The Cummins/Onan QSK78 is a 77.6 L 18-cylinder 4-cycle, industrial diesel engine rated at 1800 rpm, producing up to 2335 kW. It powers generator sets from Cummins/Onan, including the C2750 D5, C3000 D5, DQLE and DQLF. It holds 312 L (82.4 gal) of oil.
Maintenance schedule
| Item | Spec | Capacity | Part # | Interval | |
|---|
| Air filter | Replace air filter element every 250 hours or as recommended by the engine manufacturer. | | — | 250 hr | |
| Fuel filter | Change fuel filter every 10,000 hours or as recommended by the engine manufacturer. | | — | 10000 hr | |
| Oil filter | Change oil filter every 125 hours or as recommended by the engine manufacturer. | | — | 125 hr | |
| Battery | Battery maintenance and testing every 12 months or as recommended by the battery manufacturer. | | — | 43800 hr | |
| Belt | Inspect belts for wear and replace if necessary. Replace every 5,000 hours or as recommended by the engine manufacturer. | | — | 5000 hr | |
| Coolant | Change coolant every 12,000 hours or as recommended by the engine manufacturer. | | — | 12000 hr | |
| Oil | Change oil and filter every 125 hours or as recommended by the engine manufacturer. | | — | 125 hr | |
| Spark plug | Replace spark plugs every 1,250 hours or as recommended by the engine manufacturer. | | — | 1250 hr | |
Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.
Parts cataloged across the Cummins/Onan QSK78 family
6| Part # | Description | Category |
|---|
| 0140-3963 | Filter, Air - Standard Duty | air_filter |
| A034R952 | Filter, Air - Heavy Duty | air_filter |
| A034R957 | Element, Primary - Heavy Duty | air_filter |
| A034R963 | Secondary, Primary - Heavy Duty | air_filter |
| 908-0600 | Cummins QSK78 Series Engine Operation and Maintenance Manual | other_service |
| A041X654 | Element, Replacement - Crankcase Breather | other_service |
Parts cataloged across all variants of this engine family — verify the exact part fits your specific variant and serial number before ordering.
Common questions
What is the oil capacity of the Cummins/Onan QSK78?
The Cummins/Onan QSK78 engine has an oil capacity of 312.0 liters.
What is the displacement of the Cummins/Onan QSK78?
The Cummins/Onan QSK78 has a displacement of 77.6 liters with 18 cylinders.
Which generators use the Cummins/Onan QSK78 engine?
The Cummins/Onan QSK78 powers 5 generator set(s), including Cummins/Onan C2750 D5, Cummins/Onan C3000 D5, Cummins/Onan DQLE, Cummins/Onan DQLF, Cummins/Onan QSK78 series.
How often should you change the oil in the Cummins/Onan QSK78?
The Cummins/Onan QSK78 oil change interval is every 125 hours.