Diesel injector failure symptoms and rules of thumb

Diesel injector failures cluster around three symptom sets. (1) Hard start, poor low-RPM performance, blue-white smoke on startup: dribbling injector allowing fuel to leak into the cylinder at rest. Compression test may reveal a wet cylinder. (2) Rough idle, one cylinder cooler than others (measure exhaust runner temps with an IR gun), knocking sound at idle: dead or partially dead injector. Balance test with the ECU (or manual cracking of individual injector lines on mechanical engines) will identify the offender. (3) Black smoke under load, high EGT, loss of top-end power: over-fueling injector, worn nozzle spraying too much fuel too coarsely. In common-rail engines (Volvo D6, Yanmar 4JH80, MAN post-2005) always investigate the fuel filter cleanliness and rail pressure before condemning injectors, contaminated fuel is the #1 killer of high-pressure injector systems. Rule of thumb: replace all injectors as a set, not one at a time, because worn injectors change the fuel curve for the whole engine. Expect 8000-15000 hours in clean marine service; contaminated fuel or biofouling can kill a set in under 2000.