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Cummins 4BT 3.9L Engine (Specs, Reliability & Issues)

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Last Updated on: 16th November 2023, 02:14 am

The Cummins 4BT engine is 3.9L a water-cooled inline four-cylinder turbodiesel engine from the Cummins B series.

This 3.9-liter straight-four, and 5.9-liter straight-six, engines are the most popular engines in the B series (12 valve Cummins).

In this article, we’ll discuss the specs and design of the Cummins 3.9L 4BT engine, as well as its reliability and issues.


The engine is cast iron, with engine bores drilled straight into the block. With the exception of the gearbox adapter plate, which differs depending on the application, all 4BT engine blocks are similar.

The engine’s valvetrain is OHV.

A camshaft is positioned in the block and is driven by the crankshaft through gears at the front end of the engine (as well as the oil pump).

Cummins 4BT engine

The camshaft opens and closes valves by forcing them open and shut using rocker arms, pushrods, and solid tappets.

The cylinder head is similarly made of cast iron. On opposite ends of the car, the intake and exhaust ports are positioned (crossflow cylinder head).

In all, there are eight valves, two for each cylinder. The 4BT comes with mechanical direct injection technology.

In the early models, the P7100 mechanical injection pump is employed, which is controlled by the camshaft’s gear through a gear.

On the 4BT engine, the turbocharger is not intercooled. The compression ratio of the intercooled variation (called the 4BTA after the letter A, which stands for “Aftercooled”) is 16.5:1.

With low compression and cooled air, you may get a bigger boost.

The 4BTA engine offers more power and torque, making it more suitable for usage in a light truck or pickup, but keep in mind that it is an industrial engine.

Automotive charge air-cooled engines have different numbers according on the kind and year of manufacturing.

There’s also the 4B, a naturally aspirated industrial variation with an 18.5:1 compression ratio. The 4BT and the 6BT have a lot in common.

It’s built in the same way as the original, and many of the parts are interchangeable. The pistons, injectors, connecting rods, valves, pushrods, and rocker arms are all the same.

The 4BT is a four-cylinder Cummins engine with a smaller displacement and shorter length than the 12 valve Cummins engine.

The 4BT produces more vibrations and is less smooth to run than the ISB 5.9-liter Cummins since it is not automatically balanced like the six-cylinder 6BT engine.

The 3.9-liter Cummins was employed in a variety of commercial applications, including vans and other commercial vehicles.

4B engines were used in industrial equipment such as power units, drilling machines, huge water pumps, and wood chippers.

Engine swaps on Jeeps, Dodge pickup trucks, and smaller trucks/SUVs are fairly prevalent in today’s world.

Engine Specs

  • Manufacturer: Cummins
  • Production years: 1984-1998
  • Cylinder block material: Cast Iron
  • Cylinder head material: Cast Iron
  • Fuel type: Diesel
  • Fuel system: Direct Injection (DI), mechanical injection pump
  • Configuration: Inline
  • Number of cylinders: 4
  • Valves per cylinder: 2
  • Valvetrain layout: OHV
  • Bore, mm: 4.02 inch, 102 mm
  • Stroke, mm: 4.72 inch, 119 mm
  • Displacement: 239 cubic inches, 3.9 liters
  • Type: Four-stroke, turbocharged
  • Compression Ratio: 17,5:1 (4BT), 16.5:1 (4BTA), 18.5:1 (4B)
  • Power: 80-130 hp at 2,300-2,600 rpm
  • Torque: 243-391 lb-ft (330-530 Nm) at 1,600-1,700 rpm
  • Engine weight: 855 lbs, 388 kg dry
  • Firing order: 1-3-4-2
  • Engine oil capacity: 10 qts (9.5 liters) with filter
  • Oil change interval: 6,000 miles (10,000 km) or 6 months
  • Applications: Industrial equipment, vans and trucks, marine vehicles and boats

Reliability & Issues

Because of its widespread usage in a number of applications, the 3.9-liter engine can be found promptly and at a reasonable cost.

The B series engines contain a small number of electrical components, making them cheap and easy to replace or repair.

Diesels with P-pump are simpler to fix and more trustworthy than contemporary diesels with complex electric VP pumps.

On the other hand, the 4BT is a large, vintage diesel engine. It’s as noisy, shaky, and smoky as a vintage diesel engine.

The 4BT is difficult to put under the hood of a smaller vehicle because to its large size – it is tall and heavy, and it also needs room for an intercooler and pipes.

It lacks the power of a full-size truck or SUV, and its weight adds extra strain to the front axle, impacting the handling of a tiny 4×4.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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