Wife came home and parked the car, and I noticed that there was a puddle of coolant on the ground, so I put my key in, and it gave me a coolant low warning.

I added about 20 oz of water to the expansion tank. What might have caused this to happen? It's the 2nd time for me to experience it in about 1000 miles. Tried searching but my search-fu wasn't up to the task.

Car is a 2008 535xi w/75k miles.

 

easiest way to find out would probably be to pressure test the system but you might be able to find a leak if it making on obvious puddle under the car.

check the upper/lower radiator hoses for cracks, water pump seal, etc..

 

Also check the upper radiator vent tube, shown as part no. 23 here: RealOEM.com - Online BMW Parts Catalog

The hose connects the coolant tank with the radiator. It is mounted on top of the radiator underneath the metal plate. If you see alot of dried coolant, the brittle plastic tube broke from the pressurized coolant. This part has been known to fail quite often.

 

You got to find out where it is leaking. Get a flashlight and look around. Expansion tank, radiator, hoses, etc.

 

Inspect the expansion tank very carefully. My 550i did the same thing multiple times but continued to pass the pressure test. My indie finally found a small crack in the bottom of the tank.

 

Harbor Freight has a cheap coolant pressure kit. I used it on my e36 M3 to find a leak.

 

Figured it out.

11537566329 cylinder head-thermostat hose is leaking. How difficult is this to change?

 

looks like my car has the exact same hose issue. how difficult was it to fix? thanks!

 

it was a water pump replacement job.