slow coolant leak
 

2008 E70 3.0si 98K miles...

I find I'm putting about 2 quarts of coolant in after around 1000 miles of driving and receiving the coolant warning. No evidence of leaking under the car after sitting for a while following a long drive. No visible evidence of leaking around the cooling tank or under the radiator. The radiator hose is new, no leakage evidence around it. The oil filter and coolant housing also has recently replaced gaskets and there is no leakage evident there either.

The indy shop that did that work 2 months ago, along with engine valve cover and gasket replacement due to oil leaking, inspected the coolant system and found no issues.

The electric pump was replaced right around the 50K mark. I know this is a known problem with my model and am starting to wonder if this is starting to go bad? I don't recall if this kind of coolant consumption is a sign of a failing pump, or if the pump just dies without any warning.

What other things might I check with respect to the high coolant consumption?

Do a pressure test. Also check if the hoses are hard or soft when fully warmed up. Wear a glove. Check if hoses stay hard after you shut down.

Consider a UOA "used oil analysis" looking for coolant in the oil.

Blackstone-labs.com

Possible head gasket leak?

I Would check on bottom seams of the radiator. Mines is leaking from the right bottom corner. Its also a slow leak. Definitely worth checking out.

I used to have to fill my coolant reservoir almost every month with 100% BMW coolant... Then finally I did a 50/50 mix with BMW coolant and Distilled water and for some reason- I never had to refill it again...over a year now...

Weird...

The radiator hose was an initial problem with some leaking at the connection point of one of the hoses and that helped but was still getting coolant warnings around 5-7K miles so recently I finally got around to getting to the bottom of the problem once and for all.

The culprit was a small leak, very slight, on the small hose attached the main coolant hose (#1)
https://www.realoem.com/bmw/images/diag_4fx5.jpg

This is directly above the serpentine belt and was contaminating it, along with seeping oil from a crankshaft seal leak.

The replacement to #1 on the part diagram no longer includes the small hose returning to the Y adaptor. BMW must have realized that it's location and small connection was a stress point prone to failure.

The replacement hose also requires buying a cap. Here's a pic of the current hose used when replacing the original. Long story short - after replacing this hose, sans the old vent host replaced by the cap, the coolant level has remained constant.