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Cummins/Onan Onan Engine

The Cummins/Onan Onan is a 0.3 L 1-cylinder air cooled engine rated at 3600 rpm. It powers generator sets from Cummins/Onan, including the 3.6MKY, 4MKY, P4500iDF and P9500df. It holds 1.5 L (0.4 gal) of oil.

Also known as: Cummins Onan

0.304 L

Displacement

Cylinders

3600 rpm

Rated speed

1.5 L

Oil capacity

On this pageSpecs Generator sets Maintenance FAQ Service intervals Torque & specs Fluids Fault codes Procedures Wiring

Engine specifications

Oil capacity	1.5 L
Displacement	0.304 L
Cylinders	1
Aspiration	Air Cooled
Rated speed	3600 rpm

Generator sets powered by this engine

Cummins/Onan 3.6MKY Cummins/Onan 4MKY Cummins/Onan P4500iDF Cummins/Onan P9500df

How to read your Cummins/Onan engine serial & model number →

Common problems

Engine Does Not Crank
Cause: Open control fuse F1
Fix: Check fuse. If open, locate and correct cause of overload. Replace fuse.
Engine Runs Rough
Cause: Dirty air or fuel filter
Fix: Check and replace if necessary.
Engine lacks power
Cause: Air filter restricted
Fix: Clean or replace air filter
Low Battery Voltage
Cause: Weak or discharged battery due to: No battery charging
Fix: 60-Hz models require a battery charger. These models do not have a battery charge winding.
Engine Hunts or Surges
Cause: Fuel supply problem caused by: Faulty fuel pump
Fix: Check fuel pump and replace if defective.
Engine Cranks But Does Not Start
Cause: Restricted fuel supply due to: Fuel level below pickup tube in tank
Fix: Add fuel if tank is low.
No AC Output Voltage — This condition may cause the genset to stop when start switch S1 is released.
Cause: Open circuit breaker
Fix: Locate cause of overload and correct as required. Reset breaker.
Engine Overheats
Cause: Restricted airflow due to dirt, debris or insulation blocking air inlet or outlet
Fix: Clear air inlet and outlet areas. Do not store anything in compartment area.
Remote Control Run Lamp, Time Meter, or Battery Condition Meter Inoperative
Cause: Open circuit in remote control wiring
Fix: Check continuity between remote control and control assembly A1.
Generator Overheats
Cause: Generator overloaded due to defective circuit breaker
Fix: Replace circuit breaker. Do not exceed specified load when operating genset.
The engine cranks but fails to start
Cause: Empty fuel tank
Fix: Check the fuel tank and fill if necessary
High Oil Consumption (Note: New engines can have high oil consumption during break-in)
Cause: Oil viscosity too light or oil is diluted
Fix: Drain and refill with correct viscosity oil.
The engine runs and then stops or surges
Cause: Empty fuel tank
Fix: Check the fuel tank and fill if necessary
Engine Backfires Through Muffler When Running
Cause: Rich fuel mixture due to: Incorrectly adjusted carburetor
Fix: See CARBURETOR.
Engine Backfires Through Carburetor
Cause: Lean fuel mixture due to: Incorrectly adjusted/dirty carburetor
Fix: See CARBURETOR.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan Onan.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Battery	Cranking battery		—	3 hr
Oil	SAE 10W30	1.1 qt / L	—	100 hr

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.

