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Cummins/Onan QSK78 Engine

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.

Also known as: Cummins QSK78

Part of the Cummins/Onan QSK78 engine family — 5 variants share parts & fitment.

77.6 L
Displacement
18
Cylinders
1800 rpm
Rated speed
2335 kW
Gross power

Engine specifications

Oil capacity312.0 L
Displacement77.6 L
Cylinders18
Aspiration4-cycle, industrial diesel
Rated speed1800 rpm
Gross power2335 kW

Maintenance schedule

ItemSpecCapacityPart #Interval
Air filterReplace air filter element every 250 hours or as recommended by the engine manufacturer.—250 hr
Fuel filterChange fuel filter every 10,000 hours or as recommended by the engine manufacturer.—10000 hr
Oil filterChange oil filter every 125 hours or as recommended by the engine manufacturer.—125 hr
BatteryBattery maintenance and testing every 12 months or as recommended by the battery manufacturer.—43800 hr
BeltInspect belts for wear and replace if necessary. Replace every 5,000 hours or as recommended by the engine manufacturer.—5000 hr
CoolantChange coolant every 12,000 hours or as recommended by the engine manufacturer.—12000 hr
OilChange oil and filter every 125 hours or as recommended by the engine manufacturer.—125 hr
Spark plugReplace 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.

Service intervals

40
ItemActionHoursMonthsNotes
Air cleaner restriction indicatorCheck8 hrDaily 8 Hours; if indicator shows red, replace elements and reset indicator
Charge alternatorCheck8 hrDaily 8 Hours
Control panel operationCheck8 hrDaily 8 Hours
Coolant level of radiator(s) (water jacket & LTA)Check8 hrDaily 8 Hours
Crankcase breather tube/collectorDrain8 hrDaily 8 Hours; check for condensed oil
Emergency stop button operationTest8 hrDaily 8 Hours; with genset running
Engine coolant heaterCheck8 hrDaily 8 Hours
Engine oil levelCheck8 hrDaily 8 Hours
Exhaust components and hardwareCheck8 hrDaily 8 Hours
Fuel level in tankCheck8 hrDaily 8 Hours
Fuel system primary filter/water separatorDrain8 hrDaily 8 Hours; drain one cup or more
Air intake system (leaks)Inspect50 hrWeekly 50 Hours
All hardware (fittings, clamps, fasteners)Check50 hrWeekly 50 Hours
Coolant lines and radiator hosesCheck50 hrWeekly 50 Hours
Cooling fan bladesInspect50 hrWeekly 50 Hours
Drive belt condition and tensionCheck50 hrWeekly 50 Hours
Exhaust condensation trapDrain50 hrWeekly 50 Hours
Fuel lines and hosesCheck50 hrWeekly 50 Hours; check before starting and again with genset running
Generator set enclosureCheck50 hrWeekly 50 Hours
Air cleaner housingClean250 hr6 mo
Anti-freeze and DCA concentrationCheck250 hr12 mo
Battery electrolyte levelCheck250 hr12 moAlso at least every month or 100 hours of operation, whichever occurs first
Engine groundCheck250 hr6 moClean as necessary
Engine/Alternator flexible rubber couplingInspect250 hr6 mo
Fuel system filtersReplace250 hr6 mo
Lubricating oil and filtersChange250 hr6 mo
Radiator air flowCheck250 hr6 mo
Radiator coreClean250 hr12 moSchedule may be reduced depending on operating conditions/environment
Water separator elementReplace250 hr6 mo
Generator set fuel system filters (Single Fuel Filter)Replace1000 hr12 mo
BearingsLubricate1500 hr12 moRe-grease bearings
Engine mountsCheck1500 hr12 mo
Pre-lube system (if fitted)Check1500 hr12 moCheck for leaks and cracks
Starting motorCheck1500 hr12 mo
Aftercooler coreCheck2000 hr12 mo
Coolant filterChange2000 hr12 mo
Coolant heaterCheck2000 hr12 mo
Fan drive idler arm and fan belt tensionerCheck2000 hr12 mo
TurbochargerCheck2000 hr12 mo
Water pumpCheck2000 hr12 mo

