Mitsubishi 6D31T Specifications
| Engine Model: Mitsubishi 6D31T |
| Engine Type: 4-cycle inline 6-cylinder turbocharged liquid-cooled diesel |
| Total Displacement: 384 cu.in (6.29 L) |
| Rated Engine Power: 145-160 Hp (108-119 kW) at 2800 rpm |
| Fuel System Type: Direct fuel injection |
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The Mitsubishi 6D31T engine is a mid-sized,
turbocharged inline-six cylinder diesel powerplant designed for
medium-duty commercial vehicles and industrial equipment. As part of
Mitsubishi's long-running 6D series, this engine was engineered to
provide a balance of power, durability, and fuel economy for a diverse
range of applications. It features a direct injection fuel system, a
robust cast iron cylinder block and head, and an overhead valve (OHV)
valvetrain with a gear-driven camshaft.
The turbocharger enhances power density and operational efficiency,
particularly under load and at altitude. The 6D31T is characterized by
its mechanical simplicity, proven reliability, and accessible
serviceability. It served as a key power unit for Mitsubishi Fuso trucks
and buses, as well as for construction machinery, establishing itself as
a dependable workhorse in markets where low lifecycle cost and longevity
are paramount.
Engine family: Mitsubishi 6D series.
Engine type: Inline-six cylinder, four-stroke diesel cycle,
turbocharged.
Displacement: 6292 cc (384.0 cubic inches).
Bore: 105.0 mm (4.134 inches).
Stroke: 110.5 mm (4.35 inches).
Compression ratio: 17.0:1.
Maximum power output: Ranges from 145 to 160 Hp (108 to 119 kW) at
2800-3200 rpm, depending on specific model and application.
Maximum torque output: 280-310 lb-ft (380-420 Nm) in the 1800-2200 rpm
range.
Firing order: 1-5-3-6-2-4.
Engine orientation: Longitudinal for vehicle applications; variable for
industrial use.
Aspiration: Turbocharged.
Fuel type: Diesel fuel.
Recommended oil grade: SAE 15W-40, meeting API CD or higher
specifications.
Oil capacity: 16-18 liters (16.9-19.0 U.S. quarts), including filter.
Coolant capacity: 18-20 liters (19.0-21.1 U.S. quarts).
The cylinder block is a rigid, deep-skirt casting from grey cast iron.
This construction provides the necessary strength to withstand
combustion pressures and the torsional loads inherent in an inline-six
configuration. The block incorporates six cylinder bores, cast
integrally with generous water jackets. The crankshaft is supported by
seven main bearings. This seven-bearing design ensures excellent
crankshaft support, minimizing deflection and vibration for smooth
operation and extended bearing life.
The crankshaft is a forged steel component with large-diameter journals
and integral counterweights. Main bearing caps are cast iron and secured
with two vertical studs and nuts each. Connecting rods are forged from
steel, featuring an I-beam design for strength under compression.
Pistons are aluminum alloy, with a toroidal combustion bowl in the crown
designed for optimal air-fuel mixing with the direct injection spray.
Each piston is fitted with three rings.
The cylinder head is a cast iron unit. It employs an overhead valve
(OHV) layout, with a single camshaft located in the cylinder block. The
camshaft is driven by helical gears directly from the crankshaft,
ensuring precise and durable valve timing. Valve actuation follows a
direct mechanical path: camshaft lobe, cylindrical tappet, pushrod,
rocker arm, and valve. The engine uses two valves per cylinder (twelve
valves total).
Valve clearance is adjusted manually via a screw and locknut on each
rocker arm. This adjustment is a required maintenance procedure to
ensure optimal valve timing, engine performance, and to prevent valve
train damage from excessive lash. Valve seats are induction-hardened,
and valve guides are replaceable. The cylinder head is secured with
numerous high-tensile studs to maintain an effective seal.
The fuel system is a direct injection setup utilizing an in-line,
jerk-type fuel injection pump. This pump, often a Bosch or Zexel (Diesel
Kiki) model, is gear-driven from the engine's camshaft timing gear
train. It contains individual plungers and barrels for each cylinder,
providing precise metering and timing control. High-pressure steel lines
connect the pump outlets to each cylinder's injector.
The injectors are multi-hole, pencil-type units threaded directly into
the cylinder head, spraying fuel into the piston bowl at high pressure.
A mechanical centrifugal governor is integrated into the injection pump
to control maximum engine speed and provide stable idle. A transfer pump
delivers fuel from the tank through a primary filter/water separator and
a secondary fine filter to the injection pump. A hand priming pump is
incorporated for system bleeding.
