Increased fuel consumption during winter

The owner of N53B30U0 approached me. The fuel consumption of his car, when the weather got colder (when the day-time temperature in Europe dropped to +2..5 oC), has increased. First suspicions – for some reason DME has stopped to use Stratified charge. Diagnostics and test drive confirmed – the engine uses a Stratified charge regularly. In this case, there is no problem.

Some time ago, the car has had troubles with the thermostat. Although it was replaced a long time ago, the owner of the car sometimes checks the temperature of the engine, just to be sure. When the weather got colder, the owner of the car noticed fluctuations of the engine temperature in the range of 95..101 oC, and in no way reaches 106 oC typical for Normal mode, not even talking of 112 oC of Eco mode. Troubles with the thermostat again?

Several years ago, I explored how does the thermal management of these engines works, but it was done in the summertime when the outside temperature was +25..30 oC. Then all modes were performing as described in BMW aftersales materials.

Exploring DME live data after longer driving in even conditions with speed 80..100 km/h, the engine has reached following such thermal balance:

The outside cooling fan – as expected: is not used

Warming-up of the thermostat: 0%

The requested power of the water pump – unusually high: 47..52%

Checking the cooling pipe from the thermostat, I detected that it is moderately warm (40..50 oC) – so: the thermostat is slightly open.


How to understand these live data?

The temperature of the coolant is around 90..93 oC – corresponds to the threshold of the opening of the thermostat. The oil temperature was only 92..98 oC (this is low temperature for these engines), obviously, DME commands the water pump to forcibly pump the coolant to not allow the increasing of its temperature (against the oil temperature).

The driving profile corresponded to the Normal mode (for a longer time it was driving with a slight load and turned-on Cruise control to provoke DME to switch to the Eco mode), but the temperature of the engine was lowered for all the time and quite unstable.


What could be the causes of the problem?

a) damaged thermostat – however, it stucks in a half-open position and DME tries to remeasure its curve;

b) damaged oil sensor – it indicates lowered oil temperature.

Both options seemed unlikely to be possible, so I decided to check another car with exactly the same setup.

 30 minutes long test confirmed identical thermal mode of the second car’s engine!

Then I checked thermal modes of around 20 different cars (equipped with N43/N53 engines) during last two months (data, which were sent to me for performing the diagnostics) and detected – only for several cars the engine has reached temperature of Normal mode, and only for one (!) – temperature of Eco mode.

It became clear – there is no problem with the thermostat or the oil sensor.


Where the problem actually hides?

I will start with two slight off topics.

During last two-three decades (since M20/M30 series engines), the power of the engine to one liter of the engine volume has increased for almost two times (till 100 hp/liter), at the same the fuel consumption in idle and in low load conditions – decreased for almost 2 times. So, from one side – the cooling system has to able to cool down a very powerful engine, from the other side – this petrol engine is very close to the diesel engine by efficiency. And as we know, diesel engines have trouble to reach the working temperature in winter.

Other nuance – for the cars, equipped with N43/N53 series engines, cooling blinds were planned as additional equipment. Cooling blinds are placed before the cooling radiator and allow or limit the air supply to the radiator (according to the command from DME). In their aftersales materials, BMW indicates that these cooling blinds reduce fuel consumption, but they don’t explain is how this economy reached. In addition, in most complicate setup, there are two independent cooling blind modules. For one of them: even the control module, manageable via serial interface, is intended! BMW has tried indeed – there is no lack of mechanical parts or electronics.

I believe, now you presume, how the cooling blinds allow saving the fuel. Only in case, if the car with the N43/N53 series engine is equipped with cooling blinds, the engine can reach a correct working temperature (corresponding to the Normal and Eco modes) during winter.

One more interesting nuance – for N43 series engines (in idle and low-load modes they are even more economical than N53) the +10 oC of the coolant temperature is allowed (comparing to the oil temperature). Driving with low load, in such conditions (outside temperature around 0 oC) the coolant temperature of these engines reaches 95..100 oC, but the oil temperature is only 85..90 oC. Obviously, the situation with thermal management of N43 series engines in winter conditions was unsatisfactory and BMW engineers had to compromise – allow slight difference of oil and coolant temperature to be a bit closer to the acceptable engine thermal condition.

These engines are designed for higher temperatures!

Here, one of the reasons, why the fuel consumption of N43/N53 series engines in winter increases even in case if the engine is in perfect working order.

Increased fuel consumption is confirmed also by the air mass. For the N53 series engine, which works in Normal mode (but uses Stratified charge) at idle, the typical air consumption is around 18 kg/h. In the first image we see, that in current conditions the engine consumes around 19.9 kg/h or for around 20% air (and accordingly, also fuel).

Additional info. NewTIS