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Mitsubishi 4D31 Specifications
| Engine Model: Mitsubishi 4D31 |
| Engine Type: 4-cycle inline 4-cylinder naturally aspirated liquid-cooled diesel |
| Total Displacement: 201.2 cu.in (3.29 L) |
| Rated Engine Power: 110 Hp (81 kW) at 3500 rpm |
| Fuel System Type: Direct fuel injection |
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- Mitsubishi 4D30 Engine Specifications
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| General Technical Data |
The Mitsubishi 4D31 is a naturally aspirated 3.3-liter inline-four diesel engine belonging to the renowned Mitsubishi 4D3 engine family, which has long been associated with light commercial vehicles and medium-duty trucks. Introduced during the late 1980s and used through the 1990s, the 4D31 replaced earlier indirect injection diesel units with a more efficient, direct-injection design. It was primarily installed in the Mitsubishi Canter and related light trucks, offering an ideal combination of simplicity, reliability, and adequate torque for commercial transport duties.
The engine’s mechanical fuel injection, strong
cast-iron construction, and low-speed torque characteristics made it a
workhorse powerplant in markets where durability and serviceability were
paramount. Designed to operate under severe load conditions and long
service intervals, the 4D31 became well-known among fleet operators for
its dependability and ease of overhaul.
The 4D31 engine exemplifies Mitsubishi’s engineering philosophy for
diesel utility engines: conservative design, robust materials, and
accessible maintenance. With no electronic control systems, it relies
entirely on mechanical precision and fuel system calibration. The engine
was also widely exported and licensed for assembly in several markets,
solidifying its reputation as one of the most serviceable and long-lived
engines in its displacement class.
The Mitsubishi 4D31 is a four-stroke, naturally aspirated, water-cooled
diesel engine with four inline cylinders. Total displacement is 3,298 cc
(3.3 liters, or 201.3 cubic inches), achieved through a bore of 100.0 mm
(3.93 inches) and a stroke of 105.0 mm (4.13 inches). The long-stroke
configuration favors strong low- and mid-range torque delivery, ideal
for heavy-duty applications.
The compression ratio is 18.5:1, characteristic
of naturally aspirated diesel engines of its era. This high compression
allows efficient combustion of diesel fuel without forced induction. The
engine operates on the direct injection principle with a mechanical
in-line fuel injection pump and pintle-type injectors delivering fuel
directly into the combustion chambers.
The cylinder block is cast iron with deep skirts and thick cylinder
walls for strength and thermal stability. The cylinder head is also made
of cast iron, designed with crossflow ports and individual
pre-combustion chambers integrated into each cylinder. The valvetrain
consists of a single camshaft located in the block, operating overhead
valves through pushrods and rocker arms. Each cylinder has two valves –
one intake and one exhaust – ensuring simplicity and durability.
The camshaft is gear-driven from the
crankshaft, eliminating the need for a timing belt or chain. The
crankshaft runs on five main bearings, ensuring smooth operation and
long service life. Pistons are made of aluminum alloy with three rings:
two compression rings and one oil control ring. The connecting rods are
forged steel with solid big-end bearings and replaceable shell inserts.
The maximum power output of the 4D31 engine is approximately 81 kW (110
horsepower, 108 bhp) at 3,500 revolutions per minute, while maximum
torque is around 245 Nm (181 lb-ft) at 2,000 rpm. The engine’s redline
is limited to about 3,800 rpm, consistent with its commercial-duty
tuning. Idle speed is typically 750 ± 50 rpm. The firing order is
1-3-4-2, and the crankshaft rotation is clockwise viewed from the
flywheel end.
| Fuel System |
The fuel system is entirely mechanical, employing a direct-injection configuration for improved fuel efficiency and cold-start performance compared to older indirect injection designs. A mechanically driven in-line fuel injection pump (commonly manufactured by Mitsubishi or licensed Bosch-type) meters and delivers precise quantities of diesel fuel under high pressure to the injectors.
The injectors are multi-hole pintle-type, opening at approximately 17
Mpa (2,465 psi) of pressure. Each injector nozzle atomizes fuel finely
into the combustion chamber, promoting complete combustion and reducing
smoke output. Fuel is supplied from the tank by a mechanically driven
lift pump integrated into the injection pump body. Excess fuel returns
to the tank through a return line.
A replaceable cartridge-type fuel filter is installed between the fuel
tank and injection pump to remove contaminants and water. The filter
element should be replaced every 20,000 km (12,000 miles) or sooner
under severe operating conditions. The fuel used is diesel with a cetane
number of 50 or higher. The injection timing is mechanically set and
typically occurs 12° to 14° before top dead center at rated speed. The
system includes a manual priming pump for bleeding air after filter
replacement or fuel system maintenance.
| Lubrication System |
The lubrication system is a full-pressure, wet-sump type using a trochoid oil pump driven by the crankshaft through a gear train. Oil is drawn from the sump through a wire mesh strainer and delivered under pressure to the main oil gallery, which supplies oil to the crankshaft main bearings, connecting rod journals, camshaft bearings, and rocker arms.
The piston undersides are cooled by oil jets directed upward from the
main gallery, helping control piston temperature during high-load
operation. The oil filter is a full-flow, spin-on type with a built-in
bypass valve. The total oil capacity, including the filter, is
approximately 7.0 liters (7.4 US quarts, 6.2 Imperial quarts).
Normal oil pressure is around 350 to 400 kPa (51 to 58 psi) at 3,000 rpm
and about 100 kPa (15 psi) at idle. Mitsubishi recommends high-quality
diesel engine oil meeting API CF, CG, or later standards. Viscosity
grades are SAE 15W-40 for temperate climates or 10W-30 for colder
environments.
