Coolant looking brown!!
 

Coolant on X5 V8 from 2000 has a brown color!!! We´ve cleaned the system and changed the coolant, fresh new coolant was dropped, after some miles it become again VERY brownish!! What can this be, it never happend before? Engine does not get hot and there are no loss of coolant after many miles made! Thanks in advance.

Could be trans fluid is getting into the radiator and mixing with coolant. I would check the trans to make sure that coolant hasn't gotten into it.

The coolant is not so tick anymore, looks more like water! I´ve asked the new owner which is a friend of mine if he dropped water in the system and he didn´t. He his telling me that maybe the second radiator has a problem and is letting the coolant mixing with the oil!

I will tell him to check the transmission oil, thanks.

I know it's stating the obvious, but did you use distilled water + blue BMW coolant to fill it back up?