Mitsubishi 4D55T Specifications
| Engine Model: Mitsubishi 4D55T |
| Engine Type: 4-cycle inline 4-cylinder SOHC 8-valve turbocharger liquid-cooled diesel |
| Total Displacement: 143.2 cu.in (2.34 L) |
| Rated Engine Power: 84-95 Hp (63-71 kW) at 4200 rpm |
| Fuel System Type: Indirect injection |
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The Mitsubishi 4D55T represents a landmark in diesel
engine development, being the first turbodiesel unit fitted to a Japanese
passenger car upon its debut in 1980. This 2.3-liter, inline-four engine is the
forced-induction iteration of the renowned “Astron” diesel family. Engineered
for a balance of durability and efficiency, it provided a significant leap in
power and torque over its naturally aspirated counterpart, making it
instrumental in the success of early Mitsubishi Pajero (Montero), Delica L300
vans, and L200 pickup trucks. Its robust construction and indirect injection
system laid the groundwork for the brand’s reputation for reliable, high-mileage
diesel powertrains throughout the 1980s.
The 4D55T is a 4-stroke, inline 4-cylinder, water-cooled diesel engine utilizing
a single overhead camshaft (SOHC) configuration. The combustion system employs
an indirect injection (IDI) design featuring a swirl chamber. Total displacement
is 2.346 Liters (2,346 cc / 143.2 cu.in), derived from a bore and stroke
measuring 91.1 mm (3.587 in) x 90.0 mm (3.543 in). The compression ratio is set
at 21.0:1.
The firing order follows 1-3-4-2. In factory trim, the engine produces a maximum
power output ranging from 84 to 95 HP (63-71 kW) at 4,200 RPM, with peak torque
delivered between 175 and 181 Nm (129-133 lb-ft) at 2,500 RPM. Standard maximum
turbocharger boost pressure is regulated to 0.50-0.55 bar (7.2-8.0 psi). The dry
weight of the engine is approximately 195 kg (430 lbs).
The cylinder block is constructed from cast iron,
forming a rigid, deep-skirt design that provides exceptional support for the
rotating assembly. It is a siamese-bore configuration, where the cylinders are
cast without water jacket gaps between them, enhancing structural integrity. The
block houses five main bearings, with the crankshaft supported by these bearings
to ensure stability under high combustion pressures.
The bore diameter is precisely machined to 91.1 mm (3.587 in). Engine oil
passages are integrated directly into the block to supply the crankshaft,
connecting rods, and the cylinder head. The block also features provisions for a
balance shaft system - a twin-shaft setup located in the lower crankcase area -
designed to mitigate secondary vibration inherent in inline-four diesel engines.
The cylinder head is manufactured from cast iron,
aligning with the engine’s focus on durability and thermal stability under
turbocharged operation. It utilizes a SOHC valvetrain configuration operating
two valves per cylinder - one intake and one exhaust - for a total of eight
valves. The valves are arranged in a cross-flow pattern to optimize gas flow
efficiency. The indirect injection system is facilitated by swirl chambers
located within the cylinder head casting, where fuel mixes with compressed air
before combustion.
Valve clearance adjustment is performed via threaded tappets (lash adjusters)
located between the camshaft lobes and the valve stems. The valve clearance,
measured when the engine is at operating temperature, is set to 0.25 mm (0.010
in) for both intake and exhaust valves. The camshaft is driven by a rubber
timing belt, which also interfaces with the injection pump drive sprocket.
The fuel system is centered on an inline injection pump
- a Bosch VE-type distributor pump or a similar licensed unit - mounted to the
side of the engine and driven by the timing belt. This system delivers diesel
fuel under high pressure to the swirl chambers for indirect injection. Fuel
delivery is calibrated to manage combustion efficiency and boost pressure
correlation.
Glow plugs are integrated into the cylinder head to assist with cold starting;
depending on the market and model year, the engine utilizes either a 12V
standard glow system or a 6V “Super Quick Glow” system that heats the plugs
rapidly for near-instantaneous starting in cold conditions. The injection pump
is equipped with a mechanical governor to maintain stable idle speeds and
control fuel delivery under load.
Lubrication is provided by a gear-type oil pump driven
by the crankshaft, circulating oil through a full-flow filtration system. The
total oil capacity, including the filter and an optional oil cooler, ranges from
5.5 to 6.5 Liters (5.8-6.9 qt).
A critical feature of this engine is the inclusion of a piston cooling system;
oil jets (piston squirters) are installed in the crankcase to spray oil onto the
undersides of the pistons, dissipating heat absorbed during combustion and
ensuring the longevity of the piston rings and skirts under turbocharged stress.