Service intervals

Item	Action	Hours	Months	Notes
Genset	Other	2 hr	1 mo	Exercise at least 2 hours each month at approximately 1/2 rated power; a single 2-hour period is better than several shorter ones
Cylinder Cooling Fins	Clean	8 hr		Daily or every 8 hours; more often in dusty conditions
Engine	Inspect	8 hr		Daily or every 8 hours
Engine oil level	Check	8 hr		Before first start of day and after every 8 hours
Fuel Lines	Inspect	8 hr		Daily or every 8 hours
General inspection	Inspect	8 hr		Before first start of day and after every 8 hours of operation (Table 4)
Oil Level	Check	8 hr		Daily or every 8 hours
engine oil	Check	8 hr		Check oil level before each use or every 8 hours
exhaust system	Inspect	24 hr		Inspect for exhaust leaks at every startup and after every eight hours of running.
Air Filter	Clean	50 hr	6 mo	service more frequently if dry/dusty
engine oil	Change	50 hr	6 mo	First service — Change every month under heavy load or high temperatures
Fuel Filter	Clean	100 hr	6 mo	authorized Cummins service dealer; check/clean
Spark Arrestor	Check	100 hr	6 mo	check/clean
Spark Plug	Check	100 hr	6 mo	check/clean
fuel filter	Replace	100 hr	6 mo	Recommend service by authorized Cummins service dealer
spark plug	Inspect	100 hr	6 mo	Inspect/clean spark plug
Engine oil and oil filter	Change	150 hr		First service — Footnotes 1,2,3,4: more often in dusty/hot conditions; at least once a year
Spark arrestor	Clean	150 hr	6 mo	Spark arrestor muffler cleanout; Table 4 column inferred
Carburetor and combustion chambers	Clean	300 hr		Clean with Onan "4C"; Table 4 column inferred
Engine air filter	Replace	300 hr		Footnotes 2,4 — more often in dusty conditions, at least once a year
Idle Speed	Adjust	300 hr	12 mo	authorized Cummins service dealer; check/adjust
Valve Clearance	Adjust	300 hr	12 mo	authorized Cummins service dealer
air filter	Replace	300 hr	12 mo	After 300 hours or every year
spark plug	Replace	300 hr	12 mo	After 300 hours or every year
Choke vacuum sustain valve	Replace	500 hr		Footnotes 5,6 — sooner if performance deteriorates; qualified mechanic
Cylinder heads and combustion chambers	Clean	500 hr		Remove cylinder heads and clean combustion chambers; footnote 6
Engine cooling fins	Clean	500 hr		Footnotes 2,6
Engine valve clearance	Adjust	500 hr		First service — Footnote 6 — must be performed by an authorized Onan dealer
Fuel filter(s) and fuel pump screen	Replace	500 hr		Footnotes 5,6 — replace filter(s) and clean fuel pump screen
Governor linkage	Inspect	500 hr		Inspect and clean; footnote 6
Spark plugs	Replace	500 hr		Footnote 4 — at least once a year; Table 4 column inferred
Battery	Other		1 mo	Charge battery overnight once a month if generator is not run
Governor speed	Adjust			First service — Section 11 - adjust governor speed for safe no-load speed; warm up at least 15 minutes at 50%-75% of rated load and check choke fully open
Starting Battery	Check		1 mo	Monthly; see battery manufacturer instructions
Starting battery	Clean		1 mo	Clean and check starting battery; footnote 3 — perform more often in hot weather

