[TLK]

I checked my manual and it will also confuse anybody who hasn't had BMW experience. It mentions only one tank, locates the turbo tank, but shows a close up of the engine radiator tank.

The engine coolant filler on my USA driver's left side is the same cap BMW has been using on some models since the '90's. Note, it isn't the same cap still used on current models that have a big high mounted expansion tank you can see. These are pressure release caps, they are supposed to allow blow off at a certain pressure for safety. On the F10 M5, you can't see the tank and all you get is a long filler neck, kind of like a wash/wipe reservoir filler.

The other little filler tank up close to the engine with the knurled cap is for the turbo cooling circuit.

Note: although the main coolant circuit for the engine/AC/Steering/etc has a level sensor, the turbo circuit does NOT! Yes, if you get a leak or worse a hole in the turbo circuit radiator or lines, you can bleed out all the coolant in no time flat and there will be no warning. Guess how I know this.

Both tanks take the same 50/50 diluted mixture of BMW coolant.

On USA cars, the engine radiator filler neck has no min/max level (the first car, let alone BMW, I've seen like this). This is some trick BMW NA has to get all us "rich" people to keep coming back to the dealer for stupid things. In any case, a tape measure set to 6" makes a perfect gauge to stick down the neck. If it just touches the coolant in the bottom of the neck, it's perfect.

Fortunately the turbo circuit tank has a min/max level gauge inside and it's a snap to check and fill.

STAY VIGILANT! Check these levels every couple of weeks at a minimum, especially the morning before you go out for a planned hard run. Cooling is a major Achilles' Heel of this and it's 550i brother. Remember, the turbo/intercooler coolant circuit has NO low warning! You need to fix any leaks as soon as they start.

Pictured below is the main coolant circuit cap for the engine, heater core, etc.