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Cummins ISB 3.9 Engine

The Cummins ISB 3.9 is a medium-duty, inline 4-cylinder, 3.9-liter displacement turbocharged diesel engine. Engineered as an electronically managed evolution of the classic mechanical B-series architecture (4BT), the ISB 3.9 is designed for commercial vehicles, step vans, light trucks, and industrial equipment requiring a compact but highly durable footprint. The engine construction features a deep-skirted cast iron block with parent-bore cylinders (no wet liners) to maximize structural rigidity while keeping dimensions tight. A single piece cast-iron cylinder head incorporates advanced cross-flow porting to optimize thermal efficiency and breathing.

Electronic control parameters are governed via a full-authority Engine Control Module (ECM) to actively optimize fuel maps, reduce combustion noise, and manage emission profiles through precise fuel and air-handling controls. Valvetrain Layout: Overhead Valve (OHV) pushrod-driven architecture activated by a gear-driven, block-mounted camshaft. Total Valves: 16 valves total, configured with 4 valves per cylinder (two intake, two exhaust) to enhance volumetric efficiency and gas exchange over legacy 2-valve iterations.

The ISB 3.9 engine incorporates a High-Pressure Common Rail (HPCR) fuel injection architecture. High-pressure fuel delivery is generated via a timing-gear-driven Bosch high-pressure injection pump, which continuously feeds a heavy-walled fuel rail cylinder acting as an accumulator. Fuel metering and exact timing intervals are managed electronically by the ECM using centralized, vertically oriented electronic solenoid fuel injectors. This system delivers multiple precise injection events per combustion cycle (pilot, main, and post-injection). The low-pressure lift circuit relies on an electric transfer pump to pull fuel through a water-separating fuel filter, maintaining standard inlet suction limits below 20.3 kPa to prevent supply cavitation.

The lubrication system is driven by a high-capacity internal gear-type oil pump. The block casting integrates internal oil rifle drillings that deliver pressurized lubrication directly to the main bearings, camshaft, and overhead rocker assemblies. The circuit is protected by an integral liquid-to-liquid oil cooler element built into the block to mitigate oil degradation under heavy duty cycles.

Engine thermal regulation relies on a closed, forced-circulation liquid architecture managed by a high-flow, belt-driven water pump integrated into the front gear cover housing. The engine block jacket design forces localized cooling passages directly around the fuel injector sleeves and upper cylinder walls. The maximum operating static pressure threshold for the cooling circuit is specified at 100 kPa, with a maximum allowable peak coolant limit of 100C before initiating automatic electronic engine protection protocols.

General Technical Data

Engine Specifications

Engine Model ............... Cummins ISB 3.9
Engine Type ............... 4-cycle inline 4-cylinder common rail direct injection liquid-cooled diesel
Number of Cylinders ............... 4
Cylinder Arrangement ............... In-line
Engine Bore, in (mm) ............... 4.02 (102.0)
Engine Stroke, in (mm) ............... 4.72 (120.0)
Displacement, cu.in (L) ............... 239 (3.9)
Rated Engine Power, hp (kW) ............... 60-150 (45-112)
Rated Engine Speed, rpm ............... 2200-2800
Peak Torque ............... 260-540 Nm (192-398 lb-ft) at 1400-1800 rpm
Idle Speed ............... 700-800 rpm
Aspiration Type ............... Turbocharged
Compression Ratio ............... 16.5:1-17.5:1
Firing Order ............... 1-3-4-2
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 ............... Belt-driven, centrifugal
Air Cleaner Type ............... Dry dual-stage
Starting System ............... Electric Starter
Electrical System ............... 12V or 24V
Alternator Rating, Amps ............... 60-120

Fuel System

Fuel System ............... Common rail direct injection
Fuel System Control ............... Electronic
Governor Type ............... Electronic
Injection Pressure ............... 1350 bar
Fuel Type ............... Diesel Fuel

Dimensions and Weight

Overall Length, in (mm) ............... 35.3-39.4 (900-1000)
Overall Width, in (mm) ............... 27.6-31.5 (700-800)
Overall Height, in (mm) ............... 35.4-39.4 (900-1000)
Dry Weight, lbs (kg) ............... 750-840 (340-380)

