Common Mercedes OM646 Injector Failure Symptoms
OM646 injectors fail in four field-recognisable patterns. The first is a no-start or extended-crank condition on a fully-warm engine — almost always a worn pintle or seized armature on a single cylinder. The second is heavy black smoke under acceleration combined with a measurable loss of pull above 2,500 rpm — symptomatic of over-fuelling on one or more cylinders. The third is rough idle with a perceptible knock that smooths out above 1,500 rpm, often misdiagnosed as turbo or top-end mechanical play. The fourth is excessive leak-back into the rail return system, surfacing as repeat hard-starts on a cold morning after the rail has bled down overnight.
Live-data diagnosis on Mercedes-spec scan tools (Star Diagnosis / XENTRY) is the fastest route to the failed unit. Read individual cylinder fuel-trim corrections, rail-pressure deviation at idle and during cranking, and run a cylinder-by-cylinder leak-off test before tear-down. A single injector returning more than the workshop manual permits at the rail-return port is the unit to replace.
