A long time ago, Jurgen, Karl and Gunter at some Germany university came up with the sweet spot of cylindrical displacement for internal combustion engines. The number they concluded was 0.5-litres per-cylinder.
While we definitely lied about their names, the maths behind their conclusion is proving to be the new gospel for engines with its stroke length longer than its bore diameter as that minimises the engine’s internal surface-to-volume ratio as the piston approaches top dead centre (TDC).
What does all that mean to the layman? Simply put, it creates an improved combustion process.
BMW has invested heavily in this new testament with its current engine family being half-litre per-cylinder based three- (B38), four- (B48) and six-cylinder (B58) configurations that make 1.5-litre, 2.0-litre and 3.0-litre engines respectively.
All these engines share the same 82-mm bore and 94.6-mm stroke dimensions as well as direct fuel injection, variable intake- and exhaust-valve timing, and variable intake-valve lift systems. The modular approach translates into a 60 percent sharing in terms of components across the range of petrol engines.
There’s one caveat though here and that would be the short-lived 1.2-litre variant of this engine used in the Mini One that’s since been discontinued.
Background on the B38 engine
The focus here would be on the baby of this engine family, the 1.5-litre 3-cylinder B38. It replaced the notorious prince N13 4-cylinder engine shared with Peugeot that leaked oil like the Deepwater Horizon disaster.
Traditionally, the 3-banger hasn’t received much love from the motoring world due to predominant perceptions of being noisy with harsh vibrations due to the firing imbalance. While that might have been the case decades ago, the inherent fuel efficiency of these engines made the trade-off a necessary sacrifice and perfect for use in smaller kei cars for example.
However, modern tech has tamed the 3-cylinder to the point that premium carmakers such as BMW are littering the B38 across its range of compact cars.
For bragging rights purposes, the first ever BMW to use the B38 is none other than the BMW i8 hybrid sports coupe, where it was mounted in a transverse mid-mounted layout. Nonetheless, this version of the B38 was significantly modified for a higher output.
Hell, even Toyota dropped a turbocharged 3-cylinder it the GR Yaris and you’ll happily trade your mother in for one of those, wouldn’t you?
The undersquare (smaller bore than stroke) configuration with higher compression ratios and turbocharging will improve fuel efficiency by up to 15 percent over the 4-cylinder engines.
BMW worked on the fact that the 3-cylinder had some commonalities with the 6-cylinder such as neither of them generating free inertial forces, nor free moments of inertia.
Being free of first and second order inertial forces and roll torque was achieved with the use of a single balance shaft while a torsional vibration damper functioning on the principle of a centrifugal pendulum, allowed the engine to remain refined at low RPMs.
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To sum it up, the B38 delivered fuel efficiency, high power, torque, lightweight (aluminium) and was compact.
Both camshafts are equipped with Double-VANOS variable valve timing while the intake camshaft is also equipped with a Valvetronic valve lift system. A single row timing chain rotates both camshafts. Boost pressure is 1.17 bar (17 PSI).
Modularity wasn’t just limited to the number of cylinders but also meant the engine will fit in both longitudinal and transverse layouts to serve front-wheel drive (FWD) and rear-wheel drive (RWD) layouts from both BMW and MINI.
Guises / Power figures and configurations
| BMW B38 configurations and output | ||
|---|---|---|
| Engine Code | Details | Power Figures |
| B38A15U0 | FWD | 102 hp / 190 Nm |
| B38B15U0 | RWD | 109 hp / 180 Nm |
| B38A15M0 | FWD | 136 hp / 220 Nm |
| B38B15M0 | RWD | 136 hp / 220 Nm |
| B38A15M1 | Euro 6d | 140 hp / 220 Nm |
| B38K15T0 | High output | 228 hp / 320 Nm |
Which models does it power?
You’ll find variations of the B38 in a range of BMW and MINI models as well as the Karma Revero. If that last one doesn’t ring any bells, you’ll probably recognise as the Fisker Karma. A new company acquired the rights to Fisker Automotive and rebranded it as Karma Automotive. The hybrid Revero pairs the same high output B38 from the i8 with electric motors.
