Article Updated: January 18, 2023
The 3.9L Cummins 4BT engine was released in 1983 and was predominantly used in smaller commercial vans such as bread trucks and other small delivery trucks. Despite its initial use in commercial vehicles, the engine has become extremely popular in the 4×4 and off-road communities for engine swaps. Additionally, it is commonly used in farm equipment, forklifts, dozers, and other various types of small machinery.
At only 30.6in in length, 24.6in of width, and 37.7in of high, the engine can be swapped into just about any car. Making 105hp and 265lb-ft of torque, it is great engine options for Jeeps, Broncos, 4Runners, buggies, and other offroad vehicles. However, despite it’s small size, it still weighs approx. 800lbs.
The 4BT is essentially the exact same as the 5.9L Cummins 6BT (12v), just minus two cylinders. The 3.9L Cummins 4-cylinder had two variations: the 4BT and the 4BTA. The 4BTA, launched in 1998, was increased from 8 valves to 16 valves, improving performance to 170hp and 420lb-ft of torque.
3.9L Cummins Engine Specs
|Engine||3.9L Cummins||3.9L Cummins|
|Displacement||239 cu. In. (3.92L)||239 cu. In. (3.92L)|
|Bore x Stroke||4.02 x 4.72||4.02 x 4.72|
|Compression||16.5:1 to 18.5:1||17.5:1|
|Block||Cast iron||Cast iron|
|Head||Cast iron||Cast iron|
|Injection||Direct injection||Direct injection|
|Aspiration||Turbocharged, no intercooler||Turbocharged w/intercooler|
|Valvetrain||OHV, 2-valve||OHV, 4-valve|
|Oil Capacity||10 quarts||10 quarts|
|Length||30.6 in||30.6 in|
|Height||37.7 in||37.7 in|
|Width||24.6 in||24.6 in|
4B vs 4BT vs 4BTA
Both the 4BT and 4BTA are extremely similar. The “T” stands for turbocharged – both engines use the same turbo. The primary difference in the 4BTA is that it includes a factory intercooler. The “A” stands for after-cooled which is synonymous for an intercooler. The 4BTA uses an air-to-air intercooler and more aggressive tuning which allow it to produce more horsepower and torque than the non-intercooled 4BT version. Additionally, the 4BTA changed designs to use 4-valves per cylinder compared to the standard 2-valves per cylinder used in the 4B and 4BT.
There is also a less popular version of the engine, known as the 4B. This is a naturally aspirated version of the 4BT that only produces 53hp. The lack of power from this engine means it isn’t used much in vehicles and is more so reserved for stationary commercial equipment.
4BT Fuel Pumps and Power Potential
Despite the 4BTA producing more stock power, the 4BT is still the engine of choice for swaps. There are two fuel pumps that the 4BT can be equipped with, a standard inline A pump, or the P7100 pump.
When outfitted with the P7100 pump, the engine can be modified to produce material power. However, the 4BT is a very basic engine and does not feature an intercooler. Therefore turbocharger updates, fueling upgrades, etc. are going to require the additional of other power-supporting systems. Long-story short, the 4BT can produce impressive power, but it will be expensive to make it do so.
Cummins 4BT Engine Problems
- Dowel Pin Failure
- Fuel Injector Failure
- Drivetrain and Suspension Stress
- Cracked Cylinder Heads
The 4BT is virtually a bulletproof engine. Before I point out a few potential issues with the engine, I want to reiterate that these engines are brutally reliable and can withstand beating after beating. However, as is with any engine, certain things are prone to needing replacing from time to time. Fortunately, the 4BT engine is so simple with almost no electronic components, that there are very few things that can go wrong with the engine.
The caveat is that adding additional power to the any engine of course increases the likelihood of problems arising. The only major problem to consider is the cylinder heads. They generally are very reliable but adding power can increase the chances of the cylinder head cracking.
1. Cummins 4BT Dowel Pin Failure
Dowel pins on the 4BT are known to become loose or crack. Dowel pins are used to align the drive plate or flywheel with the crankshaft. On the Cummins 4BT, the dowel pins were made from steel. Because of heat properties of the steel, the pin is constantly expanding and contracting from the engine heat cycles. This can cause it to loosen, or even fall completely out of the casing.
