So, I found the source of the coolant leak on my wife's 525xi. It is leaking close to the lower hose. It looks like there is a leak right where the sensor plugs in. Does anyone have a BMW part number for the sensor? Or, perhaps, is this something I can get at NAPA/Advance?

 

I have the same problem on my 525i, I've got under the car and had a look, it seems to be leaking from the bottom right hand side of the radiator (as you look at it from the front) There is a cover around the radiator that seems to be all one peice, so looks difficult to remove, therefor I can't pinpoint the leak, but it's in this general area. Is it in the same area as the sensor?

 

My leak is from the passenger side. If you look under the hood, you can see where the lower rad. hose connects to the lower radiator neck. On the neck there is a spot for the sensor (vertical from the neck). That is where ours it leaking. I can see the coolant coming out. However, there seems to be too much coolant on the ground to be coming just from that. I wonder if there is a secondary leak somewhere. I am going to pull the lower engine cover tomorrow to have a look.

I did stop at Advance Auto and they can get the sensor for $18.39 and a two day wait.

 

Pull the sensor out this morning. I could tell that is where the leak was coming from. As I wiggled it, more would come out the sides. Of course though, I couldn't find a parts store on a sunday that carried it. It is special order through the big box parts stores. I'll have to wait until Monday to pick up one from the dealership. I'm going to just change the lower hose while the coolant is out of it and I have it up on the lift.

 

Go to pepboys or your local auto supply its like $15 max. I think I paid $9 at Kragen, basically its the o-ring thats bad.

 

i agree, the O ring is what wears off. replace the O-ring and also you can apply silicon right above the o-ring which will seal it tight and prevent further leaks.

 

does anyone know what size the o-ring is so I can buy a new one without removing the old one first?

 

i want to know the O-ring size too. i know where to buy a new O-ring , but first thing is the size. the outer diameter and the thickness, enough.

 

i believe the o-ring is green. i had the same issue and i replaced the sensor which came with the o-ring. but before i was able to replace it, i used teflon thread tape around the old o-ring for a week or so. it worked surprisingly well, not a drop on the belly pan.

 

I have the same issue in the same place! I found it the other day while cleaning the bottom of engine. It's so small that car doesn't get low on coolant though.
I'm sure I can replace the o-ring if I know the size or just order a whole new one at pepboys.