For old type engines (for example, M50/52/54), worn-out spark plugs (and also the defect of the ignition coils) at first starts “classically”. Classically means – for cold engine and warmed up engine: in range of max required load. The situation is logical – cold fuel burns poorly. Instead, in the kick-down mode, the fuel mixture lights up poorly because it is rich.
First engines, equipped with Valvetronic (for example, N52), behave similarly to the “old ones”. If (typically due to HVA problems because of not timely changed oil) the individual fuel mixture of cylinders in idle is incorrect (it can be felt by persisting vibration), misfires can also start in idle.
For N43/N53, the largest problem to ignite the fuel is directly in Stratified charge. If the engine uses Stratified charge, the mode of low required torque will be the one in which the shivering of the engine will appear. Of course, if the injector is leaking, if the NOx system is damaged, there can be problems also with the cold engine, etc., but – this already is another story.
With N54/55, the situation is more “interesting”. These engines are fundamentally different, but they are unified with similar (pitiful) exploitation conditions. N54 in Europe (more exactly – in the country where I am living) basically is imported from the USA. Typically – cars, totalized in the USA, are imported, and they are restored with max low costs, to use them on our roads. Unfortunately, I haven’t seen a really well-maintained N54. N55 specific. In our country, the diesel car cult prevails. All cheap cars with diesel engines from Germany, France, Netherlands, which can not pass the TI in the countries anymore due to DPF problems, are brought to us to damage our lungs. Specific of N55 in our country – the owners of these cars have chosen max new car, but they can really afford to maintain it. Those N54 and N55, which I have seen, were unattended and already had misfires in all possible modes. I assume that for N55, some slight misfires could appear in the range of low required torque, similar to for B58. Why such an assumption?
In this mode, the mixing of fuel and air is the worst due to very weak airflow (inlet valves are almost closed, it is ensured by the Valvetronic). In addition, to not allow the knocking, the fuel is injected in a concise moment before ignition – it has no time to mix with air, and the fuel mixture in the Combustion chamber is far from Homogeneous.
Note: all previously mentioned refers to situations when/if all other engine systems are in good working order. Of course, if there are problems with the fuel mixture (not only the total but also the individual of cylinders), VANOS and other additional hubs are incorrectly performing – misfire problems will be (also) in other work modes.
But, now, let’s return to B58 and its misfires.
On idle, such an engine (with worn-out spark plugs) works slightly unevenly. As it has no “full-fledged” misfires, it can be felt that the engine’s performance is uneven. In addition, both Rough run data and their graphs (by ignition cycles) Expert mode indicates similar (but subjectively increased) unevenness of all cylinders.
Expert mode ignition times in idle also indicate increased jittering, but there are no registered misfires!
Also, the results of the exhaust gas test are good. CO = 0.01 (0.30); HC = 5 (200) ppm. Note: in brackets – the allowed limit.
Not looking to as if acceptable test results, the uneven performance of the engine, when partly releasing the accelerator pedal (for example, driving with 90 km/h, reducing the throttle for 1/3 .. 2/3 of required to maintain this speed) can be felt. So – when slightly breaking with the engine, the shivering can be felt. It is possible that if the car has an automatic gearbox, the problem can not be felt so strongly. This unevenness would not be felt if the gearbox will switch to Neutral or will switch off the Torque converter. But, if the car is equipped with a manual gearbox – the defect can be felt very well.
In the entry regarding misfire counter surprises (here), I identified a thorough density exactly in this – a range of very low required torque. I will put the image here also:
Here, on the left side of the image, the throttle opening corresponds to the speed of 40 .. 70 km/h, on the right side (178 .. 188 second): almost closed throttle.
Note: opening of the throttle – the red curve, 3-rd value scale.
And here is the same (minimal, only around 10%) throttle opening after replacing the spark plugs:
Interestingly, a wider scattering of the cylinder performance can be seen directly after starting this mode (73 .. 75 seconds). Exactly scattering, not unevenness (scattering = the curves of cylinders are shifted vertically, one – higher, the second one – lower; unevenness = curves are hacksaw shaped)! And then DME evens out the mechanical performance till practically perfect. The image is taken directly after the repair; it is clear – DME needs time to update the adaptation maps of the injectors (which in these modes are completely incorrect due to past misfires).
In time period 82 .. 92 seconds, when DME has evened out the efficiency of cylinders, cylinders work perfectly even! All shivering symptoms have completely disappeared. So – yes, the spark plugs were the culprits. The economizer indicates 3 l/100km by throttle openings 5 .. 10%, even more (completely) releasing the accelerator pedal; the engine very smoothly switches to Overrun mode.
And here: evenness of the idle before the repair (the unevenness on second 172 can be ignored: at that moment, I switched the gearbox in the Neutral).
And there: idle after repair.
I believe, even without additional zooming of the Y-axis, we can see that in idle, both short-term unevenness of cylinders and the offset of the average mechanical efficiency from ideal are reduced.
Let’s continue the repair event itself.
I purchased the spark plugs from the official dealer. On the bodies of the spark plugs brands – “Champion” and BMW. The price is around EUR 27/piece, and also the box with BMW symbols.
And here, the old spark plugs, taken off the engine:
First cylinder: starting from the top.
As we see, in cylinders No.2, 5, and 6 – BMW NGK spark plugs, in cylinders No.1,3 and 4 – “Champion” spark plugs. Interestingly, the side electrode of the BMW “Champion” spark plugs is different from the “simple” “Champion”! Visually the air gaps of all sparkplugs seem of correct size; here is the visual comparison:
It is interesting; what could be the weak spot of these spark plugs?
I note again: the existing (BMW NGK) spark plugs have served for 85’000 km of 1700 motor hours (the average speed is around 50 km/h). At least the dealer (as far as I remember from the report) did not make any remarks regarding replacing the spark plugs. I don’t think that the problem appeared just now. Accordingly, it becomes clear that in the city mode, if the average speed is, for example, 25km/h, the spark plugs of these engines serve 20 .. 25.000 km, not more! So – replace the spark plugs together with the oil. In the maintenance, this is not the cheapest engine.
P.S. Thinking logically, I have to conclude that the part of the spark plugs (the “simple” “Champion”) are already replaced spark plugs – it explains the story of different suppliers. Unfortunately, it is not possible to find it out.
If the spark plugs have been replaced only in part of cylinders, such a “repair” fails essentially. Not even taking into account that this DME incorrectly registers misfires, and the spark plugs are replaced “by guessing”! But, I’m not ready to blame anyone because the truth will not be found.
The main in this entry: in the “misfire theme”, B58 behaves very untypical. It has to be kept in mind!