Cummins 4BT 3.9L Engine: Specs, Problems, and Reliability. The 4BT engine is inline four-cylinder water-cooled turbodiesel engine in the Cummins B series. The 3.9-liter strait-four along with 5.9-liter straight-six (12 valve Cummins) are the best popular engines in B series. There are 4.5-liter and 3.3-liter four-cylinder engines also available.
The engine has a cast-iron engine block with engine bores device directly into the engine block. All 4BT engine blocks are the same, and they have a different transmission adapter plate only depending upon the application. The engine valvetrain is OHV. A camshaft is mounted into the block and driven by a crankshaft through gears in the front end of the engine (as well as the oil pump).
Table of Contents
Cummins 4BT 3.9L Engine: Specs, Problems, and Reliability
The camshaft opens and closes valves pushing through rocker arms by pushrods and solid tappets. The cylinder head is made of cast iron too. The intake and exhaust ports are on opposite sides (crossflow cylinder head). There are eight valves, 2 per cylinder. The 4BT features the mechanical direct injection system. The early versions are equipped with the P7100 mechanical injection pump driven through a gear by camshaft’s gear.
The 4BT engine has a turbocharger, but it is not-intercooled. Intercooled version (named as the 4BTA, where letter A means “Aftercooled”) has a lower compression ratio of 16.5:1. Low compression and cool air represent a higher boost. The 4BTA engine produces more power and torque and is more suitable for installation in a tiny truck or pickup, but keep in mind what the 4BTA is an industrial engine. Automotive charge air-cooled engines have different numbers depending upon model and year of production. There is the industrial naturally aspirated version named the 4B with an 18.5:1 compression ratio also available.
Cummins 4BT 3.9L 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, cc | 239 cubic inches, 3.9 liters |
| Type of internal combustion engine | Four-stroke, turbocharged |
| Compression Ratio | 17,5 : 1 (4BT) 16.5 : 1 (4BTA) 18.5 :1 (4B) |
| Power, hp | 80-130 hp /2,300-2,600 |
| Torque, lb ft | 243-391 lb-ft(330-530 Nm)/1,600-1,700 |
| Engine weight | 855 lbs, 388 kg dry |
| Firing order | 1-3-4-2 |
| Engine oil weight | – |
| Engine oil capacity, liter | 10 qts (9.5 liters) with filter |
| Oil change interval, mile | 6,000 (10,000 km) or 6 month |
| Applications | Industrial equipment, commercial vehicles (step vans, trucks), marines and boats |
The 4BT is incredibly similar to the 6BT. It has the same architecture, and many parts are interchangeable. The pistons, injectors, connecting rods, valves, pushrods, and rocker arms are identical. The 4BT is a smaller displacement and shorter in length four-cylinder version of the 12 valve Cummins engine.
The 4BT is not naturally balanced like the six-cylinder 6BT engine, so it produces a lot of vibrations and not so smooth effective compared to the 5.9-liter Cummins. The 3.9-liter Cummins was the popular choice for many commercial step van applications (Chevy Step Vans, for example), including bread vans and other commercial vehicles. The 4B engines were commonly used in industrial equipment such as power units, drilling makers, big water pumps, wood chippers, etc. In our day, it has massive popularity as engine swaps in Jeeps, Dodge pickups, and smaller trucks/SUVs.
4BT Cummins Problems and Reliability
The 3.9-liter engine was extensively used in different applications, which means it could be found easily and for a meager price. The B series engines require really few electronics, and they are straightforward and cheap to rebuild or repair. Diesels with P-pump are more comfortable to modify and more reliable compared to newer diesels with modern electric VP pumps. The fact is the 4BT is an old, massive diesel engine.
It is noisy, shaky, smoky as an old diesel. The 4BT is quite difficult to install under a hood of a tiny vehicle as a result of its large dimensions – it is tall and weighs a lot, also needs a place for an intercooler and pipes. It is not enough power for a full-size truck or SUV, concurrently its weight puts more strain on a front suspension and make an adverse effect on the handling of a small 4×4 vehicle.
