Rough run. Trim errors. Hidden cause

N43 and N53 series engines are the most advanced everyday use engines in BMW history. They are No.1 in all aspects: they are the most complicated, but also – the most responsible, with the largest power (100 hp/1 liter) and an excellent torque curve.

These engines can work in 3 completely different fuel injection modes:

• Homogeneous mode (in this mode all “regular” petrol engines are working);
• Homogeneous lean mode;
• Stratified charge fuel injection.

The last two fuel injection modes in the BMW engine family are used only for N43 and N53 series engines. These injection modes allow us to save fuel (15 .. 25%) and provides an excellent responsibility for the engine. Unfortunately, while working in these specific modes, the engines are emitting an increased amount of NOx content in the exhaust. For the engine to correspond to Euro4/Euro5 ecology requirements, they are equipped with a NOx catalytic converter, which reduces the amount of the NOx (emitted in the environment), and with NOx sensor, which continuously measures the amount of NOx in the exhaust and follows up the performance and well-being of the NOx catalytic converter.

If at least one of the NOx system elements is damaged, the engine management disables Homogeneous lean and Stratified charge modes. The engine switches to a “simple” (actually – emergency) – Homogeneous mode, because in it the NOx content in the exhaust is low enough even without using the NOx system. Fuel consumption increases. But, if the Stratified charge mode is turned off, the owner of the car can expect a series of “pleasant” symptoms:

• the idle of the engine becomes uneven – persistent vibration can be felt;
• when driving – vibration can be felt; the feeling, that some of the cylinders are not working properly, is not leaving;
• quite often serious problems with a cold start of the engine appear – the engine shivers, shakes;
• in the DME error message memory the error messages regarding fuel mixture, misfires, Lambda probe trims appear;
• typically – in KOMBI the EML (Check Engine) lights up;
• very often – also the error messages regarding reduced efficiency of the CO catalytic converter are stored;
• the engine doesn’t give full power, it is clumsy and unresponsive;
• if the vehicle is equipped with an automatic gearbox, it is switching very awkward;
• the engine tends to shiver even switching on and off AC; switching on and off any powerful consumer (blower, heating of rear window or seats, etc.).

All these symptoms stubbornly don’t disappear after replacing all possible culprits, for example: ignition coils, spark plugs, injectors.

Cause of all these symptoms – problems with the NOx system!

Why exactly the NOx system causes this series of problems?

A stratified charge is the main fuel injection mode of these complicated engines. Exactly due to this mode, these engines have a list of fundamental innovations:

• this is Direct Injection engine with a Rail pressure 150 .. 200 Bar;
• for fuel injection, this engine uses piezo injectors.

Piezo injectors are very fast – this is a necessity, which allows executing the injection of Stratified charge and very economical performance. Unfortunately, these injectors have an Achilles heel – their week spot is a wide parameter dispersion in idle, low load and middle load modes. For example, in idle their parameters (amount of injected fuel) can differ for 3 .. 4 times!

To solve this problem of parameter difference, DME regularly performs measurements of each injector for each driving condition (creates multi-dimensional adaptations maps for each injector), and, when the data regarding each injector are known, compensates the differences of their parameters. DME is very smart, if, for example, can even detect, if the rich fuel mixture in some of the cylinders is due to leaking of the injector in a closed state, or the injector is slightly late with the closing!

But all most advanced measurements are possible only in Stratified charge mode. If this mode (due to problems of the NOx system) is prohibited – there are no necessary injector measurements, there are no data regarding differences of their parameters, the compensation of the parameter difference is not possible.

If a Stratified charge is prohibited, with each driving sessions the existing situation gets more and more different from an ideal one (the last injector measurements and data, which are given by the manufacturer). With each driving session:

• the fuel mixture disbalance between cylinders increases – in several cylinders the fuel mixture is rich, at the same time in some – lean: due to this, vibrations and misfires appear;
• DME can not predict the behavior of the engine in conditions of variable loads; due to this – vibrations appear when some consumer is switched on or off;
• DME can not create the long-term fuel adaptation maps (LTFT) – for this reason, the problems with a cold start appear, also the shivering of the engine on the run;
• yes, DME sees, that the fuel mixture is not correct, so it records the error messages regarding it, but is not able to fix the situation because there are no data regarding injectors and their behavior;
• due to this incorrect fuel mixture, the load of the CO catalytic converter increases, they are overloaded – so there are the error messages regarding them too;
• due to incorrect fuel mixture, the car has problems to pas the TI: when the exhaust gasses are checked, an increased amount of CO/HC is detected.

Unfortunately, even the replacement of the injectors don’t solve this problem – after a short moment, all previously mentioned symptoms and problems return!

Only restoring of the working capacity of the NOx system and restoring of the Stratified charge fundamentally will prevent the problems, mentioned before.

The NOx system has to be maintained not only for this reason, that it is one of the conditions for driving to be comfortable and economical. If the fuel mixture is not optimal, the following problems appear:

• the workload of the CO catalytic converters significantly increases, they get damaged quickly. Unfortunately, these catalytic converters are critically necessary for the car to pass the TI (for the exhaust gasses to fit in the norm);
• if the car is shivering (misfire problem), the unburned fuel is thrown in the exhaust and explodes there – both CO catalytic converters and Lambda probes are getting damaged.

CO catalytic converters for these cars are expensive, there are no alternative products. If you want to save yourself from the bill of several thousands of EUR or at least “zoom-out” the problems with CO catalytic converters – repair the NOx system immediately!