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Caterpillar 3126E Engine

The Caterpillar 3126E is a medium-duty, 7.2-liter, inline 6-cylinder, four-stroke cycle diesel engine introduced as an evolutionary step within the 3126 engine family. Released to meet tightening emissions standards, the "E" variant relies heavily on refined advanced electronic architecture rather than structural mechanical redesign. It retains the deep-skirt, parent-bore cast-iron block of its predecessors while utilizing an 18-valve crossflow cylinder head design. The engine is widely deployed in medium-duty trucks, buses, RVs, emergency vehicles, and marine applications due to its high power-to-weight ratio and precise electronic governor controls.

The 3126E utilizes a Hydraulic Electronic Unit Injection (HEUI) system. Fuel delivery is decoupled from engine speed and driven mechanically via high-pressure hydraulic oil force. Fuel is supplied from the fuel tank by a mechanical gear-type fuel transfer pump, passing through a primary fuel filter/water separator into low-pressure galleries inside the cylinder head. Simultaneously, a high-pressure hydraulic pump (HEUI pump) increases engine oil pressure up to 4,000 psi under the direction of an Injection Actuation Pressure Control Valve (IAPCV) controlled by the Electronic Control Module (ECM). This high-pressure oil travels through a separate cylinder head rail to each individual injector. When the ECM signals an injector solenoid, an internal poppet valve opens, allowing the high-pressure oil to act upon an internal intensifier piston. This piston multiplies the force, pressurizing the underlying diesel fuel up to 22,000 psi for fine mechanical atomization directly into the combustion chamber.

The engine features a full-pressure, wet-sump lubrication system driven by a gear-type oil pump integrated into the front timing gear train. Oil is drawn from the pan, forced through an integral water-cooled engine oil cooler block, and passed through a spin-on primary oil filter. Pressurized oil is then distributed through internal block galleries to the main bearings, connecting rod journals, camshaft bearings, and overhead valve mechanism. Crucially, the lubrication circuit serves a dual purpose: it acts as the primary hydraulic fluid media required to charge the high-pressure injection system. Normal operating oil pressure at full throttle is maintained between 44 psi and 65 psi.

The cooling loop uses a closed, pressurized system featuring a belt-driven centrifugal jacket water pump mounted to the front housing. Coolant circulation is regulated by a capsule-type water temperature regulator (thermostat) housed at the top front of the cylinder head. During warm-up, the thermostat restricts flow, bypassing the radiator and routing coolant directly back to the water pump inlet. Once opening temperature is reached, the thermostat directs full flow through the external radiator or heat exchanger system. The cooling circuit also encompasses an integrated oil cooler core and pathways surrounding the cylinder walls and internal cylinder head passages.

General Technical Data

Engine Specifications

Engine Model ............... Caterpillar 3126E
Engine Type ............... 4-cycle inline 6-cylinder turbocharged ATAAC liquid-cooled diesel
Number of Cylinders ............... 6
Cylinder Arrangement ............... In-line
Engine Bore, in (mm) ............... 4.33 (110.0)
Engine Stroke, in (mm) ............... 5.00 (127.0)
Displacement, cu.in (L) ............... 439 (7.2)
Rated Engine Power (Truck/Industrial), hp (kW) ............... 170-350 (126-260)
Rated Engine Power (Marine), hp (kW) ............... 350-450 (260-333)
Rated Engine Speed, rpm ............... 2200-2600
Peak Torque ............... 770-1160 Nm (570-860 lb-ft) at 1400-1600 rpm
Aspiration Type ............... Turbocharged and aftercooled
Operating Speed ............... 2000-2600 rpm
Idle Speed ............... 650-750 rpm
Compression Ratio ............... 17.3:1
Firing Order ............... 1-5-3-6-2-4
Crankshaft Type ............... Forged steel
Piston Type ............... Cast-aluminum alloy
Con Rod Type ............... Forged steel
Cylinder Block Type ............... Cast iron
Cylinder Head Type ............... Cast iron
Lubrication System ............... Full-pressure
Oil Pump Type ............... Gear-driven
Oil Filter Type ............... Full-flow, spin-on
Cooling System ............... Liquid-cooled
Water Pump Type ............... Gear-driven, centrifugal
Air Cleaner Type ............... Dry two-stage
Starting System ............... Electric Starter
Electrical System ............... 12V or 24V
Alternator Rating, Amps ............... 75-120

Fuel System

Fuel System ............... High-Pressure Electronic Unit Injection (HEUI)
Fuel Pump Type ............... Gear-type, transfer
Governor Type ............... Electronic
Injection Pressure, bar ............... 800-1100
Fuel Type ............... Diesel Fuel

