In the internet forums, a frequent question is: who is the cheapest BMW engine in maintenance and exploitation? Or, at least, which engines should be avoided?

Of course, if you buy a used car, no one will be able to tell how much you will need to invest in the vehicle (and in the engine), but certain tendencies can be predicted.
It is clear that, for example, 8-cylinder or S series engines wouldn’t be the cheapest ones, so in this entry, I will, in short, summarize the most popular N series 4 and 6-cylinder engines.

4-cylinder engines
N40. Relatively exotic engine. It was manufactured in 1.6 liters version N40B16, with limited sales in some countries. This engine has no Valvetronic – poorly equipped, cheap in all comprehensions. The only problem could be to get some used spare parts.
N45. Uncommon low-power engine without Valvetronic. The situation is similar to N40.
N42, N46. These engines are already equipped with Valvetronic. Contrary to the widespread myths regarding wearing-out eccentric shafts and other Valvetronic hubs, I can not confirm such legends. In my evaluation – the reason for permanent vibration in idle (with the reduced mechanical efficiency of the 4th or 3rd and 4th cylinders) – is problems with the inlet HVA. These problems occur due to rare oil change intervals.
All previously mentioned are the cheapest possible BMW 4-cylinder engines during the last 10+ years.
N43. Direct injection engine with Stratified charge. Much more expensive if any repairs are needed. Piezo injectors cost 500+ EUR/piece; they tend to start to leak in cold temperatures. HPFP costs around EUR 350, but its defects are pretty rare. NOx system – in the OEM performance serves around 100 k km (catalytic converter) or about 10 .. 20 k km (sensor); its repair is expensive (5000+ EUR). The catalytic converter – is around EUR 4000, and the sensor – is around EUR 500. Specific of the engine: regular driving with a speed above 110 k/km is required. Very fuel-saving; powerful; responsive; nuanced engine, but – undoubtedly much more expensive repair than previously mentioned engines. A complicated engine and only a tiny number of diagnostics specialists can handle it.
N20, N26. Turbocharged engines. Good news: these engines don’t have Piezo injectors anymore. Bad news – also, the new, robust injectors turned out problematic. Follow the factory recalls (injectors, wastegate management, DME, if I remember correctly, HPFP also was replaced). Of course, nothing is eternal, but these engines are durable.
B48. This is the last BMW internal combustion engine (and B38 in a 3-cylinder version). As in the B58 6-cylinder version – already known technical units are polished. Nothing innovative/interesting, but at the same time – current usage experience shows that these are long-lasting engines without marked weak spots. Of course, the turbo aggregate is not eternal, and the maintenance also doesn’t cost cheap (OEM BMW spark plugs cost EUR 25/piece).

6-cylinder engines
M54. Present in the first years of the E60 body, with MS45 management, I included it in the list. The good, old “workhorse”. In terms of maintenance – the cheapest engine. Accordingly, this engine has no Valvetronic – even clicking compensators won’t cause problems. Unbeatable engine.
N52 is equipped with a Valvetronic, but this engine will not cause any problems if the oil is changed promptly. The cheapest of all possible options in the 5th series of the latest years and in other BMW limos.
N53. Direct injection engine with Stratified charge. The most expensive in the maintenance of all 6-cylinder engines. Piezo injectors; HPFP (which, unfortunately, can let down) costs EUR 700, NOx system (with a catalytic converter, which costs EUR 6000), its lifespan – identical to previously described N43. The injectors tend to leak; the CO catalytic converters are damaged if they are not replaced promptly. The same destiny awaits them if the NOx system is not repaired for a long time. Costs of the CO catalytic converters – around EUR 4500, no alternatives. The engine requires regular driving with a speed above 110 km/h. Excellent engine – fuel-saving; responsive; nuanced; powerful, with a momentous reaction to the accelerator pedal. Suppose it is in technical order. Unfortunately, this engine is expensive and complicated, and only several specialists can perform the correct repair jobs on it.
N54 is a modified version of N53 with a turbocharger. Accordingly, these engines don’t have a Stratified charge – no problems with the NOx system. Although BMW has tried to reduce the leaking of the injectors in idle, the injector still gives problems; these engines have the same expensive Piezo injectors as the N53 series. In addition, N54 has two turbo aggregates (which wear out), and it has no Valvetronic – these engines are powerful but not economical. Those, who are trying to save money on fuel, have to remember it. Also, HPFP is identical to N53; these pumps are expensive and are not eternal.
N55. Thus is already modernized turbo 6-cylinder power aggregate with one turbine, cheaper and more robust injectors, and HPFP, similar to the one used in N43, with a longer life span. This engine has Valvetronic, so the remark regarding oil changes and HVA is valid. Otherwise – durable and long-lasting engine.
B58. The actual engine replaces N55. Experience to date shows that this engine is long-living and without weak spots. Real workhorse. Yes, maybe someone will be surprised that, for example, OEM spark plugs for these engines cost EUR 25/piece, but – this is BMW. BMW is not the cheapest car among its competitors.
P.S.: I mention that B38/B48/B58 is nothing innovative – it is not so bad, actually. These engines work with compression 11:1; even in the mode of the Stoichiometric fuel mixture, these engines use ”Layer” fuel mixture creation principle (it means the fuel mixture is not evenly mixing with the air as in “old” engines but is ignited directly in the injection moment, to not to get detonated). The essence of this paragraph – these engines require high-quality spark plugs; they have to be replaced relatively often. The injectors should be in perfect order, and requirements regarding their performance are much higher than in the “old” (N42/N46/N52) engines. Oils should be of high quality and replaced regularly; you will not use water in the cooling contour.