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

Type	Item	Value	Unit	Condition
Clearance	Brush wear limit (starter)	0.315	inch (8 mm)
Clearance	Camshaft Lobe Height (factory min/max)	1.4035 / 1.4059	inch (35.65 / 35.71 mm)	70 deg F (21 deg C)
Clearance	Connecting Rod Journal O.D. (factory min/max)	1.3177 - 1.3181	inch	70° F (21° C)
Clearance	Connecting Rod Journal O.D. (factory min/max)	1.3177 / 1.3181	inch (33.47 / 33.48 mm)	70 deg F (21 deg C)
Clearance	Connecting Rod Large Bore Clearance (factory min/max)	0.0008 / 0.0022	inch (0.020 / 0.055 mm)	70 deg F (21 deg C)
Clearance	Connecting Rod Side Clearance on Crankshaft (factory min/max)	0.0157 / 0.0433	inch (0.40 / 1.10 mm)	70 deg F (21 deg C)
Clearance	Crankshaft Side Clearance (allowable limit)	0.0098	inch (0.25 mm)	70 deg F (21 deg C)
Clearance	Cylinder Bore I.D. (allowable limit)	3.1138	inch (79.09 mm)	70 deg F (21 deg C)
Clearance	Cylinder Bore I.D. (factory min/max)	3.1102 - 3.1110	inch	70° F (21° C)
Clearance	Cylinder Bore I.D. (factory min/max)	3.1102 / 3.1110	inch (79.00 / 79.02 mm)	70 deg F (21 deg C)
Clearance	Cylinder Head Distortion (allowable limit)	0.0157	inch (0.4 mm)	70 deg F (21 deg C)
Clearance	Exhaust Valve Stem to Guide Clearance (factory min/max)	0.0016 - 0.0030	inch	70° F (21° C)
Clearance	Exhaust Valve Stem to Guide Clearance (factory min/max)	0.0016 / 0.0030	inch (0.04 / 0.075 mm)	70 deg F (21 deg C)
Clearance	Generator brush minimum length	0.375	inches (9.5 mm)
Clearance	Intake Valve Stem O.D. (factory min/max)	0.2740 / 0.2746	inch (6.960 / 6.975 mm)	70 deg F (21 deg C)
Clearance	Intake Valve Stem to Guide Clearance (factory min/max)	0.0010 - 0.0022	inch	70° F (21° C)
Clearance	Intake Valve Stem to Guide Clearance (factory min/max)	0.0010 / 0.0022	inch (0.025 / 0.055 mm)	70 deg F (21 deg C)
Clearance	Piston Pin O.D. (factory min/max)	0.7087 / 0.7089	inch (18.000 / 18.005 mm)	70 deg F (21 deg C)
Clearance	Piston Skirt O.D. (factory min/max)	3.1089 / 3.1094	inch (78.965 / 78.98 mm)	70 deg F (21 deg C)
Clearance	Slip ring out-of-round limit	0.002	inches
Clearance	Tailpipe extension beyond perimeter of vehicle	at least 1 inch (25.4 mm)
Clearance	Valve Springs Free Length (allowable limit)	1.6850	inch (42.8 mm)	70° F (21° C)
Clearance	Valve Springs Free Length (factory min/max)	1.8031 - 1.8228	inch	70° F (21° C)
Clearance	Valve Springs Free Length (factory min/max)	1.8031 / 1.8228	inch (45.8 / 46.3 mm)	70 deg F (21 deg C)
Gap	Ignition Coil Air Gap	0.009 - 0.015	inch (0.23 - 0.38 mm)
Gap	Spark Plug Gap	0.025	inch (0.64 mm)
Gap	Spark Plug Gap	0.035 inches (0.89 mm)	inches (mm)
Gap	Spark Plug Gap (50 Hz)	0.020	inch (0.51 mm)
Gap	Spark Plug Gap (60 Hz)	0.025	inch (0.64 mm)
Gap	Spark plug electrode gap	0.024 - 0.032	in (0.60 - 0.80 mm)
Gap	Spark plug gap	0.025 inches (0.64 mm)
Gap	Spark plug gap	0.027-0.032	in. (0.70-0.80mm)
Injection timing	Ignition Timing (magneto ignition)	25	° BTDC	non-adjustable
Other	Altitude power derate	3.5	% per 1000 feet above sea level	above sea level
Other	Ambient temperature power derate	1	% per 10°F (6°C) above 60°F (16°C)	above 60°F (16°C) ambient
Other	Battery electrolyte specific gravity — recharge below	1.215
Other	Battery electrolyte specific gravity — stop charging at	1.260		at approximately 80° F (27° C)
Other	Compression ratio	7.0 : 1
Other	Cylinder bore	3.250 inches (83 mm)
Other	Cylinder bore	3.563 inches (90 mm)
Other	Displacement	48 inches3 (782 cc)
Other	Displacement	60 inches3 (980 cc)
Other	Engine oil capacity (with filter)	3.5 quarts (3.3 l)
Other	Fuse F1 (control circuit)	5 amperes
Other	Fuse F2 (autochoke/fuel pump; fuel solenoid on LPG)	10 amperes mini-bayonet
Other	Genset power (BGE LPG, 50 Hz)	4800 watts; 120 volts; 40 amperes
Other	Genset power (BGE, 60 Hz)	4000 watts; 120 volts; 33.3 amperes
Other	Genset power (NHE, 50 Hz, 1-phase)	5000 watts; 110/220 or 120/240 volts; 45.5/22.7 or 41.7/20.8 amperes; 1500 RPM
Other	Genset power (NHE, 60 Hz, 1-phase)	6500 watts; 110/220 or 120/240 volts; 54.2 amperes; 1800 RPM
Other	Ignition Timing	23° BTDC	degrees BTDC	Non-adjustable, electronic, magneto ignition
Other	Ignition Timing (magneto)	25 deg BTDC	degrees	non-adjustable
Other	Ignition timing (electronic ignition)	12° BTDC, non-adjustable
Other	Leak down test - excellent	0 - 10	percent leak down
Other	Leak down test - normal	10 - 20	percent leak down
Other	Leak down test - service limit	20 - 30	percent leak down
Other	Maximum oil capacity	0.63	US qt (0.60 L)
Other	Minimum battery cold cranking capacity above freezing	360 amperes		above freezing
Other	Minimum battery cold cranking capacity below freezing	450 amperes		below freezing
Other	Power decrease with altitude	approximately 3.5 percent of rated power each 1000 feet (305 m)
Other	Stroke	2.875 inches (73 mm)

