In this entry, as if it were a non-standard situation, the cause/reason of which is actually quite common.
The owner of the car with an N53 series engine approached me. The performance of the engine is uneven and shivering. The engine sometimes uses Homogeneous lean but never – Stratified charge. In the DME error message memory, the error message 3104 is recorded regularly.
I checked the live data. Everything looks correct. Cold start – without problems.

Several years ago, new injectors were installed.
At the same time, new spark plugs and injectors have been installed. Since this significant repair/maintenance, 40’000 km are driven.

The reason for the error message 3104 should be clarified. Misfires are the consequences! Misfire counters indicated problems in all cylinders.

I have been reminded several times that according to my (and stories, know to me) experience:
a. the average resource of BMW (manufacturer NGK) spark plugs is around 25 .. 30,000 km (on condition that the most part of mileage is driven outside cities);
b. NGK spark plugs are good to go for 10 .. 15,000 km, not more.
The owner of the vehicle has NGK spark plugs installed because – ” why overpay”?

Let’s see these spark plugs!

What do these spark plugs tell us?
a. all spark plugs “behave” correctly; strong leakage of injectors is not confirmed;
b. sometimes DME uses a lean fuel mixture (works in at least Homogeneous lean mode);
c. the electrodes of the spark plugs are significantly burned. The air gap from the initial 1.2 mm has increased to close to 2.00 mm;
d. the insulator of the spark plugs is significantly burned – it is a grave sign of worn-out.

These spark plugs are catastrophically worn out! Their correct performance finished a long, long time ago! Again, we return to mileage around 10 .. 15,000 km, which I already mentioned before.
After installing new spark plugs (of course, the owner again chose not to overpay = the cheapest NGK spark plugs were installed), it turned out that several cylinders continued to misfire. What happened?
If the spark plugs are worn out and their air gap is increased, the load on ignition coils significantly increases. The voltage in the secondary winding of the ignition coil significantly increases. Increased voltage damages the ignition coils very swiftly.
The second reason that damages the spark plugs swiftly is misfires. If a misfire (due to a worn-out spark plug) happens, the voltage in the secondary winding of the ignition coil increases even more dramatically. The larger the voltage overload, the faster the current leakage will appear somewhere (not intended place) in the ignition coil.
The current leakage damages the insulation material of the ignition coil (under the influence of high energy and temperature), and in place of this leakage, the “bridge” is created – all future energy will be conducted precisely here, not via electrodes of the spark plug.
When trying to save on spark plugs, unfortunately, the ignition coils will have to be replaced now, too. These are around EUR 200 extra expenses.

Are these the only extra expenses? Unfortunately, no.

If DME has not used Stratified charge during the last 20,000 km, additional fuel costs (assuming that the fuel consumption increase was only 10%) were at least EUR 150.
The most important – misfires damage the CO catalytic converters. The price of these catalytic converters is around EUR 4000 (offered by BMW – alternatives, unfortunately, are not available). Even if the resource of the catalytic converters is unnecessarily shortened by 10%, their end will come one year faster.
Here is an example of how to “save” around EUR 70 on spark plug replacement, for a year, you drive a car that shivers; indirect expenses, at least EUR 350, are lost, and the remaining lifespan of the expensive CO catalytic converter is reduced. In my opinion, such an “economy” is not a smart choice.