The 4 Most Common 3.9L Cummins 4BT Engine Problems

The 4BT is virtually a bulletproof engine. It is extremely simple and has virtually no electronic components leaving very few things that can go wrong. However, there are a couple of common problems with these engines worth mentioning.

Cracked cylinder heads is the main problem but the Cummins 4BT does also suffer from fuel injector issues and possible dowel pin failure. We’re going to discuss each of these problems in depth and provide some additional information on specs and engine power limits as added power is the primary cause of all these engine problems.

Engine Specs

	4BT	4BTA
Engine	3.9L Cummins	3.9L Cummins
Displacement	239 cu. In. (3.92L)	239 cu. In. (3.92L)
Configuration	Inline-4	Inline-4
Bore x Stroke	4.02 x 4.72	4.02 x 4.72
Compression	16.5:1 to 18.5:1	17.5:1
Firing Order	1-3-4-2	1-3-4-2
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
Horsepower	105hp-130hp	170hp
Torque	265-385lb-ft.	420lb-ft.
Length	30.6 in	30.6 in
Height	37.7 in	37.7 in
Width	24.6 in	24.6 in

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 engine 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, these engines 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

1. Dowel Pin Failure

Dowel pins 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. Fuel Injector Failure

The 4BT fueling system is a direct injection system with a mechanical injection pump. Fuel system issues 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 it 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.