The Cummins QSB6.7 is one of the most common diesels in recreational and light commercial boats, built around a Bosch high-pressure common-rail (HPCR) fuel system with electronically controlled injectors. It's known for quiet, efficient, reliable operation — but the injectors and high-pressure fuel components are still serviceable wear items, and marine duty (water, long sits, variable fuel) adds stress.

Common symptoms

• Hard starting or extended cranking, especially after sitting.
• Rough idle or a miss at low RPM.
• White or black smoke under load.
• Power loss or inability to reach rated RPM when trying to get on plane.
• Fuel-pressure or injector fault codes on the diagnostic system.

How to diagnose it

1. Check the fuel first — inspect the primary filter/water separator for water or growth, a frequent marine culprit.
2. Scan for fault codes and review cylinder contribution data to flag a specific injector.
3. Verify high-pressure common-rail pressure against spec to separate a pump/supply issue from an injector.
4. Check the lift/supply side and lines for air leaks that cause hard starting.
5. Check oil for fuel dilution from a leaking injector.

Repair notes

QSB6.7 injectors are commonly serviced as a set when several are worn. Because the QSB shares its Bosch common-rail architecture with other 6.7-family Cummins engines, the correct injector or pump is often available — but it must be confirmed by part number, since marine calibrations can differ from on-road versions.

Before you order: QSB6.7 injectors and pumps are application-specific. Verify the exact part by OEM cross-reference for your engine before purchasing.

Browse our Bosch QSB6.7 common-rail injectors and QSB6.7 injection pump, or send us your engine serial/model details and we'll confirm fitment.