When the pin becomes loose, it will eventually fall out of the casing. When this happens, there are a few different scenarios. Best case, the pin falls and lands in the oil pan without causing any damage. Secondly, the pin can fall into the timing gearcase, hit one of the gears, and then eject out of the crankcase. When this happens, oil pressure will drop instantly and the engine will experience significant internal damage from a lack of lubrication.
Worst case, the pin gets stuck in the gears, throwing the timing off and breaking the camshaft. This is commonly referred to as the “Killer Dowel Pin” because it virtually destroys the engine.
Killer Dowel Pin Prevention
Because of the commonality of dowel pin issues across the 4BT and 6BT Cummins, there are dozens of prevention kits on the market. Prevention kits include a small tab that is placed over the pin to prevent it from falling out of place. Alternatively, the pin can be JB-welded into place.
Killer Dowel Pin Kits are inexpensive, but installing them requires the valve cover, cylinder head, and camshaft to be removed. Additionally, to access to timing gear case, you will also need to remove the lower pulley, fan and shroud, and overflow containers.
2. 4BT Fuel Injector Failure
The 4BT fueling system is a direct injection system with a mechanical injection pump. Fuel system issues with the 4BT are generally less common and less expensive to fix compared to the common-rail fuel systems that newer diesels use.
However, as is with any direct injection system, the fuel injectors themselves are prone to leaking and clogging. Because fuel injectors are highly pressurized, over time they can begin to lose their pressure-holding capabilities which can cause the injectors to leak fuel. When this happens, you will experience a lot of misfires, poor fuel economy, and poor idling.
Alternatively, the injectors can get gunked up and clogged from bad diesel fuel and other small particles and contaminates running through your fuel system. Gunked injectors will also cause misfires, poor idling and performance, but can also cause hard start or no start issues.
Fuel Injector Failure Symptoms
- Cylinder misfires
- Rough idling
- Hard start / no start
- Decreased fuel economy
- Poor acceleration and overall performance
Fortunately, a new set of 4BT fuel injectors will run you $300-$500, compared to the typical $1.5k-$2k on 6 cylinder or 8 cylinder diesels with more advanced fueling systems. While injectors tend to fail individually we recommend replacing all of them at the same time since they aren’t very expensive.
3. Drivetrain and Suspension Stress
Despite being a very small engine in size, the 4BT is extremely heavy at nearly 800lbs. To put that into comparison, my 7.3L V8 Powerstroke only weighs about 920lbs. So, despite being nearly half the size, it barely weighs less.
While this isn’t an issue if you are swapping the 4BT into a normal sized truck, it can cause some issues and wear and tear on smaller vehicles. The added weight can put extra stress on the drivetrain as well as the front suspension which can lead to frequent maintenance and repair on components not capable of handling the weight.
At 800lbs, the weight of the engine is about 1/3 that of a complete Jeep Wrangler which can cause poor handling. Additionally, it will put a lot of strain on suspension components like bushings, shocks/struts, control arms, etc. along with the axles themselves.
While this isn’t necessarily a “problem” you will need to factor the weight into any swaps and consider upgrading your suspension components and drivetrain.
4. Cracked Cylinder Heads
While the 4BT cylinder heads generally aren’t an issue, it is estimated that about 3% of engines develop a cracked cylinder head. Between the 3rd and 4th holes, the cylinder head can crack and burn pistons, causing the need for a new head and potentially new pistons.
Cummins eventually changed the heads from 9mm injectors to 7mm due to cracking occurring with the 9mm’s. Smaller injector holes makes the thickness between the holes greater, causing for less cracking. When a crack forms, you can either replace the head, or continue as if nothing is wrong. Plenty of 4BT owners claim that small cracks won’t effect the performance of the engine and therefore aren’t necessary to replace.
However, if you have large cracks, a new head is going to be required. Fortunately, new heads can be found for less than $1,000.
Cummins 4BT Reliability
The 4BT engine is a workhorse and is nearly bulletproof. The Cummins 6BT has continually been claimed as the most reliable diesel engine to ever be built. Considering the 4BT is built with the exact same 6BT parts, these engines are just as reliable and dependable.
Additionally, the 4BT has very few electronics, has a simple fuel system setup, and doesn’t have an intercooler or other systems commonly found in turbocharged engines. Because of the simplicity of the engine, the only real problems that can arise relate to the block itself, the head, and the internals. Fortunately, these parts are all extremely well manufactured and frequently last beyond 500,000 miles without any issues.
Overall, this engine is a beast when it comes to reliability and dependability.