I've had a coolant drip since I bought the car ('06 530i N52) I've had to fill the overflow tank about once a week but finally got it solved yesterday. It was dripping from the water hose that goes into the radiator and Y's off to two other connections to the engine block/thermostat.



Once you get under the car and remove the cover (phillips head screws hold the cover in place) and it is easier to see some of the connections than from the top. This hose is not on the Radiator parts diagram or in the radiator section at all on RealOEM.com I later found it on the Engine Cooling diagram under the Engine section.

The connection pictured above is part of this hose (part number 11537522999 about $50):


BMW sells the entire hose, you can't just buy a replacement connector or O-rings for the connections. One connection is quick disconnect, the other two are hose clamps (re-used hose clamps from original hose they were in good shape)

I also replaced the hose the Engine Coolant Temperature sensor sits on and the sensor itself since the O-ring on it was leaking. The hose was $45 (part #17127521778) and the sensor was $28 (part #13621433077). To remove the hose, take some needle nose plier or a pick and pull the metal wire tab on the top of the connection. Once you get it, it will stay "pulled up" and you can wiggle the hose off.



The ECT sensor in the picture is the old one, note the rubber O-ring is almost flush with the sensor, it should be sticking out (look fatter). Sorry I don't have a New/Old side by side, I removed the old one and put the new one in immediately as I covered the hole with my finger to keep as little coolant leaking out as possible. The O-ring seems to be a non-standard size, I have a box of about 40 different replacement O-rings I got at Pep Boys but none are the right size. It is about 9mm x 4mm but it is the old one that has shrunk some. Luckily the sensor is cheap and can be replaced from the top of the car without removing any hose, there are two prongs on the side to push to pull it out.

I bought the parts from the local BMW dealership. He originally quoted list prices and I simply asked if he could do any better and he gave me everything at wholesale, so it pays to ask! The wholesale prices were within a dollar or so of the prices on RealOEM, so I was happy to get a good price, they were in stock, and not have to pay shipping.

She is not leaking a drop now. Hope this helps!

 

Thanks for the pictures! It really helps to know which parts are prone to leaks so we know where to look.

 

I had the same problem like you and after inspection I changed the same hose that you changed but in the end it was the radiator.

 

Just found out I have a leak coming from the coolant temperature sensor(pic#4). My first trip tomorrow would be at autozone to see if I can find an o-ring for the sensor, If this don't work, paln B is to buy the OEM sensor. Thanks for the info