Maintenance Data

Service Information

Thermostat begins opening ............... 82C (180F)
Thermostat fully open ............... 95C (203F)
Cooling system pressure ............... 70-100 kPa (10-15 psi)
Operating oil pressure ............... 275-380 kPa (40-55 psi)
Idle oil pressure ............... 70-100 kPa (10-15 psi)
Intake Valve Clearance, in (mm) ............... 0.254 (0.010)
Exhaust Valve Clearance, in (mm) ............... 0.508 (0.020)

Engine Oil and Coolant

Engine Oil Type .................. SAE 10W-30, 15W-40
API Service Classifications ......................... CI-4, CJ-4, CK-4
Engine Oil Capacity, qts (L) .................. 9.5-12.6 (9.0-12.0)
Coolant Capacity, qts (L) ............... 10.5-14.7 (10.0-14.0)

Service Intervals

Change oil and filter ............... Every 250 hours
Replace fuel filters ............... Every 300 hours
Inspect air filter ............... Every 100 hours
Replace air filter ............... Every 300 hours
Change coolant ............... Every 2000 hours
Valve-lash adjustment ............... Every 1000 hours
Injector inspection ............... Every 1500-2000 hours
Compression test ............... Every 2500 hours
Drain water separator ............... As required
Belt inspection ............... As required

Service Specifications

Cylinder Bore Inside Diameter, in (mm) ............... 4.0157-4.0169 (102.000-102.030)
Crankshaft Journal Outside Diameter, in (mm) ............... 3.2673-3.2685 (82.991-83.021)
Crank Pin Outside Diameter, in (mm) ............... 2.7161-2.7173 (68.987-69.013)
Crankshaft End Play, in (mm) ............... 0.0043-0.0130 (0.109-0.330)
Intake Valve Stem Outside Diameter, in (mm) ............... 0.2740-0.2748 (6.960-6.980)
Exhaust Valve Stem Outside Diameter, in (mm) ............... 0.2736-0.2744 (6.950-6.970)
Intake Valve Guide Inside Diameter, in (mm) ............... 0.2756-0.2768 (7.000-7.030)
Exhaust Valve Guide Inside Diameter, in (mm) ............... 0.2756-0.2768 (7.000-7.030)
Intake Valve Stem to Guide Clearance, in (mm) ............... 0.0008-0.0028 (0.020-0.070)
Exhaust Valve Stem to Guide Clearance, in (mm) ............... 0.0012-0.0031 (0.030-0.080)
Intake Valve Spring Free Length, in (mm) ............... 1.878 (47.70)
Exhaust Valve Spring Free Length, in (mm) ............... 1.878 (47.70)
Intake Cam Height, in (mm) ............... 1.8488-1.8654 (46.960-47.380)
Exhaust Cam Height, in (mm) ............... 1.8681-1.8846 (47.450-47.870)
Piston Pin Outside Diameter, in (mm) ............... 1.5746-1.5750 (39.994-40.006)
Piston Pin to Piston Bore Clearance, in (mm) ............... 0.0002-0.0006 (0.005-0.015)
Second Ring Side Clearance, in (mm) ............... 0.0016-0.0033 (0.040-0.085)
Oil Ring Side Clearance, in (mm) ............... 0.0012-0.0028 (0.030-0.070)
Top Ring End Gap, in (mm) ............... 0.0110-0.0189 (0.280-0.480)
Second Ring End Gap, in (mm) ............... 0.0177-0.0276 (0.450-0.700)
Oil Ring End Gap, in (mm) ............... 0.0098-0.0217 (0.250-0.550)

Tightening Torque Specs

Cylinder head bolts ............... 135-150 Nm (100-111 lb-ft)
Main bearing caps ............... 175-190 Nm (129-140 lb-ft)
Connecting rod bolts ............... 65-75 Nm (48-55 lb-ft)
Flywheel bolts ............... 220-260 Nm (162-192 lb-ft)
Crankshaft pulley bolt ............... 200-225 Nm (148-166 lb-ft)
Oil drain plug ............... 40-50 Nm (30-37 lb-ft)
Injector hold-down bolts ............... 20-25 Nm (15-18 lb-ft)

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• Cummins Engines Specs
  
• Cummins ISB 4.5 Specs
  
• Cummins ISB 5.9 Specs
  
• Cummins ISB 6.7 Specs
  

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