Actually, this topic is much wider – it doesn’t correspond to N63 series engines only.
N63 series engine is chosen because on the Internet you can find an infinite amount of entries regarding these engines, that they tend to overheat, it is impossible to aerate them and fill the cooling system.

Several nuances have to be taken into account:

  • these engines have 3 (!) electrical additional pumps of the cooling system. The error messages regarding these pumps most often can not even be seen in the OBD system. It means when using the “universal” diagnostics systems, the diagnostics simply is not possible. Instead, if these pumps are not working, a local overheating of the engine elements (for example, in the turbochargers cooling contour) is possible and the steam will press out all cooling liquid;
  • BMW requires (obligatory) using special equipment, which sucks the cooling liquid in the cooling system, using the vacuum. Unfortunately, most “specialists” try to fill the system without special equipment, and the consequences are sad.

I was always interested, why? Why it is so hard to fill the cooling system of these engines?

In the image:

  • with a red curve: heating of the thermostat;
  • with a blue curve: the power of the main water pump.

Upper curves: N43/N53/N54/N55, etc. with an electrically manageable main water pump.
Bottom curves: N62/N63 with a mechanical main water pump.
In both graphs – the opening temperature of the thermostat is marked.
Note. Both curves (for heating of the thermostat and management of the water pump) are shown in a simplified way. DME works in a “closed-loop” mode, managing thermal management. Each component (thermostat, cooling radiator, water pump, engine) has a separate energetic model, adaptation maps. DME continuously calculates the emitted heat of the engine, required cooling energy and manages the actuators: pump, thermostat, cooling blinds.

As you see, the opening temperature of the thermostat for N43/N53/N54/N55 series engines is “only” 96oC, which corresponds to “high power” mode. Reducing the power of the water pump, the temperature of the engine increases to 106oC (Normal mode) and even 112oC (Eco mode). Instead, when increasing the power of the pump and warming up the thermostat, the temperature is lowered to 80oC (Mapped mode).
N62/N63 (with a mechanical main water pump) the opening temperature of the thermostat is 105oC (!). The main water pump works in a max flowrate mode for all the time, DME is able only to reduce the temperature of the cooling agent (warming up the thermostat).

Here also is the main problem of filling up the cooling system of these engines – as in the large contour (warming radiator) has no bleeding screw, the engine would have to be heated above 105 .. 110oC. By this temperature, the thermostat will open up and the cooling agent will fill the radiator. But, to reach such a temperature in the area of the thermostat, close to the turbochargers (N63) and in several areas of the engine head (also N62) the temperature of the cooling agent under the pressure of the atmosphere will reach/overreach 120 .. 125oC – its boiling temperature. It will form the steam, which will press the cooling agent out via the filling opening.

Several “tuning” companies offer to install the thermostats with an opening temperature of 90oC, to reduce the thermal stress to the engine. Unfortunately, in such mode, the engine will never reach temperature, required for Normal mode. The exhaust system will suffer the most from such reduced temperature (the CO catalytic converters will get damaged very fast). Mostly it will happen due to increased load to the catalytic converters. The load of the exhaust system also will be increased, because of the DME will not be able to perform correct adaptations (measurements) of the injectors – the fuel mixture in cylinders will not be optimal (even, if the average fuel mixture of the bank will be perfect). Additional side effects: increased vibration of the engine, misfires, and shivering of the engine (which can not be avoided with replacing the spark plugs, ignition coils or even injectors).

It goes without saying that trying to replenish (not even talking about refilling) the cooling system with plain water is a completely inadequate maneuver.

One more interesting nuance: ISTA has a “bleeding procedure”, during which the additional, electrically manageable water pumps are activated to ease the refilling of the cooling system, but the thermostat is not warmed up! If the thermostat would be warmed up, the refilling of the cooling system would be much easier!