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

Fluid	Grade	Standard	Ambient	Capacity	Notes
Engine oil	10W-30			0.63 US qt (0.60 L)	Recommended type for typical use. Viscosity chart also lists 5W-30, 10W-40 and 5W-30 Synthetic for extreme ambient temperatures.
Engine oil	10W-30 (SAE 10W-30)			0.6 L	Chart: 10W-30, 5W-30 Synthetic, 10W-40, 5W-30 by ambient temperature; 0.63 US qt (0.60 L) capacity; do not use 2-stroke oil
Engine oil	10W30			1.1 L	4-stroke/cycle engine oil only, never 2-stroke; synthetic oil acceptable substitute
Engine oil	5W-30	API Class SG or SH (also SG/CD, SG/CE, SH/CD or SH/CE)	-28°C to 10°C	3.5	-20° F to 50° F
Engine oil	10W-40	API Class SG or SH (also SG/CD, SG/CE, SH/CD or SH/CE)	-18°C to 27°C	3.5	0° F to 80° F
Engine oil	15W-40 (OnaMax)	API Class SG or SH (also SG/CD, SG/CE, SH/CD or SH/CE)	-12°C to 38°C	3.5	10° F to 100° F
Engine oil	SAE 30	API Class SG or SH (also SG/CD, SG/CE, SH/CD or SH/CE)	above 0°C	3.5	32° F (0° C) and higher; single-grade SAE 30 preferable when temperatures consistently above freezing
Fuel	87–93 octane unleaded gasoline			3.4 Gal (12.8 L)	Up to 10% ethanol acceptable; DO NOT use E15 or E85. Also operates on LPG/propane.
Fuel	HD-5 grade liquified petroleum gas or equivalent, at least 90 percent propane				Commercial LPG may contain more than 2.5 percent butane, causing poor vaporization below 32° F (0° C)
Fuel	Unleaded gasoline	minimum octane rating (Anti-Knock Index) of 87			Do not use gasoline or gasoline additives containing methanol.
Fuel	Unleaded gasoline, 87-93 octane, up to 10% ethanol			12.8 L	Do not use E15 or E85; do not use gas/oil mix; fuel stabilizer recommended; tank capacity 3.4 Gal (12.8 L)
Fuel	Unleaded gasoline, max 10% ethanol, 87 octane or higher			6.6 Gallons	tank size
Fuel	Unleaded gasoline, minimum octane rating (Anti-Knock Index) 87				Do not use gasoline or additives containing methanol; leaded gasoline may be used but increases maintenance
Fuel	gasoline				Fuel connection is a 1/4 inch barb fitting for gasoline hose
Other	LPG/propane	Tank must have Overfilling Prevention Device (OPD) valve			Use only approved LPG/propane tanks; liquid withdrawal tanks cannot be used

