I would suggest a 50/50 mix of BMW coolant and distilled water. Drain from the bottom of the radiator and look for a drain or 2 on the side of the engine block, on the 520i I would guess that there would only be one drain, towards the rear of the block on the exhaust side, but thats just a guess.
My standard COOLANT blurb:
If you are going to spend the $ on the BMW coolant (82.14-1 467 704) consider using Distilled water ($1/gallon) as the 50% mix suggested by BMW. I just say that because the label on the BMW coolant touts how it was made with no impurities like nitrates or phosphates, why take the chance at adding contaminates back in with city water? I can guarantee you'll at least be adding chlorine and fluoride into your cooling system with city water. I have no experience with well water, but I’m guessing it would contain minerals that would precipitate out into your cooling system.
BMW does not specify the use of distilled water, they also don't specify any service interval for the power steering fluid, but I change that too. Why? Because both are cheap prevention. It cost me $2 more to use distilled water when I did the coolant service. $2 is not excessive. When paying $14 a bottle for the BMW Coolant that claims to be Nitrate and Phosphate free, if an additional $1 will indeed keep that bottle Nitrate or Phosphate (and who know what else) free in my cooling system, then I think I can spare it. If distilled water was expensive, I wouldn't use it, but it is so inexpensive I couldn't justify not using it.
BMW Coolant Product Description:
“This product is exclusively formulated and produced by BMW to protect your engine all year round.
It protects against freezing, boilover, corrosion and excessive silicate gel preciptation. This ethylene glycol based antifreeze contains no nitrites or phosphates, greatly reducing the possibility of harmful deposit formation.
This coolant is superior to off the shelf brands that contain phosphates. Your cooling system is expensive to replace so use Genuine BMW Coolant. “
To ensure the cooling system is properly vented, it is absolutely essential to follow the steps below:
1. Undo cap but do not remove from expansion tank. This prevent excessive discharge of coolant from the expansion tank.
2. Turn on ignition.
3. Set heating controller to max. temperature.
4. Set blower to low level. This opens the heating valves and sets the auxiliary water pump in operation.
5. Run engine and briefly press accelerator pedal three to four times (approx. 4500 to 5000 rpm) to flush engine cooling circuit.
6. In doing so, do not run engine for longer than approx. 30 seconds; otherwise coolant will heat up and expand.
7. If the coolant level in the expansion tank drops in the process, top up the expansion tank to the max. cold fill level.
8. Screw on cap tightly and allow engine to warm up until thermostat opens.
9. Check coolant level. Pay attention to differing expansion tank designs.
Transparent expansion tank:
Allow engine to cool down before checking coolant level. Coolant temperature must not exceed 30C. If ambient temperature exceeds 30C, allow engine to cool down at least to ambient temperature. Check coolant level and if necessary top up coolant to max. cold fill level (see marking on expansion tank). Note: The tank mark indicates the fluid level at approx. 20C. Use recommended coolant only.
Black expansion tank:
Allow engine to cool down before checking coolant level. Coolant temperature must not exceed 30C. If ambient temperature exceeds 30C, allow engine to cool down at least to ambient temperature. Check coolant level and if necessary top up coolant to max. cold fill level (the ball on the float needle is on the same level as the top edge of the expansion tank). Note: The tank mark indicates the fluid level at approx. 20C. Use recommended coolant only.
Draining and topping up coolant
Unfasten cap on expansion tank.
Unfasten cap on expansion tank.
Release vent screw.
If necessary, remove front underbody protection.
Unscrew drain plug at bottom of radiator.
Unfasten drain plug(s) on engine block.
Top up coolant!
Replace all screws/plugs
M62 Slacken vent plug on expansion tank.
On vehicles with M52 engines, also unfasten additional vent screw.
- Switch on ignition.
- Set heater control to maximum temperature.
- Set blower to low setting. This opens the heating valves and sets the auxiliary water pump in operation.
Perform filling operation slowly.
Fill up expansion tank fully and wait until bubble-free coolant emerges from vent point. Close vent plugs.