Quote:
Originally Posted by BMW_Diesel_Owner
How do the 2 reservoirs work? Does the vehicle use the active reservoir first (the one located on the passenger side), and then give a warning message when it starts using the passive reservoir (the one located on the driver side, under the air intake)?
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The X5 uses DEF from the active tank and fills it up from the passive as needed to keep active tank filled. IIRC, the warning sign comes when the level in the active tank is low (and the passive tank is empty or the transfer pump is not working).
I plan to keep both tanks filled up (filling both of them up every 5,500 miles or so) and not wait for the tanks to empty. My thinking is that as the levels go down in the tanks, it leaves residual DEF on the tank walls, which then dries and crystallizes and over time falls down on the bottom plugging things up. Don't know if this will work, but I've seen this happen in industrial applications with other chemicals.