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

Code	Name	Meaning	Severity	Action
3	Service Check Fault	First-level fault code - indicates fault with second-level fault code	Warning	Check the second-level fault code by pressing STOP once; the second-level code will have two digits
4	Overcrank Fault	First-level fault code - cranking exceeded 20 seconds without engine starting	Shutdown	Check fuel, open fuel valves, prime fuel system, check ignition, service air cleaner, replace fuel filter, fuel shutoff solenoid, fuel pump, governor, choke, leak down test, carburetor
12	Overvoltage Fault	Controller unable to maintain rated voltage	Shutdown	Service brushes and slip rings; replace controller A1
13	Undervoltage Fault	Controller unable to maintain rated voltage	Shutdown	Reduce connected appliances; service brushes/slip rings and test generator field, stator and quadrature windings; replace controller A1
14	Overfrequency Fault	Engine governor unable to maintain rated frequency	Shutdown	Readjust and repair the governor as necessary
15	Underfrequency Fault	Engine governor unable to maintain rated frequency	Shutdown	Service air cleaner; repair blocked exhaust; replace fuel filter; check fuel; fuel pump tests; governor linkage; choke; carburetor/intake gaskets; valve lash; leak down test; carburetor; controller A1
27	Voltage Sense Fault	Controller unable to sense output voltage	Shutdown	Check AC sense circuitry; on 50 Hz and 100V/60 Hz gensets check and repair transformer T2 and voltage trim potentiometer R5; replace controller A1
29	High Battery Voltage Fault	Voltage across battery system greater than 19 volts	Warning	Check battery bank connections so 12 volt batteries are in parallel (12 volt) not in series (24 volt); select a lower battery boost charge rate
32	Low Cranking Speed Fault	Cranking speed less than 180 rpm for more than 2 seconds	Shutdown	Clean/tighten battery cables; recharge or replace battery; replace engine oil with proper viscosity; check starter; service brushes/slip rings and test generator field/quadrature windings; replace controller A1
35	Control Card Failure Fault	Microprocessor EEPROM error during self-test	Shutdown	Replace controller A1
36	Engine Stopped Fault	Engine stopped without command by controller	Shutdown	Check fuel; secure spark plug lead; check mechanical damage; repair blocked exhaust; service air cleaner; replace spark plug; remove cooling air restrictions; replace fuel filter; fuel shutoff solenoid; fuel pump tests; governor linkage; brushes/slip rings and field/quadrature windings; replace controller A1
37	Invalid Genset Configuration Fault	Genset configuration is preprogrammed at the factory	Shutdown	Replace controller A1
38	Field Overload Fault	Low power factor loads	Shutdown	Reduce appliances running at the same time, especially high motor starting loads; have air conditioners and appliances checked
41	Generator Rotor Fault	Controller unable to sense field voltage	Shutdown	Service brushes and slip rings and test generator field windings; replace a rotor with faulty windings; replace controller A1
42	Processor Fault	Microprocessor ROM error during self-test	Shutdown	Replace controller A1
43	Processor Fault	Microprocessor RAM error during self-test	Shutdown	Replace controller A1
45	Speed Sense Fault	Controller unable to sense quadrature frequency	Shutdown	Service brushes and slip rings and test generator field and quadrature windings; replace rotor or stator with faulty windings; replace controller A1
48	Generator Field Sense Fault	Controller unable to sense field voltage	Shutdown	Replace controller A1

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

Procedures

Carburetor Throttle Stop Screw Adjustment adjustment

Tools: Frequency meter

Connect a frequency meter and run the genset until warmed up.
Disconnect all loads.
Pull the governor linkage toward the front so the throttle lever tang bears against the throttle stop screw.
Adjust the stop screw to obtain 54-56 Hz on 60 Hz gensets (44-46 Hz on 50 Hz gensets).

Checking and Adjusting Valve Lash adjustment

⚠ Must be done when engine is cold

Tools: feeler gauge

Remove rocker arm cover and carefully remove gasket, replace if torn or damaged
Remove spark plug so engine can be rotated more easily
Rotate engine to top dead center (TDC) of compression stroke
Confirm both rocker arms are loose at TDC on compression stroke; if not, rotate engine 360 degrees
Insert feeler gauge between rocker arm and push rod to check clearance
If adjustment required, hold adjusting nut and loosen jam nut
Turn adjusting nut to obtain correct valve lash
Hold adjusting nut and tighten jam nut to 106 in-lb (12 N.m)
Recheck valve lash after tightening jam nut
Perform for both intake and exhaust valves
Install rocker arm cover, gasket and spark plug

Choke Adjustment adjustment

⚠ The choke housing becomes hot during operation and can cause severe burns if touched

Allow genset to cool down; housing temperature must equal ambient temperature.
Loosen the adjustment screw holding the adjustment plate.
Slowly rotate adjustment plate until bimetal choke shaft connection is visible in the sight window (correct at 70 deg F [21 deg C]).
For other ambient temperatures, make a mark opposite the center mark; each mark equals 10 deg F (6 deg C) difference.
Rotate plate clockwise if warmer, counterclockwise if cooler than 70 deg F (21 deg C); tighten screw.
Move choke lever back and forth to check for free movement; verify choke does not bind or stick.