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Service specifications

12
TypeItemValueUnitCondition
OtherBattery CCA (minimum)2200CCAbattery specification
OtherBattery electrolyte specific gravity charge threshold1.215SGcharge battery if reading is below
OtherBattery electrolyte specific gravity, fully charged1.260SGcorrected to 25 °C (77 °F)
OtherWater quality - Calcium, Magnesium (hardness) maximum170ppm (as CaCO3)cooling system make-up water
OtherWater quality - Chloride maximum40ppm (as Cl)cooling system make-up water
OtherWater quality - Sulfate maximum100ppm (as SO4)cooling system make-up water
SpeedDefault cranking/rest start cycle15 seconds cranking, 30 seconds restsecondsdefault 3 start cycles
SpeedIdle speed range to avoid in cold ambient temperatures650 to 1000rpmlow operating temperatures
TemperatureElectrolyte temperature limit during boost charge43°Cboost charge
TemperatureMaximum permissible electrolyte temperature during charge45°Cduring charge
VoltageBoost charge battery volts limit (12 V battery)15Vboost charge; reduce charge rate if reached
VoltageStarting system voltage24VDCbattery type specification

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Recommended fluids

FluidGradeStandardAmbientCapacityNotes
Coolant50/50 mixture of high-quality water (distilled or deionized) and fully formulated antifreeze (ethylene glycol or propylene glycol); Fleetguard® coolant containing DCA4Cummins Engineering Standard 14603If not using Fleetguard Compleat antifreeze or equivalent, add DCA4
Engine oilhigh-quality multi-viscosity lubricating oil (e.g. Cummins Premium Blue® or equivalent)Consult authorized distributor for correct specification for operating conditions; special running-in oils not recommended
FuelASTM No. 2D diesel, minimum Cetane number 40; alternative BS 2869:2010+A1:2011ASTM No. 2DHigher cetane numbers may be needed at high altitude or very low ambient temperature

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Fault codes

42
CodeNameMeaningSeverityAction
111Internal ECM FailureShutdown
1131Battle Short ActiveControl is in Battle Short mode.WarningIf possible, investigate any fault codes being bypassed.
115Eng Crank Sensor ErrorShutdown
1416Fail To ShutdownRecords a bypassed shutdown fault while the genset was in Battle Short mode.WarningTroubleshoot the shutdown fault causing the generator set to shut down.
143Low Oil Rifle PressureEngine oil pressure has dropped below the warning trip point.WarningIf powering critical loads and cannot be shut down, wait until the next shutdown period and follow fault code 415 procedure.
1433Local Emergency StopA local Emergency Stop has been activated.ShutdownDe-activate the Emergency Stop button; press Stop; press Reset; select desired operating mode.
1434Remote Emergency StopA remote Emergency Stop has been activated.ShutdownDe-activate the Emergency Stop button; press Stop; press Reset; select desired operating mode.
1435Low Coolant TemperatureEngine coolant heater not operating or not circulating coolant; warning occurs when coolant temperature is 21 °C (70 °F) or lower.WarningCheck coolant heater power supply, blown fuse or disconnected heater cable; check for coolant leaks; check and replenish coolant.
1438Fail To CrankPossible fault with control, speed sensing or starting system.ShutdownSee code 441 for corrective action; ensure the Emergency Stop button is disengaged.
1439Low Day Tank Fuel SwitchDay tank fuel supply running low.WarningLook for fuel leaks and repair; check and replenish fuel level.
144High Coolant 1 TempEngine has begun to overheat and coolant temperature has risen to an unacceptable level.WarningReview fault code 151 correction list for other possible causes.
1441Low Fuel LevelFuel level has gone below the threshold setting for low fuel level against the time setting.WarningLook for fuel leaks and repair; check and replenish fuel level.
1442Weak BatteryDuring cranking the battery voltage is at or below the weak battery warning trip point.WarningSee code 441.
1443Dead BatteryDuring cranking the battery voltage dropped below the control operating voltage, resetting the control.ShutdownSee code 441.
1445Short CircuitShutdown
1446High AC VoltageShutdown
1447Low AC VoltageShutdown
1448Under FrequencyGenerator set frequency dropped below 90% of nominal for approximately ten seconds.ShutdownCheck the fuel supply; check the air intake supply; check the load and correct any overload.
1449Over FrequencyGenerator set frequency exceeded the Over Frequency threshold for the set time.ShutdownCheck the fuel supply; check the air intake supply; check the load and correct any overload.
145Low Coolant 1 TempEngine coolant temperature below the low coolant temperature warning threshold.WarningIf powering critical loads and cannot be shut down, wait until next shutdown and follow fault code 1435 procedure.
146Pre-High Engine Coolant TemperatureEngine operating near cooling system capacity.WarningReview fault code 151 correction list for other possible causes.
1465Ready To LoadParalleling event indicating the genset is ready to load.Info
151High Coolant TempEngine has overheated (coolant temperature above shutdown trip point or coolant level low).ShutdownLook for coolant leaks and repair; check and replenish coolant; check for cooling airflow obstructions; check fan belt; check blower fan and circulation pumps; reset and restart.
155High Intake Manf 1 TempIntake manifold temperature has risen above the shutdown trip point; large load or high ambient temperature may be the cause.ShutdownReview fault code 151 correction list.
1852Pre-High H2O In FuelWater in the fuel is above normal and has reached the warning trip point.WarningCheck fuel in tank (local or remote); drain and re-fill if necessary.
196Low Coolant 1 LevelEngine coolant level has fallen below the trip point.WarningLook for coolant leaks and repair; check and replenish coolant; reset and restart.
197Low Coolant LevelEngine coolant level has fallen below the trip point.WarningLook for coolant leaks and repair; check and replenish coolant; reset and restart.
2335AC Voltage Sensing Lost (Excitation Fault)Loss of voltage sense - critical shutdown fault that suspends Battle Short mode.Shutdown
234Crankshaft Speed HighOverspeed - critical shutdown fault that suspends Battle Short mode.Shutdown
235Low Coolant LevelLow Coolant Level input active and threshold response set to Shutdown.ShutdownLook for coolant leaks and repair; check and replenish coolant; reset and restart.
2678Charging Alternator FailWarning
2977Low Coolant 2 LevelEngine coolant level in the second radiator has fallen below the trip point.WarningLook for coolant leaks and repair; check and replenish coolant; reset and restart.
3416Start System FaultStart system fault condition exists.WarningRefer to fault codes 359 and 1438.
359Fail To StartPossible fault with control or starting system (engine cranks but fails to start).ShutdownCheck for empty fuel tank, leaks or blocked lines; check/replace dirty fuel filter; check/replace blocked air filter; clean and tighten battery connections; recharge or replace battery; reset and restart.
415Low Oil Rifle PressEngine oil pressure has dropped below the shutdown trip point.ShutdownCheck oil level, lines and filters; replenish oil if low; reset and restart.
427CAN Data Link DegradedCommunication between the engine control (ECM) and the PCC is severed.WarningEnsure the E-Stop button is not active; reset the Local/Remote Emergency Stop.
441Low Battery 1 VoltageBattery voltage below 24 VDC approaching a low level at which unpredictable operation will occur.WarningClean battery terminals and tighten connections; check electrolyte level, top up and recharge; recharge or replace battery.
442High Battery 1 VoltageBattery voltage approaching a high level (exceeding 32 VDC) at which control damage can occur.WarningClean battery terminals and tighten connections; check battery charge float level and lower if applicable.
488High Intake Manf 1 TempIntake manifold temperature has risen above the shutdown trip point.DerateReview fault code 151 correction list.
5135Genset OverloadShutdown
611Engine Hot Shut DownAn engine hot shutdown occurred (cooldown timers bypassed).WarningAvoid hot shutdown; run genset at 10% or less load for three minutes before shutdown.
781CAN Data Link FailureCommunication between the engine control (ECM) and the PowerCommand 3.3 control severed.ShutdownEnsure the E-Stop button is not active; reset the Local/Remote Emergency Stop.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Procedures