Oil and filter should be replaced every 10,000 km (6,000 miles) or 6 months, whichever occurs first. Severe service such as prolonged idling, dusty environments, or heavy loads may require shorter intervals.
| Cooling System |
The cooling system is a pressurized, forced-circulation type using a
centrifugal water pump driven by a V-belt from the crankshaft pulley.
The pump circulates coolant through the block, cylinder head, radiator,
and heater core. The thermostat begins to open at approximately 82°C
(180°F) and is fully open by 95°C (203°F). The radiator is a cross-flow
type with a pressure cap rated at 88 kPa (13 psi). Cooling is assisted
by a belt-driven fan equipped with a viscous coupling to reduce noise
and energy loss at high speed. The total coolant capacity is around 10.5
liters (11.1 US quarts, 9.2 Imperial quarts).
The recommended coolant mixture is 50% ethylene glycol antifreeze and
50% demineralized water, providing freeze protection down to –35°C
(–31°F). A coolant temperature sender provides input to the dashboard
gauge. Proper maintenance includes periodic inspection of the radiator
core, hoses, and thermostat, as well as coolant replacement every 40,000
km (25,000 miles) or two years. The cooling system’s reliability is
critical for the cast-iron head, which is sensitive to localized
overheating.
| Intake and Exhaust Systems |
The intake system consists of a dry-type air cleaner with a replaceable paper element, connected to the intake manifold through a flexible rubber duct. Air passes through the filter and intake manifold directly into each cylinder through individual ports. Intake air temperature affects performance, and some models include a warm-air duct from the exhaust manifold to improve cold-weather driveability.
The exhaust system includes a front pipe, muffler, and tailpipe designed
for low restriction while maintaining acceptable noise levels. Exhaust
gas recirculation (EGR) was not standard on early versions but appeared
on later 1990s units for emission control compliance.
Lubrication of the valve train is achieved through oil galleries leading
to the rocker arms. Valve clearance is adjustable via screw-and-locknut
mechanisms, ensuring ease of service. Typical clearances (cold) are 0.25
mm (0.010 inches) for intake and 0.30 mm (0.012 inches) for exhaust
valves.
| Electrical System |
The electrical system is conventional 12-volt DC, featuring a gear-reduction starter motor rated at 2.5 kW and an alternator rated at 24/60 amperes, depending on vehicle application. The engine uses glow plugs for cold starting, powered through a relay and timer unit.
Pre-heating time ranges from 5 to 10 seconds under normal ambient conditions. The battery capacity should be at least 90 ampere-hours for reliable cold starts. The charging system maintains approximately 14.0 volts at normal operating conditions. The engine grounding is provided by braided copper straps between the block and chassis.
| Dimensions and Weight |
The Mitsubishi 4D31 engine measures approximately 860 mm (33.9 inches) in length, 720 mm (28.3 inches) in width, and 830 mm (32.7 inches) in height. Dry weight is about 275 kg (606 lbs). The engine’s robust cast-iron construction contributes to its high weight, but also ensures rigidity and vibration resistance. The mounting system uses four rubber-insulated brackets to minimize vibration transfer to the chassis.
| Maintenance Data |
Oil and filter changes should occur every 10,000 km (6,000 miles), with air filter replacement every 20,000 km (12,000 miles). The fuel filter element should be renewed every 20,000 km (12,000 miles). Valve clearances must be checked every 40,000 km (25,000 miles). The cooling system should be flushed and refilled every two years.
The fan belt tension must be inspected every 10,000 km and replaced if cracked or frayed. The injection pump calibration should be verified every 80,000 km (50,000 miles). Compression pressure should be approximately 3,000 kPa (435 psi) per cylinder, with a minimum allowable limit of 2,550 kPa (370 psi). The maximum permissible difference between cylinders is 300 kPa (43 psi).
| Tightening Torques |
Cylinder head bolts: 39 Nm (29 lb-ft) initial, 88 Nm (65 lb-ft) final,
plus 90 degrees.
Main bearing cap bolts: 100 Nm (74 lb-ft).
Connecting rod bolts: 55 Nm (41 lb-ft) plus 90 degrees.
Flywheel bolts: 120 Nm (89 lb-ft).
Crankshaft pulley bolt: 245 Nm (181 lb-ft).
Rocker shaft bolts: 25 Nm (18 lb-ft).
Oil pan bolts: 9 Nm (7 lb-ft).
Intake manifold bolts: 25 Nm (18 lb-ft).
Exhaust manifold nuts: 35 Nm (26 lb-ft).
Glow plug torque: 15 Nm (11 lb-ft).
Oil drain plug: 45 Nm (33 lb-ft).
Injector retaining nuts: 30 Nm (22 lb-ft).
| Vehicle Applications |
The Mitsubishi 4D31 naturally aspirated
diesel engine was mainly used in the following vehicles:
– Mitsubishi Canter FE series (light-duty trucks, 1988–1997)
– Mitsubishi Rosa (minibus variants, early 1990s)
– Mitsubishi Delica Cargo (commercial van models)
– Industrial applications such as small construction machinery and
generators.
The engine’s simple mechanical nature and long service life made it
especially popular in Asian, African, and Middle Eastern markets, where
ease of repair and fuel economy were prioritized over high power output.
Even decades after production ended, many 4D31 engines remain in
operation, often rebuilt multiple times due to the availability of parts
and straightforward overhaul procedures.
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