Nissan HR12

Nissan's three-cylinder petrol engines — the 1.2 HR12, including the supercharged DIG-S version with a Miller-cycle intake, and the later 1.0 turbocharged HR10DDT of the current Micra and Juke.

The DIG-S is the interesting one and the one most often misdiagnosed. It uses a supercharger rather than a turbocharger, runs an unusually high compression ratio with late intake valve closing, and its behaviour under load reads very differently from a turbo engine's — a performance complaint diagnosed with turbo expectations will look for the wrong things. Carbon on the intake valves is heavy because of the direct injection and the Miller cycle together.

Both engines sit in small cars often fitted with a CVT, which supplies the usual caution: hesitation and flaring described as engine symptoms are frequently transmission behaviour, and separating an engine-speed symptom from a road-speed one comes before any component test.