12
Air Cleaner Installation installation
  1. Use the two air filter straps to install a new air cleaner on the mounting platform.
  2. Reinstall the hose clamp to secure the air cleaner to the hump hose.
Air Cleaner Removal maintenance

⚠ Holes, loose-end seals, dented sealing surfaces or corrosion render the air cleaner inoperative.

  1. Loosen the hose clamp securing the air cleaner to the hump hose.
  2. Remove the air cleaner and the two air filter straps from the mounting platform.
  3. Remove the air filter straps from the air cleaner.
  4. Dispose of the old air cleaner in accordance with local environmental requirements.
Crankcase Breather Oil Drain Cup maintenance

⚠ Avoid prolonged skin contact with used oil.

Tools: Suitable container

  1. With the generator set not in operation, open the drain valve and drain oil into a suitable container.
  2. Close the drain valve when completed.
  3. Clean the area of any spills and dispose of waste per local regulations.
Engine Oil Level Check maintenance

⚠ Crankcase pressure can blow out hot oil; do not check oil while running.

Tools: Lint free cloth, Funnel (where possible)

  1. Remove the dipstick and wipe it clean with a lint free cloth.
  2. Fully replace the dipstick.
  3. Remove the dipstick and check oil level against the High and Low marks.
  4. Top up to the High mark with the correct grade of oil.
  5. Replace the dipstick and oil filler cap.
  6. Start the engine, idle for five minutes, switch off and wait five minutes.
  7. Check oil level and repeat if oil needs to be added.
Fuel Filter Element Removal maintenance

⚠ Do not allow fuel to drain onto the ground.