Engine Speed Adjustments adjustment

⚠ Adjusting engine speed above equipment manufacturer's specification could cause equipment to operate at speeds in violation of Federal and State Standards for Safety.

⚠ Moving parts can cause severe personal injury or death. Take care when measuring engine speed with tachometer. Must be qualified mechanic.

Tools: Accurate tachometer

Adjust only by qualified mechanic using accurate tachometer.
Set low-idle and high-idle speeds to values in equipment Operator's Manual. If absent, adjust low-idle to 1400 RPM and high-idle to 3400 RPM.
Start engine and move throttle lever to lowest speed position.
Adjust low-idle speed stop screw on carburetor to obtain specified low-idle speed.
For cable controlled engines: move throttle lever to highest speed position. Loosen high-idle speed stop plate clamping screw and rotate stop plate while holding throttle control arm against it until
For lever controlled engines: adjust lever stop screw while holding throttle lever against it until specified high-idle speed is obtained.
Check for full, smooth movement of throttle control arm between stop positions.

Governor Adjustment adjustment

⚠ Voltage/frequency-sensitive equipment may be damaged by power line frequency variations

Tools: Digital frequency/voltmeter (0.3% freq accuracy, 0.5% voltage accuracy), Digital ammeter, Variable load bank with 4 kW capacity

Run genset at least 10 minutes at 3/4-rated load; check choke completely open.
Check governor linkage for binding or excessive looseness; check motion spring.
At no-load, turn speed adjustment screw to obtain 62-63 Hz (60 Hz) or 51.5-52.5 Hz (50 Hz).
Check frequency and voltage with load and no load against Table 8-1 limits.
Adjust governor sensitivity for closest regulation without hunting.
Recheck speed setting and readjust if necessary.
Set carburetor throttle stop screw as specified in FUEL SYSTEM.

Governor Adjustments adjustment

⚠ Disconnect voltage/frequency-sensitive equipment before adjustments.

Tools: Digital frequency/voltmeter, Digital ammeter, Variable load bank 4 kW

Run the genset at least 10 minutes at 50% to 75% of rated load; check choke is fully open.
Check governor linkage for binding or looseness; check motion spring for damage.
At no-load turn the speed adjustment screw to obtain 62 Hz for a 60 Hz genset (output 123-132 VAC).
For a 50 Hz genset adjust to 51.5-52.5 Hz (220-231 VAC if rated 220 VAC).
Check frequency and voltage with load then no load; verify within Table 8-1 limits.
Adjust governor sensitivity to give closest regulation without hunting; turn CCW to increase.
Recheck the speed setting and readjust if necessary.
Set the carburetor throttle stop screw as specified in Fuel System.

Reconnecting and Reclamping the Choke and Throttle Cables adjustment

⚠ Accidental starting can cause severe injury or death. Always disconnect spark plug and battery before resetting engine control cables.

Stop engine and disconnect spark plug and battery (negative cable).
Push choke knob in to full open-choke position and throttle lever to lowest speed position.
Loosen cable clamp at engine bracket if necessary and hook cable wires into hook holes in choke and throttle control arms.
Rotate choke and throttle control arms clockwise, hold against stops, pull slack out of cables and tighten cable clamp.
Check for smooth movement and full travel of choke knob and throttle lever. Remove air cleaner element to see choke plate opens and closes fully. Check throttle control arm touches stop plate and thro
Reassemble air cleaner and reconnect spark plug and battery.

Valve Clearance adjustment

⚠ Checking and adjusting valve clearance must be done when the engine is cold.