For the 1.2-litre version of the engine, you’ll find it in early models of the base-level F55/F56 Mini One.
Apart from that, the common 1.5-litre version can be found in varying levels of tunes in the following models sporting a BMW emblem:
- F45/F46 216i Active Tourer / Gran Tourer
- F20/F21 118i
- F22/F23 218i coupe / convertible
- F45/F46 218i Active Tourer / Gran Tourer
- F30/F31 318i LCI
- F48 X1 sDrive18i
- F39 X2 sDrive18i
- F45 225xe Active Tourer (PHEV)
- BMW F40 118i
- F44 218i Gran Coupé
- F39 X2 xDrive25e
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Mini had them powering the following models:
- F55/F56/F57 Cooper
- F54 Clubman
- F60 Countryman
- F55/F56 Mini One
Common problems
No engine is perfect and although BMW ironed out plenty of the 3-cylinder’s shortcomings, there are some common problems you should look out for if you plan to get a model with this engine or already own one.
Some of the common engine problems include:
1. Carbon build-up
The use of direct injection to help with fuel economy has the occasional drawback, and carbon build-up is one of them.
This causes carbon deposits to form on the intake ports and over time, the build-up will lead to the valves not opening and closing properly.
To be fair, most direct injection cars suffer from this and therefore, it’s recommended that you carry out walnut blasting to clear the carbon deposits every 80,000 km or so.
Many independent workshops that cater to continental cars will be able to carry out this procedure and it helps lengthen the engine’s lifespan.
2. Turbo coolant leak
Another common issue with the B38 is coolant leaks from the turbo. There are 2 coolant lines running to the turbo and a rubber O-ring keeps them sealed.
Over time the rubber can harden and the seal is compromises, leading to coolant leaks from the turbo. If you notice that your coolant levels are getting low but can’t find the source, the turbo might be the culprit. You can look for signs of coolant contaminants on the hot and cold side of the turbo.
Replacing the O-rings is an easy job.
3. Water pump & thermostat failure
Again, this isn’t exclusive to the B38 given that most older BMW engines suffer from premature water pump and thermostat failures.
The official service schedule recommends the replacing the water pump every 5 years but the probability of them degrading and starting to leak before that is quite high. If this happens, you’ll lose coolant and the engine will overheat.
If you notice the water temperature tends to spike and there are bluish leaks of coolant on the ground, it’s best to replace the water pump and thermostat. Furthermore, they don’t cost that much as well.
4. VANOS failure
VANOS is BMW’s variable valve timing system and like other systems, it’s regulated by solenoids. If the solenoids fail, it’ll kick up a check engine light or even throw the car into limp mode.
A diagnostics tool will easily pick up the fault code for a quick fix.
5. Crankshaft Guide Bearing Failure
Now this one’s a pretty common issue with the B38 to the point that it resulted in a recall issued by the NHTSA. However, the bright side is that it’s only for models equipped with a manual transmission and by our reckoning, BMW Group Malaysia didn’t officially sell any models with a B38 engines mated to a manual box.
However, it’s worth pointing out, nonetheless. The crankshaft guide bearing failure sees the part experiencing premature wear that causes the clutch pedal to malfunction while shifting.
Symptoms include a check engine light and noise from the transmission bell housing.
The affected models are restricted to the F55 and F56 Mini Cooper 5-door hardtop models produced between December 2014 and June 2015.
Conclusion
The B38 family of engines are modern powerplants with some of the latest tech and therefore, largely foolproof. For the small volume it brings to the table, the power, torque and usability are equivalent to a 4-cylinder but with more fuel efficiency.
It’s arguably one of the more reliable small displacement engines from BMW without any major drawbacks, at least not any major issues exclusive to the B38 family but more of the technology implemented in it.
Overall, it’s a reliable engine with decent fuel economy and respectable performance.
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Source: BMW B38 engine maintenance and common problems – 3’s definitely not a crowd