Why do some cars consume excessive oil?

Auto industry sources and Consumer Reports engineers believe that some new lower-friction engine designs meant to improve fuel economy have had the side effect of increasing oil consumption.

One automaker’s instructional engineer—who spoke on condition of anonymity because he didn’t want to comment on competing automakers’ troubles—said there are several factors at play.

The use of super-low-viscosity oils, also known as zero-weight oils, reduces the hydraulic friction of moving liquid oil around in the engine. They also reduce the effort needed to turn the oil pump, contributing to efficiency improvements.

However, the lighter, thinner oil is more able to sneak past engine parts such as piston rings, valve guides, gaskets, and seals that have tiny gaps between the sliding surfaces. Those tolerances are extremely precise, to within a few thousandths of an inch between a piston and its cylinder, leaving little margin for error.

Subaru’s recent engine changes involved improving the assembly process to reduce those tolerances, a Subaru insider said. However, an inverse problem is that too tight a fit between sliding parts can increase friction and shorten the life of an engine significantly.

It’s not always the oil or engine tolerances. Several years ago, BMW discovered its “N63” V8 engine had an oil leak from the engine’s vacuum pump that was causing increased consumption. It also revised the oil sump design with new ventilation hoses and increased capacity.

A different problem is known as “positive crankcase ventilation.” When that happens, oil gets sucked into the engine’s emissions control system, and from there into the cylinders. Both BMW and GM have issued technical service bulletins to address this problem.

For more insights, read our special report on excessive oil consumption.

Share your experiences in the comments below.

—Eric Evarts

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