Tools: feeler gauge

Remove the rocker arm cover and carefully remove the gasket.
Remove the spark plug so the engine can be rotated more easily.
Rotate the engine to top dead center (TDC) by pulling the recoil handle slowly.
Both rocker arms should be loose at TDC on the compression stroke.
Insert a feeler gauge between the rocker arm and valve stem to measure clearance.
If adjustment is necessary, hold the rocker arm pivot and loosen the pivot adjusting nut.
Turn the rocker arm pivot to obtain the specified clearance.
Hold the pivot and re-tighten the pivot adjusting nut to the specified torque.
Perform this procedure for the other valve.
Install the gasket, rocker arm cover, and spark plug.

Valve Clearance Adjustment adjustment

⚠ Adjust the valve clearance only when the engine is at ambient temperature.

Tools: Feeler gauge, Wrench

Remove the head cover and inspect valve stems for alignment with the rocker arms.
Advance the engine until both valves are closed (piston at top dead center).
Check clearance with a feeler gauge between the top of the valve stem and rocker arm.
Loosen the outer locking nut and turn the adjusting nut to obtain the correct clearance.
Tighten the locking nut after adjustment.
Recheck the clearance and check the rocker arm bolts have not loosened.
Reinstall the head cover and torque the bolts to specification.

Valve Lash (Clearance) Adjustment adjustment

⚠ Adjust valve clearance only when engine is at ambient temperature

Tools: Feeler gauge, Wrench set

Remove head cover; inspect valve stems for alignment with rocker arms.
Advance engine until both valves are closed and no pressure on lifters (piston at TDC).
Check clearance with feeler gauge between valve stem top and rocker arm.
Loosen outer locking nut and turn adjusting nut as needed to obtain correct clearance.
Tighten locking nut after adjustment.
Recheck valve clearance and check rocker arm bolts have not loosened.
Reinstall head cover and torque bolts.

Field Voltage Test diagnostic

⚠ Attach and remove meter leads only when genset is not operating.

Tools: DC voltmeter, AC voltmeter

Test A: Connect DC voltmeter positive to J4 pin 7 and negative to J4 pin 3.
Push start switch; approximately 12 VDC indicates field flash voltage is available; proceed to Test B.
Test B: Move positive lead to pin 9 and negative to pin 10; start the genset and allow speed to stabilize.
Measure field voltage no load then full load; 40 VDC no load to 100 VDC full load is normal.
Test C: Disconnect J4 from P4, attach AC voltmeter to L1 and L2, and apply 12 VDC to P4 pins 9 (+) and 10 (-).
Start the genset; approximately 40 VAC indicates brushes, rotor and stator are OK.

Fuel Pump Test diagnostic

⚠ Do not substitute an automotive electric fuel pump for the Onan-supplied fuel pump

Tools: Pressure gauge

Make sure fuel tank has sufficient fuel.
Check genset starting battery voltage; pump will not work properly below 6 VDC.
Remove fuel line from carburetor inlet and install a pressure gauge.
Press the stop switch and hold for several seconds until pressure stabilizes.
Reading should be 2.5-4.00 psi (17-27 kPa); pressure should hold constant or drop off slowly.

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

Voltage reconnection

Voltage	Phase	Connection	Leads	Notes
120V	Single phase	Floating neutral (neutral wire isolated from grounding fastener and AC receptacle ground pins)		Grounding System (AC): Neutral Floating, per schematic

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

Common questions

What is the oil capacity of the Cummins/Onan Onan?

The Cummins/Onan Onan engine has an oil capacity of 1.5 liters.

What is the displacement of the Cummins/Onan Onan?

The Cummins/Onan Onan has a displacement of 0.304 liters with 1 cylinders.

Which generators use the Cummins/Onan Onan engine?

The Cummins/Onan Onan powers 4 generator set(s), including Cummins/Onan 3.6MKY, Cummins/Onan 4MKY, Cummins/Onan P4500iDF, Cummins/Onan P9500df.

How often should you change the oil in the Cummins/Onan Onan?

The Cummins/Onan Onan oil change interval is every 100 hours.

What is a common problem with the Cummins/Onan Onan engine?

A commonly reported issue on generators using the Cummins/Onan Onan is: Engine Does Not Crank. Cause: Open control fuse F1. Fix: Check fuse. If open, locate and correct cause of overload. Replace fuse..