I've had a coolant leak for the past few weeks. Been adding distilled water from time to time as the low coolant warning appears. Decided this weekend to find out where the leak was coming from and attempt a fix. Turns out it's leaking from the return hose where the temperature sensor is located. I guess judging by some of the other 'radiator problem' posts, this is a fairly common problem and a good place to start your search when looking for the leak.

I bought a new hose, thermostat and some new coolant. I also took a close up picture of the thermostat. It may not look like much, but this was the source of the leak. See how the new thermostat on the left has an o-ring that's nice and round and sticks out past the edge, and the old one on the right is flat? After years of heat and pressure, it flattened out and coolant was getting past.

All I really needed to do was buy a new o-ring, but they only sell it with the thermostat. I chose to spend a little and buy a new hose too. Total was $30 for thermostat, $50 for the hose and $20 for the coolant. I know I could have saved some by avoiding the dealership for parts, but it's near my house and I wanted to get it done today.

Pretty easy fix... I placed the car on ramps and used a bucket to catch the coolant. Unclip thermostat wire, remove thermostat, unclip both ends of the hose (one at pump and one at radiator), remove hose. Reverse everything to install new hose/thermostat, fill with coolant and distilled water, check for leaks and check coolant level to make sure it's not under/over filled. Took about 25 minutes total.

 

good job!

 

Thanks for sharing. How many miles you have on yours?

 

73k on a 2007 550i. Leak started around 70k or so. Overall a great car!