife's '07 530i with appx 127k miles had the dreaded engine getting too hot (or something like that msg) today. After letting the car rest for about 20 minutes, completed the 40 mile drive home with no issues. Leading up to this event, the radiator symbol along with the low coolant msg was coming on when car was initially started even though the coolant level was fine. For the past few mornings, have experienced the airplane revving rise and fall sound (from the cooling fans I presume) for the first couple of minutes.

The radiator was replaced under CPO warranty about 30k miles ago. Preasure test was recently done on the rediator & showed no leak. The expansion/reservoir tank definately showing showing signs of leak.

Assumptions/questions -
1. Could the reservoir be causing all of these problems.
2. Water pump bad? (The well documented water pump test dosen't seem to work for me I hear nothing, but I do see coolant pumping thru the return while looking in the reservoir fill hole when engine is runnig).
3. Could the low coolant level msg be due to something simple as radiator need bleeding.

First steps/suggestions much welcomed - Just would like to elimate minor fixes before diving strait into water pump replacement, etc...
 

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Maybe a bad thermostat or air in the system. Monitor the coolant temperature using the hidden menu on the OBC. Leaking expansion tank isn't great. If the cooling system can't build up enough pressure due to a leak the boiling point of the coolant drops.
 

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'18 BMW X1, '14 ML350, '07 C280
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Based on what I see in that picture you posted, either the cap is not tightened properly (arrows on the tank and the cap need to be aligned) OR the cap is leaking.
Cap can be replaced separately (it's a separate part number). For peace of mind, you can go ahead and replace both the cap and and tank.

It's possible there is more than one problem - coolant leak at the tank/cap being one problem. There may be other problem with the water pump.

If it was my car, I'd go ahead and replace the tank, cap, and water pump, bleed the system.
 
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