Though six-cylinder, inline engines are not as popular today as they were just 25 years ago, their sound and layout still translates well into high performance, durability and smoothness. The six-cylinder M50 series was originally used in 5 series cars in the early 1990s, and is now the basis for the 3 and 5 series cars in displacements from 2.5L to 3.2L. All of these engines share basic design characteristics, with different forms of intake, exhaust and valve control determining specific power and use parameters.

This series of engines is smooth running, powerful and still very frugal with fuel. Originally designed with dual, chain-driven, overhead camshafts, this design has evolved into a world-class expression of technological progress. The engine now has variable camshaft timing on both camshafts and can easily approach 2 hp per cubic inch.

Unfortunately, BMW has found it necessary to move to extended service intervals, involving condition-based oil changes and excessively separated maintenance inspections. Oil sludging, overheating problems and emissions faults are the direct result of the lack of reasonable maintenance. We've found an unusually large amount of cooling system failures on these cars just after the initial no-service, free-service period ends.

The BMW M50 engine series is a very durable powerplant. The frequency of actual failure is very low considering the number of these cars that we servi...