1965–1985 Onan MDJE Generator Sets Service Manual (P/N 974-0750)

• Onan P/N 974-0750, December 1985: A 92-page PDF for MDJA one-cylinder, MDJB and MDJE two-cylinder, and MDJC and MDJF four-cylinder marine diesel GenSet engines.
• Marine cooling and exhaust repair: Covers direct sea-water, heat-exchanger, and keel-cooler systems; water-pump overhaul; thermostat testing; exhaust back-pressure checks; and above- or below-waterline wet-exhaust layouts.
• Fuel-system diagnosis and injection timing: Includes sediment-bowl and transfer-pump service, filter bleeding, injector-nozzle testing, and timing procedures for American Bosch PLB, PSU/Model 50, Bryce, and Diesel Kiki equipment.
• Internal engine repair and overhaul: Details compression testing, cylinder-head and valve work, piston and ring inspection, connecting rods, camshaft bearings, crankshaft and main-bearing service, timing-gear alignment, and post-rebuild checks.
• Starting, lubrication, and governor service: Provides starting-circuit fault tests, Prestolite and Mitsubishi starter overhaul, oil-pump and pressure-control checks, breather and shutdown-switch service, and governor adjustments for units prior to and beginning with Spec R.
• Factory reference depth: Nine numbered system tables, technical figures running through Figure 118, assembly torque specifications, wear limits, clearances, capacities, electrical test data, timing charts, and Onan special-tool numbers.

Match the Engine by Model and Spec

The manual is written for engine mechanics and is best suited to technicians comfortable with precision measuring tools, electrical meters, pullers, bearing drivers, and diesel-injection test fixtures. Model selection is based on the complete Model and Spec number plus the engine serial number, not on year alone. The model-code section also explains R remote-start and E plant-only electric-start types.

Production changes are kept visible instead of being folded into one generic procedure. The repair sections distinguish pre-Spec P and later components, pre-Spec R and Spec R governor arrangements, early and late heat-exchanger layouts, and water-pump applications by model, Spec, and production date. Those breaks affect piston and tappet data, valve springs, pump hardware, crankcase breathers, and timing methods.

Marine Systems and Fuel-Injection Work

The cooling chapter separates direct sea-water cooling from captive-water systems using a heat exchanger or keel cooler. It covers draining, reverse flushing, filling and venting, water-flow testing, zinc-pencil inspection, and thermostat and high-temperature cutout tests. The various metal- and neoprene-impeller pumps receive removal, disassembly, seal replacement, reassembly, and installation procedures. Front- and side-mount heat exchangers, skin coolers, expansion tanks, and dry-stack or water-injected exhaust combinations are identified.

Wet-exhaust coverage includes the water-injection elbow, Onan Aqualift muffler, riser arrangements, siphon breaks, marine muffler support, and back-pressure testing. The manual warns that a blocked siphon-break vent or a scoop-type water inlet can force water back through the exhaust and into the engine cylinders - a marine-specific failure mode that can cause major damage.

Fuel-system repair goes well beyond filter replacement. The manual covers the sediment bowl, dual filters, diaphragm transfer pump, fuel-solenoid checks, injector removal and installation, nozzle cleaning, spray-pattern inspection, and leakage, chatter, and atomization tests. Injection-pump sections explain port-closing timing by depth measurement, flow timing, shim selection, and coded timing buttons for the American Bosch PLB and PSU/Model 50 systems. A separate Bryce/Diesel Kiki MDJE section handles its adapter, timing, and nozzle tests. The pump mounting shim stack must remain matched to the engine because changing it alters gear backlash; if the Bryce/Kiki control-rack ball misses the yoke fingers, engine speed can become uncontrollable.

Diagnostics, Overhaul, and Scope

The one-page Diesel Engine Troubleshooting Guide cross-references symptoms against starting, fuel, lubrication, governor, cooling, and internal-engine causes. Supporting procedures cover fuel bleeding, cold starting, maintenance scheduling, out-of-service protection, oil-pump and bypass-valve inspection, restricted rocker-box oil lines, low-oil and centrifugal switches, the preheat circuit, and the optional belt-driven charging alternator.

Major overhaul content starts with valve-clearance setting, compression testing, cylinder-head removal, valve and seat refinishing, guide replacement, valve-rotator inspection, and the MDJA decompression release. It continues through flywheel and ring-gear work, governor-cup service, cylinder reboring, pistons and connecting rods, camshaft bearings, crankshaft grinding and shot-peening, main bearings, and break-in after repair. Particularly useful visuals include the Diesel Engine Troubleshooting Guide, Direct Sea Water Cooling, Keel Cooler Installation, Injection Pump Cutaway View, Starting System, and Head Bolt Torque Sequence.

Onan special tools are identified by part number, including the diesel nozzle tester 420-0184, pintle-nozzle cleaning set 420-0208, combination main/cam bearing driver 420-0326, and flywheel puller 420-0100. This is the engine service manual, not a replacement parts catalog: detailed exploded assemblies and individual replacement-part numbers are assigned to separate parts catalogs. Internal injection-pump repair and calibration likewise require qualified diesel-injection personnel and suitable test facilities.

Disconnect the starting battery before service, and follow the manual's precautions for high-pressure fuel spray, hot pressurized coolant, rotating machinery, and carbon monoxide exposure.