Fiat MultiAir

Fiat's 1.4-litre turbocharged four with MultiAir — an electro-hydraulic system that controls the intake valves directly rather than through a fixed camshaft profile, letting the engine vary lift and timing continuously and dispense with a conventional throttle in some conditions. It is genuinely distinctive technology, fitted across Fiat, Alfa Romeo, Lancia and Jeep.

The MultiAir actuator unit itself is the characteristic failure. It sits on top of the head, is oil-pressure driven, and is acutely sensitive to oil condition and level — degraded or low oil produces rough running, misfires and limp mode, with codes that name the system rather than the cause. On any MultiAir complaint, oil condition is the first diagnostic input, and the unit is expensive enough that this is worth establishing before ordering one.

The turbocharged versions also bring the usual intake-valve carbon and, in the Abarth applications, a duty cycle that finds any weakness in the cooling system quickly.