Cat 3126 Specifications
| Engine Model: Caterpillar 3126 |
| Engine Type: 4-cycle inline 6-cylinder turbocharged and aftercooled diesel |
| Total Displacement: 439 cu.in (7.2 L) |
| Rated Engine Power: 275-330 Hp (205-246 kW) at 2200 rpm |
| Fuel System Type: Hydraulic electronic unit injector (HEUI) |
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The Caterpillar 3126 is a 7.2-liter inline
six-cylinder diesel engine representing a significant evolution within
the 3100 family, designed primarily for medium-duty truck and industrial
applications. This electronically controlled engine was developed to
meet stringent emissions standards while providing improved power, fuel
economy, and diagnostic capabilities over its mechanical predecessors.
Its core design retains the robust architecture of wet cylinder liners
and a gear-driven camshaft but integrates the advanced Caterpillar HEUI
fuel system. Offered in a range of horsepower ratings, the 3126 is
characterized by its air-to-air aftercooling, electronic governing, and
a reputation for durability in demanding service cycles. It became a
mainstay in North American trucking and served reliably across diverse
markets until its production cessation.
Engine Type: Inline 6-Cylinder, Four-Stroke Cycle Diesel.
Aspiration: Turbocharged and Air-to-Air Aftercooled (ATAAC).
Displacement: 7.2 Liters (439 cubic inches).
Bore: 110 mm (4.33 inches).
Stroke: 127 mm (5.00 inches).
Engine Power (Medium Duty Truck): 170-250 HP (127-186 kW).
Engine Power (Bus & Industrial): 275-330 HP (205-246 kW).
Engine Power (Marine): 300-420 HP (224-313 kW).
Compression Ratio: 17.0:1.
Firing Order: 1-5-3-6-2-4.
Rotation (Viewed from Flywheel End): Counterclockwise.
Governor Type: Electronic, controlled by the Engine Control Module
(ECM).
Emission Certification: Originally certified to U.S. EPA Tier 2 and EU
Stage I standards. Later production met Tier 3/Stage II limits.
The cylinder block is a high-rigidity cast iron structure. It
incorporates replaceable wet-type cylinder liners. The liner flange is
precision-machined and sealed at the top of the block with a gasket. The
block design provides extensive support for the crankshaft through
cross-bolted main bearing caps.
Each main bearing cap is secured by two vertical bolts and two
horizontal side bolts that thread into the block’s lower skirt, creating
an exceptionally rigid lower end structure capable of withstanding high
combustion pressures. The camshaft rotates in bearings machined directly
into the block and is gear-driven from the crankshaft.
A single cast iron cylinder head is used, secured by four bolts per
cylinder. The cylinder head employs four valves per cylinder - two
intake and two exhaust - to facilitate efficient airflow required for
high power output and clean combustion.
Valve seat inserts are hardened for extended service life. Valve guides
are replaceable. The valve train uses hydraulic lash adjusters,
eliminating the need for manual valve lash adjustment. Rocker arms are
shaft-mounted. The cylinder head gasket is a multi-layer steel (MLS)
design.
The 3126 engine is equipped with the Caterpillar Hydraulically Actuated
Electronically Controlled Unit Injector (HEUI) system. This system uses
high-pressure engine oil, supplied by a dedicated high-pressure oil
pump, to actuate the fuel injector plungers. An electronic control
module (ECM) precisely controls the timing and duration of fuel
injection by actuating a solenoid on each injector.
Fuel pressure generation is separated from engine camshaft speed,
allowing for greater injection pressure control across the entire engine
operating range. A gear-type transfer pump supplies fuel from the tank
to the injectors.
The lubrication system features two pumps: a standard gear-type pump for
engine lubrication and a high-pressure gear pump dedicated to the HEUI
fuel system. Oil is drawn from the sump through a pickup screen, passed
through a full-flow spin-on filter, and then routed to the engine’s main
oil gallery and the high-pressure pump inlet.
The high-pressure pump raises oil pressure to levels required for
injector operation (up to 3000 psi / 207 bar). Engine oil lubricates the
crankshaft, camshaft, and valve train components. Piston cooling jets
spray oil onto the underside of each piston.
Coolant circulation is managed by a gear-driven centrifugal water pump.
Coolant flows through the cylinder block, cylinder head, and an
integrated jacket water aftercooler (JWAC). The JWAC uses engine coolant
to lower the temperature of the turbocharged intake air.