The lubrication system is a pressure-fed, full-flow filtration wet-sump
design. A gear-type oil pump is driven by the crankshaft or from the
timing gear train. Oil is drawn from the oil pan through a pickup tube
and strainer screen. The oil is pressurized and forced through a
full-flow, replaceable cartridge-type oil filter. On some variants, an
additional bypass centrifugal filter provides supplementary oil
cleaning.
From the filter head, oil feeds the main gallery in the cylinder block.
It supplies the crankshaft's seven main bearings and connecting rod
bearings via drilled passages. A separate gallery feeds oil to the
cylinder head to lubricate the rocker arm shafts, rocker arms, and valve
stems. The camshaft bearings in the block are lubricated from the main
gallery. The turbocharger center bearing receives oil via a dedicated
feed line and drains back to the sump. An oil cooler is standard.
The cooling system is a pressurized, forced-circulation system. A
centrifugal water pump is gear-driven from the timing train. Coolant
flow is regulated by a wax-pellet thermostat. The system employs a
heavy-duty tube-and-fin radiator. Engine cooling is assisted by a
belt-driven fan, often with a thermostatically controlled viscous fan
clutch to reduce parasitic loss when cooling demand is low.
The turbocharger is a wastegate-controlled unit. Some later or
high-output applications may incorporate an air-to-air intercooler
(charge air cooler) to lower intake air temperature. The engine may be
equipped with an exhaust brake, either a flap-type or compression
release brake, for auxiliary retardation. The electrical system is
24-volt for commercial vehicle applications. A pneumatic starting system
may be used in certain industrial configurations.
The external dimensions of the 6D31T engine are substantial due to its
inline-six configuration and all-iron construction. Approximate length
is 1200 mm (47.24 inches). Approximate width is 700 mm (27.56 inches).
Approximate height is 1000 mm (39.37 inches). These dimensions include
the turbocharger, injection pump, and oil cooler assembly. The dry
weight of the engine is approximately 450-470 kg (992-1036 lbs). This
mass is indicative of its heavy-duty design and construction materials.
Engine oil and filter changes are required every 250-400 operating hours
or 10,000 km (6,214 miles) for on-road service, whichever occurs first.
The centrifugal oil filter (if equipped) requires cleaning at every
second oil change. Fuel filter elements (primary and secondary) must be
replaced every 200-300 operating hours. Valve clearance inspection and
adjustment is a critical service task, required every 1000 operating
hours or 40,000 km (24,855 miles).
Injector spray pattern and opening pressure should be tested and
calibrated every 2000 operating hours. Coolant should be tested for
inhibitor strength annually and replaced every 2-3 years or 3000 hours.
The fan drive belt tension should be checked and adjusted every 250
hours. Air filter service is environment-dependent but must be monitored
closely; elements should be replaced when restriction indicators
activate or according to schedule.
Torque values are for clean, dry threads unless specified. Cylinder head
stud nuts are tightened in a specific multi-stage sequence to a final
torque of 22-24 kgm (159-174 lb-ft / 216-235 Nm). Main bearing cap nuts:
18-20 kgm (130-145 lb-ft / 177-196 Nm). Connecting rod cap nuts: 10-12
kgm (72-87 lb-ft / 98-118 Nm).
Crankshaft pulley bolt: 35-40 kgm (254-290 lb-ft / 343-392 Nm). Fuel
injection pump mounting bolts: 5-6 kgm (36-43 lb-ft / 49-59 Nm).
Injector clamp bolt: 6-8 kgm (43-58 lb-ft / 59-78 Nm). Rocker arm shaft
bracket bolts: 4-5 kgm (29-36 lb-ft / 39-49 Nm). Oil drain plug: 6-7 kgm
(43-51 lb-ft / 59-69 Nm).
The Mitsubishi 6D31T engine was widely used in medium-duty commercial
vehicles and selected industrial machinery. Its primary vehicle
applications include: Mitsubishi Fuso Fighter series medium-duty trucks.
Mitsubishi Fuso Rosa medium-duty bus. In the industrial sector, it
powered equipment such as the Kobelco SK200-III excavator. It also found
use in other construction equipment, stationary generator sets, and
possibly in marine auxiliary applications.
The engine's power output and durability profile made it suitable for
applications requiring reliable power in the 150 Hp range with robust
construction for extended service intervals. This engine was a common
sight in Asian, Australian, and other global markets throughout the
1980s and 1990s, eventually succeeded by more modern, electronically
controlled engine families within the Mitsubishi Fuso lineup.
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