Cat 3126B Specifications
| Engine Model: Caterpillar 3126B |
| Engine Type: 4-cycle inline 6-cylinder HEUI direct injection liquid-cooled diesel |
| Total Displacement: 439 cu.in (7.2 L) |
| Rated Engine Power: 170-330 Hp (127-246 kW) at 2400 rpm |
| Aspiration Type: Turbocharged |
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The Caterpillar 3126B is a mid-displacement,
electronically controlled, turbocharged inline-six diesel engine widely
used in medium-duty trucks, buses, construction machinery, marine
vessels, and industrial power equipment. Introduced as an evolution of
the earlier 3116 and 3126 engines, the 3126B incorporates improved
fuel-delivery technology, enhanced electronic control, stronger
mechanical components, and improved emissions performance. The 3126B is
part of Caterpillar’s HEUI (Hydraulically Actuated Electronically
Controlled Unit Injector) family, designed to deliver precise
fuel-injection timing and quantity for increased power output, better
fuel efficiency, and reduced exhaust emissions.
The engine is valued for its versatility, consistent power delivery,
smooth operation, and balanced combination of mechanical durability and
electronic refinement. Thousands of 3126B engines remain in service due
to their reliability, straightforward maintenance requirements, and
long-term operational endurance in harsh working environments.
The Caterpillar 3126B is a four-stroke, water-cooled, turbocharged,
inline six-cylinder diesel engine with electronic fuel control. Total
displacement is 7.2 liters (439 cubic inches). Bore measures 110 mm
(4.33 inches), and stroke is 127 mm (5.00 inches). The compression ratio
typically ranges from 16.0:1 to 17.5:1 depending on the rating.
Power output varies widely by application:
– Truck versions: 190–330 horsepower (142–246 kW) at 2,200–2,400 rpm
– Bus and RV versions: 210–330 horsepower (157–246 kW) at 2,200–2,400
rpm
– Marine auxiliary ratings: 170–300 horsepower (127–224 kW) at
2,200–2,400 rpm
Peak torque generally falls between: 700 and 1,050 Nm (516–774 lb-ft) at
1,400–1,600 rpm. Idle speed is typically 650–750 rpm. Maximum governed
speed is usually 2,200–2,400 rpm. Firing order is 1-5-3-6-2-4.
The crankshaft is forged steel, fully counterweighted, and supported by
seven main bearings. Pistons are cast-aluminum alloy with steel struts
and machined bowls for efficient direct-injection combustion. Connecting
rods are forged steel with replaceable bearing inserts. The camshaft is
gear-driven and located in the block for long-term timing stability.
The 3126B block is cast iron and designed for durability under
medium-duty and vocational service conditions. Cylinders use parent-bore
construction rather than removable liners. Thick casting ribs reinforce
the block to minimize flex and vibration. Coolant passages surround each
cylinder to ensure even temperature control during prolonged operation.
Structural strength supports high cylinder pressures produced by
turbocharging and electronic fuel injection.
The cylinder head is a one-piece cast-iron structure featuring
direct-injection combustion chambers. Each cylinder uses four valves—two
intake and two exhaust—for optimized airflow efficiency. Valves are
actuated by rocker arms driven from the gear-driven camshaft. Hardened
steel valve seats withstand high combustion temperatures.
Cold valve-lash specifications:
– Intake: 0.38–0.43 mm (0.015–0.017 in)
– Exhaust: 0.64–0.69 mm (0.025–0.027 in)
Electronic systems control timing, fuel shot quantity, and throttle
response while maintaining emissions compliance.
The fuel system uses Caterpillar’s HEUI (Hydraulically Actuated
Electronically Controlled Unit Injector) system. This system uses
high-pressure engine oil as the hydraulic force to actuate injectors
while the electronic control module controls timing and quantity.