A separate air-to-air aftercooler (ATAAC) core, mounted in front of the
vehicle’s radiator, provides a second stage of intake air cooling.
Engine temperature is regulated by a wax-pellet thermostat. The system
is designed for connection to a radiator of appropriate heat rejection
capacity.
Starting System: 12-volt or 24-volt electric starting motor. Charging
System: A brushless alternator, commonly rated between 105 and 150
amperes for truck applications. Air Intake System: Dry-type air cleaner.
The turbocharger compresses air, which then passes sequentially through
the jacket water aftercooler and the air-to-air aftercooler. Exhaust
System: A cast iron exhaust manifold routes gases to the turbocharger
turbine housing. A wastegate regulates boost pressure.
Electronic Control: The engine is governed by an Engine Control Module
(ECM) that monitors sensors for boost pressure, coolant temperature, oil
pressure, and crankshaft position to optimize performance and
diagnostics.
Physical dimensions vary with the specific application configuration
(industrial, truck, marine). For a truck engine with accessories,
approximate length is 1250 mm (49.2 in), width is 800 mm (31.5 in), and
height is 1100 mm (43.3 in). The net dry engine weight, excluding fluids
but including standard accessories like alternator and starter, ranges
from 722 kg to 822 kg (1592 lb to 1812 lb). The weight range depends on
the specific horsepower rating, with higher-output models having heavier
components like pistons and connecting rods.
Engine Oil Grade: Use engine oil that meets Caterpillar ECF-1
specification, equivalent to API CH-4 or CI-4. Oils meeting API CG-4 or
earlier specifications are not approved for the HEUI system. SAE 15W-40
is standard.
Oil Change Interval: Service intervals are variable and based on duty
cycle, with ranges from 250 to 500 hours or 15,000 to 25,000 miles for
on-highway applications. Oil analysis is a recommended practice.
Oil System Capacity: Total system refill capacity is 25.0 Liters (6.6 US
gallons), including filter change. Cooling System Capacity: Total
cooling system capacity, including engine, aftercooler, and radiator, is
approximately 39.0 Liters (10.3 US gallons).
Fuel Filter: Replace both primary and secondary fuel filter elements at
every oil change interval or as indicated by the ECM. Air Filter:
Service or replace based on restriction gauge indication. Valve Lash
Adjustment: The hydraulic lash adjuster system requires no scheduled
adjustment.
Engine Coolant: Use a 50/50 mixture of ethylene glycol antifreeze and
deionized water. Maintain proper concentration of Cat Extended Life
Coolant (ELC) or a Supplemental Coolant Additive (SCA) if using a
conventional coolant.
Cylinder Head Bolts: Tighten in a specific three-step sequence. Final
torque: 160 Nm (118 lb-ft). Main Bearing Cap Bolts (Vertical): 190 Nm
(140 lb-ft) plus an additional 90-degree turn. Main Bearing Cap Cross
Bolts: 75 Nm (55 lb-ft). Connecting Rod Cap Bolts: 120 Nm (89 lb-ft)
plus an additional 90-degree turn.
Flywheel Housing-to-Block Bolts: 75 Nm (55 lb-ft).
Flywheel-to-Crankshaft Bolts: 115 Nm (85 lb-ft). High-Pressure Oil Pump
Mounting Bolts: 50 Nm (37 lb-ft). Oil Pan Bolts: 27 Nm (20 lb-ft). Oil
Filter Cartridge: Hand-tighten until the gasket contacts the base, then
tighten an additional three-quarters turn.
The Caterpillar 3126 engine saw extensive
application across multiple industries:
Medium-Duty Trucks: The primary application, found in numerous
Freightliner, International (Navistar), Ford (Cargo, F-Series with CAT),
Kenworth, and Peterbilt trucks for delivery, refuse, utility, and
beverage service. Construction Equipment: Used in Caterpillar 416C,
420D, 426C Backhoe Loaders; 924G, 928G Wheel Loaders; and various soil
compactors and asphalt pavers.
Marine: Employed as a propulsion engine for commercial workboats, crew
boats, and pleasure craft, often in higher horsepower ratings.
Industrial: Served as a prime mover for generator sets (100-175 kVA),
fire pump drives, and hydraulic power units. Buses: Powered a
significant number of transit and shuttle buses for various North
American manufacturers.
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