⚠ Do not permit any open flame near the fuel system.

Tools: Collar/vent cap wrench, Suitable container

  1. Shut down the engine.
  2. Close the fuel supply shutoff valve.
  3. Place a suitable container under the filter to be replaced.
  4. Use the collar/vent cap wrench to open the vent cap and break the air lock.
  5. Open the drain valve and drain fuel below the collar.
  6. Close the drain valve.
  7. Loosen the collar with the collar/vent cap wrench.
  8. Remove the clear cover, filter spring, fuel filter element, o-ring and sealing grommet.
Fuel Filter Element Replacement maintenance

⚠ Do not permit any open flame near the fuel system.

  1. Install a new o-ring, filter element (with sealing grommet), filter spring and clear cover.
  2. Install the vent cap and collar onto the clear cover.
  3. Hand tighten; do not use tools to tighten the collar.
Fuel/Water Separator Drain maintenance

⚠ Diesel fuel is a fire and explosion hazard.

⚠ Drained liquids must be disposed of per local environmental regulations.

Tools: Suitable container

  1. Shut off the engine.
  2. Place a suitable container under the fuel filter.
  3. With the fuel supply valve closed, open the vent cap to break the airlock.
  4. Turn the valve counterclockwise until it drops about 25 mm (1 in). Drain until clear fuel is visible.
  5. Push the valve up and turn clockwise to close the drain valve.
  6. Open the fuel supply valve before starting the engine.
  7. If more than 60 ml (2 oz) is drained, refill the filter to prevent hard starting.
Immobilizing for Safe Working maintenance

⚠ Accidental or remote starting can cause severe personal injury or death.

Tools: Insulated wrench, Warning notices

  1. Press the Off mode switch on the operator panel to shut down the generator set.
  2. Press the Emergency Stop button to prevent starting from any source.
  3. Thoroughly ventilate the plant room before disconnecting any leads.
  4. Isolate and lock off the supply to the heater, where fitted.
  5. Isolate and lock off the supply to the battery charger, where fitted.
  6. Isolate the fuel supply to the engine.
  7. Using an insulated wrench, disconnect the negative (–) battery cable first.
  8. Fit warning notices to indicate Maintenance in Progress – Plant Immobilized.
Oil Sampling Valve maintenance

⚠ Contact with hot oil can cause severe burns; allow engine to cool before maintenance.

Tools: Oil sampling kit

  1. Remove the cap from the sample bottle and attach the live sampling cap.
  2. Remove the oil sampling drain valve cap from the valve.
  3. With the engine running at normal operating temperature, screw the live sampling cap onto the drain valve.
  4. Fill the sample bottle as directed by the sampling kit/laboratory instructions.
  5. Remove the sample bottle and replace the valve cap.
  6. Remove the live sampling cap from the sample bottle and replace the bottle cap; send for analysis.
  7. Clean up any spillage per local regulations.
Emergency Stop Reset (Code 1433/1434) operation

⚠ Do not use the Emergency Stop button for normal shut-down; it prevents a cooling down run.

  1. Pull, or twist and pull, the button out.
  2. Press the Stop button on the operator panel to acknowledge.
  3. Press the Reset button.
  4. Press the Auto or Manual Run button as previously determined.
Starting at Operator Panel (Manual Run Mode) operation

⚠ Make sure all pre-start checks are carried out before starting.

⚠ Warn all others in the vicinity that the generator set is about to start.

  1. Make sure the main circuit breaker is in the open position.
  2. Press the Manual button, then press the Start button within ten seconds.
  3. Allow the engine to warm up and reach rated speed and voltage.
  4. Close the main circuit breaker and apply load as required.
Stopping at Operator Panel (Manual Mode) operation

⚠ Avoid hot shutdown under load to help prolong reliability.

  1. Remove the load.
  2. Open the main circuit breaker.
  3. Press the stop button on the HMI operator panel to initiate cool down.
  4. Once the cool down timer expires, the generator set shuts off and enters Off mode.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Parts cataloged across the Cummins/Onan QSK78 family

6
Part #DescriptionCategory
0140-3963Filter, Air - Standard DutyAir filter
A034R952Filter, Air - Heavy DutyAir filter
A034R957Element, Primary - Heavy DutyAir filter
A034R963Secondary, Primary - Heavy DutyAir filter
908-0600Cummins QSK78 Series Engine Operation and Maintenance ManualService part / kit
A041X654Element, Replacement - Crankcase BreatherService part / kit

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.