BMW M70 / M73 / N73

BMW's naturally aspirated V12 — the 5.0 M70, 5.4 M73 and 6.0 N73 — in the 7 Series and 8 Series.

⚑ The M70 and M73 are two straight-sixes on a common crankcase, and that is not a metaphor. Each bank has its own engine management, its own throttle body, its own set of sensors and its own idle. A fault on one bank leaves the other running normally, so the car can be running on six cylinders and feel merely rough rather than broken.

So diagnosis means treating it as two engines and comparing them. Fuel trims, live data and even the ignition system are bank-specific, and a reading taken from one bank says nothing about the other.

The N73 is a modern single-ECU engine with Valvetronic, and almost nothing about the older pair transfers to it.

Age and complexity are the practical problem rather than any one failure: twice as many coils, twice as many sensors, and a wiring harness whose insulation on the earliest cars degrades. The N74 twin-turbo that replaced them has its own page here.