Dimensions and Weight

Overall Length, in (mm) ............... 42.8-46.6 (1085-1185)
Overall Width, in (mm) ............... 26.5-36.9 (675-935)
Overall Height, in (mm) ............... 35.7-36.5 (905-925)
Dry Weight, lbs (kg) ............... 1250-1760 (570-800)

Maintenance Data

Service Information

Thermostat begins opening ............... 82C (180F)
Thermostat fully open ............... 95C (203F)
Cooling system pressure ............... 70-90 kPa (10-13 psi)
Operating oil pressure ............... 300-450 kPa (44-65 psi)
Idle oil pressure ............... 100-150 kPa (15-22 psi)
Intake Valve Clearance, in (mm) ............... 0.015 (0.38)
Exhaust Valve Clearance, in (mm) ............... 0.025 (0.64)

Engine Oil and Coolant

Engine Oil Type .................. SAE 10W-30, 15W-40
API Service Classifications ......................... CH-4, CG-4, CI-4, or higher
Engine Oil Capacity, qts (L) .................. 27.3-29.4 (26.0-28.0)
Coolant Capacity, qts (L) ............... 26.3-42.0 (25.0-40.0)

Service Intervals

Change oil and filter ............... Every 250 hours
Replace fuel filters ............... Every 500 hours
Inspect air filter ............... Every 100 hours
Change coolant ............... Every 2 years
Valve-lash adjustment ............... Every 1000 hours
Injector inspection ............... Every 1000-1500 hours
Compression test ............... Every 2000 hours
Drain water separator ............... As required
Belt inspection ............... As required

Service Specifications

Cylinder Bore Inside Diameter, in (mm) ................ 4.3307-4.3323 (110.000-110.040)
Crankshaft Journal Outside Diameter, in (mm) ................ 3.4642-3.4650 (87.990-88.010)
Crank Pin Outside Diameter, in (mm) ................ 2.9524-2.9531 (74.990-75.010)
Crankshaft End Play, in (mm) ................ 0.0030-0.0130 (0.070-0.320)
Intake Valve Stem Outside Diameter, in (mm) ................ 0.3126-0.3134 (7.940-7.960)
Exhaust Valve Stem Outside Diameter, in (mm) ................ 0.3118-0.3126 (7.920-7.940)
Intake Valve Guide Inside Diameter (Installed), in (mm) ................ 0.3146-0.3161 (7.990-8.030)
Exhaust Valve Guide Inside Diameter (Installed), in (mm) ................ 0.3146-0.3161 (7.990-8.030)
Intake Valve Stem to Guide Clearance, in (mm) ................ 0.0012-0.0035 (0.030-0.090)
Exhaust Valve Stem to Guide Clearance, in (mm) ................ 0.0020-0.0043 (0.050-0.110)
Intake Valve Spring Free Length, in (mm) ................ 2.441 (62.00)
Exhaust Valve Spring Free Length, in (mm) ................ 2.441 (62.00)
Intake Cam Height, in (mm) ................ 2.6322 (66.857)
Exhaust Cam Height, in (mm) ................ 2.5855 (65.671)
Piston Pin Outside Diameter, in (mm) ................ 1.5745-1.5748 (39.993-40.000)
Piston Pin to Piston Bore Clearance, in (mm) ................ 0.0003-0.0011 (0.008-0.027)
Second Ring Side Clearance, in (mm) ................ 0.0024-0.0043 (0.060-0.110)
Oil Ring Side Clearance, in (mm) ................ 0.0016-0.0035 (0.040-0.090)
Top Ring End Gap, in (mm) ................ 0.0138-0.0217 (0.350-0.550)
Second Ring End Gap, in (mm) ................ 0.0236-0.0315 (0.600-0.800)
Oil Ring End Gap, in (mm) ................ 0.0098-0.0217 (0.250-0.550)

Tightening Torque Specs

Cylinder Head Bolts ............... 170-190 Nm (125-140 lb-ft)
Main Bearing Bolts ............... 200-230 Nm (148-170 lb-ft)
Connecting Rod Bolts ............... 120-140 Nm (89-103 lb-ft)
Flywheel Bolts ............... 180-210 Nm (133-155 lb-ft)
Crankshaft Pulley Bolts ............... 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)

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• Caterpillar Engines Specs
  
• Cat 3126 DITA Specs
  
• Cat 3126B Specs
  
• Cat 3176 Specs
  
• Cat 3176B Specs
  
• Cat 3176C Specs
  

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