Increased fuel consumption. Mystery

In this time – quite a different story. Different, because the problem occurred in my colleague’s car. He is technically a very educated person, uses ISTA with his eyes tied, has an honorable experience in BMW diagnostics, and knows the specific of the N43/N53 series engines very well.
Why do I mention all this? Because this entry is about that, that your senses can mistake you. That there are situations when the routine checks should be done, and all possible causes of the problem should be observed, step by step. In this entry will be no diagnostics images, because problems, seen in them, have been seen for many, many times already. For the entry to be more compact, I will mention the shortcut to the appropriate INPA menu.


Description of the situation.
A colleague bought E90 with an N43 series engine. The maintenance of the engine has been done: new spark plugs (BMW) and ignition coils (Bosch); NOXEM installed. Re-adapting of the engine performed.
Problem: the average fuel consumption (70/30 driving outside the city) is close to 7 liters/100km. Without any doubt – it is increased. And in this case, we don’t talk about additional expenses – obviously, the car has some technical problems!


We started to discuss possible causes.
Does the engine use a Stratified charge? Yes, it does. It can be seen by the unstable fuel consumption, driving outside the city (the econometrics arrow indicates 6 or 7 liters on 100km), after each 50 .. 60 seconds the arrow hops up for 2 liters – the regeneration of the NOx catalytic converter is in progress.
In idle (when the engine warms up) the sound of the exhaust is more “bassy”.
Is there any wheel wedging observed? No, a colleague has paid attention to this obstacle several times, the problem was not identified!
All that remains are the tires with increased rolling resistance, completely wrong wheel alignment. No, the tires are not “wearing”, the car is not “pulled” anywhere. The car as if drives easy.


When the initial ideas were finished, we started the check-up.

1. We performed the driving session. It turned out, that in the most “popular” range of the torque (driving with speed 55 .. 95km/h), the engine runs in Homogeneous lean mode, not in Stratified charge. Injection mode can be easily seen in ../F9/F1. What is the cause of this problem? Misfires! Yes, sometimes they can be felt, but – quite rare. In addition, in the DME error message memory, the error message 3104 is not recorded!

Note: when developing the N43/N53 series engines, BMW engineers noticed the problem: DME identifies false misfires too often when driving on uneven road. During the development of the SW, the sensitivity to misfires was reduced several times. This ”upgrade” finished with the fact that misfires (as in this case) are identified very reluctantly. For granted – it makes it difficult to perform the diagnostics (finding the guilty cylinder).

In the existing situation: DME detects misfires, DOES NOT record any error messages in the error message memory, but at the current torque/RPM segment switches to the less economic Homogeneous lean mode for some time! Yes, the regeneration of the NOx catalytic converter remains, in idle (at least sometimes) the engine really works in Stratified charge, but – driving evenly, the fuel consumption has increased!
For a moment, we managed to reach the performance of the engine in Stratified charge, and it could be seen very clearly, that the fuel consumption drops for around 1 liter/100km. The first problem found!

2. What could be the causes of misfires? As I already mentioned, recently, new spark plugs and ignition coils were installed – everything should be in order. When checking the injector’s data, I noticed that the tests of chemical efficiency of injectors (../F5/Shift+F6/F3)in Homogeneous mode are never performed! Why so? It turned out that previously the error message regarding indications of the LP fuel contour sensor was recorded. Not taking into account that HPFP pressure was correct and stable, DME has stopped any measurements of injectors in Homogeneous mode! Yes, live data of the injectors (../F5/Shift+F6/F1) clearly indicated that no individual corrections are applied to the injectors. What are the consequences of this problem? For cold, partly warm engine, also in all cases, when the engine runs in Homogeneous mode (for example, by required torque above 50% of max), the fuel mixture in cylinders is not evened out. Even if the flowrate data of the injectors fit in a range of +/-5% r, this 5 % of fuel will be wasted – burned in CO catalytic converters. More additional losses will appear due to the uneven performance of the engine – this energy will be suppressed as heat in the double-mass flywheel. Scattering of the injector reaction time is much larger, which means – in idle, the problem is more pronounced. It is possible that the “bassy” sound, mistakenly taken for Stratified charge, actually was badly evened idle in Homogeneous mode. Yes, in Stratified charge mode DME as if measures flowrate of injectors (../F5/Shift+F6/F5), but there is no doubt – applying of normal corrections of injectors is impaired. This could be a possible cause of misfires.
The repair works should be started with replacing of LP pressure sensor, then – re-adapting of the engine and evaluation of the performance. The second problem found!

3. After checking the engine, a colleague drove home. After a moment, the call went out: one more cause was found! It turned out that one brake disc was heated up significantly. Yes, without a doubt – the problem was sporadic and was hiding very well. During several tests, everything was perfect. Here also the 3-rd cause of increased fuel consumption – a wedging brake caliper.


I believe this entry is a good example that sometimes even large experience and intuition do not help. Routine checks have to be done; all possible causes should be checked. Senses tend to lie.