Oil pressure is regulated by a relief valve to prevent over-pressurization
during cold starts or high-RPM operation.
The cooling system is a water-cooled, pressurized setup
utilizing a centrifugal water pump driven by the timing belt. Coolant circulates
through the cast-iron block and cylinder head, with flow directed to critical
areas such as the exhaust valve seats and the turbocharger bearing housing. A
thermostat regulates operating temperature, maintaining a range between 76°C and
82°C (169°F-180°F) for standard operation.
The system is designed with a large-capacity radiator to manage the additional
thermal load generated by the turbocharger. Coolant passages in the cylinder
head are specifically configured to prevent localized boiling near the swirl
chambers, ensuring consistent thermal management under heavy load conditions.
Turbocharger System: The engine is equipped with a
single, wastegated turbocharger mounted to an exhaust manifold. The wastegate
actuator controls maximum boost pressure to prevent over-boost conditions. Oil
is supplied to the turbocharger center housing from the main lubrication system
via a dedicated feed line, with a return line allowing oil to drain back to the
oil pan by gravity.
Balance Shaft System: A dual balance shaft assembly is located within the
cylinder block, rotating at twice the crankshaft speed to counteract the
inherent vertical and horizontal vibrations of the inline-four engine
configuration. These shafts are gear-driven and require precise timing relative
to the crankshaft during assembly.
Starting System: The engine utilizes a high-torque starter motor mounted to the
bell housing. The glow plug system integrates with a controller that manages
pre-glow and after-glow cycles to reduce white smoke and improve cold-start
reliability.
The length from the front of the timing cover to the
rear of the flywheel housing measures 680 mm (26.8 in), while the width across
the valve cover and injection pump is roughly 550 mm (21.7 in). The height from
the oil pan rail to the top of the cylinder head is 720 mm (28.3 in). The dry
weight of the fully dressed engine - including the injection pump, turbocharger,
and manifolds - is 195 kg (430 lbs).
Regular maintenance intervals are critical for the
longevity of the 4D55T. The timing belt is a high-priority service item, with a
mandatory replacement interval of 100,000 km (60,000 miles) to prevent
catastrophic valve-to-piston interference. Engine oil and filter changes are
recommended at 5,000-7,500 km (3,000-4,500 miles) under normal driving
conditions, with a strong emphasis on using diesel-rated oil to withstand soot
loading.
Valve clearance inspections are scheduled every 30,000 km (18,000 miles), with
adjustments made to the 0.25 mm (0.010 in) hot specification. The fuel filter
should be replaced annually or every 20,000 km (12,000 miles) to protect the
injection pump from contamination. Coolant should be replaced every two years to
maintain corrosion inhibition and proper thermal transfer properties.
Compression pressure testing should yield a standard value of 3.04 MPa (441
psi), with a minimum service limit of 2.26 MPa (327 psi).
Cylinder Head Bolts (Cold): 105-115 Nm (77-85 lb-ft).
Main Bearing Bolts: 75-85 Nm (55-63 lb-ft).
Connecting Rod Bolts: 45-48 Nm (33-35 lb-ft).
Flywheel Bolts: 130-140 Nm (96-103 lb-ft).
Camshaft Sprocket Bolt: 60-70 Nm (44-52 lb-ft).
Balance Shaft Bearing Cap Bolts: 10-12 Nm (89-106 lb-in).
Injection Pump Drive Gear Nut: 50-60 Nm (37-44 lb-ft).
Rocker Arm Shaft Bolts: 20-25 Nm (15-18 lb-ft).
The Mitsubishi 4D55T served as a cornerstone powertrain
for a range of vehicles across global markets, particularly throughout the 1980s
and early 1990s.
Key applications include:
Mitsubishi Pajero / Montero (L044, L045): First-generation models, providing the
necessary torque for off-road capability. Mitsubishi Delica / L300 (L038, L039):
Utilized in both passenger van and commercial configurations, valued for its
power-to-weight ratio in a compact chassis. Mitsubishi L200 / Forte / Triton
(L045, L046): Employed in pickup truck variants, often marketed under the
“Diesel Turbo” designation.
Dodge Ram 50 / Plymouth Arrow Truck: Sold in North America as captive imports,
these vehicles utilized the 4D55T as a premium diesel option. Hyundai D4BF
(License-built variants): The engine design continued in production under
license in later years, powering the Hyundai Porter and Galloper, demonstrating
the robustness of the original Mitsubishi architecture.
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