Key components include:
– High-pressure oil pump
– HEUI injectors
– Electronic Control Module (ECM)
– Low-pressure fuel pump
– Dual fuel filters
Injection pressure is dynamically varied based on load and speed, often
exceeding 1,000 bar in peak conditions.
Fuel system maintenance:
– Replace fuel filters every 10,000–15,000 km or 250–300 hours
– Inspect HEUI injectors at major service intervals
– Drain water separator as required
– Check oil condition, as HEUI operation depends on clean, high-quality
oil.
The lubrication system is a full-pressure wet-sump design powered by a
gear-driven oil pump. Oil is routed through a full-flow filter before
entering the main gallery. The HEUI system also depends heavily on oil
pressure, making lubricant condition and viscosity crucial.
Oil capacity: 22–25 liters (23–26 US quarts)
Normal oil pressure at operating speed: 300–450 kPa (44–65 psi)
Idle oil pressure: 100–150 kPa (15–22 psi)
Recommended oils: SAE 15W-40 or 10W-30 meeting API CH-4 or higher.
Cooling is provided by a high-flow centrifugal water pump circulating
coolant through the block, head, and aftercooler (if equipped). A
thermostat begins opening at 82°C (180°F) and is fully open by 95°C
(203°F). Cooling capacity varies by equipment but typically ranges from
20 to 28 liters (21–29 US quarts). Radiator cap pressure is 70–90 kPa
(10–13 psi).
Air-to-air aftercoolers (ATAAC) reduce intake-air temperature in many
versions, improving performance and durability.
| Intake and Exhaust Systems |
Air intake filtration depends on
application:
– Trucks use dry-element paper filters
– Industrial units may include oil-bath or cyclone systems
Turbocharged versions use a single turbocharger feeding into an
aftercooler. Exhaust manifolds are cast iron and designed to withstand
repeated thermal cycling. Marine versions often include water-cooled
exhaust components.
| Electrical and Starting Systems |
The 3126B uses a 12- or 24-volt electrical system depending on
application.
Electronic control is managed by the ECM,
which receives input from sensors monitoring:
– coolant temperature
– oil temperature and pressure
– boost pressure
– intake-air temperature
– engine speed
– throttle position
Alternators typically produce 75–120 amperes. Starter motors range from
3.5–5.0 kW.
The 3126B may support:
– PTO (power-take-off) drives
– Hydraulic pump drives
– Air compressors
– Transmission control modules (truck applications)
– Marine cooling systems (jacket-water or heat-exchanger type).
Approximate dimensions:
– Length: 1,050–1,150 mm (41.3–45.3 in)
– Width: 780–880 mm (30.7–34.6 in)
– Height: 1,000–1,080 mm (39.4–42.5 in)
Dry weight varies: 850–950 kg (1,874–2,094 lbs).
– Oil and filter changes: every 250 hours or 10,000–15,000 km
– Fuel filters: 250–300 hours
– Air filters: inspect every 50 hours
– Coolant replacement: every 2 years
– Valve lash adjustment: every 1,000 hours
– HEUI injector inspection: every major service interval
– Compression test: every 100,000 km
– Belt inspection: each service cycle.
– Cylinder head bolts: 170–190 Nm (125–140 lb-ft)
– Main bearing caps: 200–220 Nm (148–162 lb-ft)
– Connecting-rod bolts: 120–140 Nm (89–103 lb-ft)
– Flywheel bolts: 180–200 Nm (133–148 lb-ft)
– Crankshaft pulley bolt: 350–400 Nm (258–295 lb-ft)
– Oil drain plug: 40–55 Nm (30–41 lb-ft)
– Injector hold-downs: 25–30 Nm (18–22 lb-ft).
The Caterpillar 3126B is widely used in:
– Medium-duty trucks (Freightliner, Sterling, Ford)
– School buses and city buses
– Motorhomes and RVs
– Marine auxiliary engines
– Construction equipment
– Municipal and utility vehicles
– Industrial pumps and compressors.
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