Written By, Mike McGlothlin
Here at Big Bear Engine Company, we offer Long Block and Extended Long Block versions of the legendary 3.9L Cummins 4BT. And while it’s easily one of the best light-duty diesel engines ever produced, there is no denying that this industrial-grade four cylinder has its share of shortcomings. In fact, a few of the 4BT’s flaws can result in catastrophic engine failure. But fortunately, even the 4BT Cummins’ most severe pitfalls can be addressed before destruction strikes—and they can even be avoided altogether. In this guide, we break down some of the most common Cummins problems owners encounter with the Cummins 4BT engine and how to fix them. From the killer dowel pin to cracked cylinder heads, and injector issues to the ill-effects of NVH, we’ll detail every major 4BT weak link in this article—and then we’ll provide a permanent solution.
Problem #1: Killer Dowel Pin (KDP)
What Is It?
We’ve taken a deep dive into the killer dowel pin issue—and screamed from the
mountaintops why it’s so important to address it (sooner rather than later)—on many occasions. If our message reaches just one 4BT Cummins owner, we’ll consider it a resounding success. The killer dowel pin (or KDP, for short) is the 5/16-inch (8mm) diameter piece of steel that was used on the factory assembly line to mount the front gear cover to the block, and it earned its nickname for the carnage it causes if it works itself loose from the crankcase. The KDP affects all ’83-’98 model year 4BT Cummins engines, as well as 5.9L’s (6BT and early ISB) built from 1989 to 2002.
How It Fails
While the KDP may never strike in a 4BT Cummins, until it’s addressed the potential for it
to work itself free from the block is always on the table. High mileage and operating hours, age, vibration, or a mix of all of the above can eventually allow the dowel pin dislodge itself from its bore. When this happens, it can drop into the front gear train (first hitting the cam gear), which can lead to immediate piston-to-valve contact, a ruined front gear cover, and shrapnel circulating throughout the engine. In some cases, an owner lucks out in that the KDP manages to avoid the gear train fiasco by making its way safely down into the oil pan. Left unchecked, Cummins problems like this can quickly turn a running Cummins 4BT engine into a non-runner.
The Solution
A bit of labor is involved in eliminating the KDP threat, but it’s a task that’s 100-percent worth the peace of mind it brings. Installing a stake that pins the KDP in place (i.e. covering it in a way that permanently keeps it in the block) is much more common than KDP removal, but either method is a solution. The job calls for a new crank seal and front cover gasket, as well as tearing into the front of the vehicle. The fan, fan shroud, and front gear cover all have to be removed to gain access to the KDP, and it’s a job that normally takes a veteran mechanic 4 to 6 hours to complete.
Problem #2: Blown Head Gasket
What Is It?
Given the design of a traditional style engine, where the cylinder head and cylinder block
are separate pieces, head gasket issues are never out of the question. It’s a gasket that relies on properly machined, flat surfaces and sufficient clamping force (via head bolts, or studs) to maintain that perfect seal. Once this vital head-to-block seal is broken, and depending on where the seal is lost, the engine’s cooling system can become pressurized, rampant coolant loss can occur, or the engine oil can become highly contaminated. Age, added performance, abuse, or even a single overheat scenario can contribute to a blown head gasket on a 4BT Cummins.
Why It Fails
Beyond age and structural integrity, added performance can often contribute to head
gasket failure. And the failure of an owner to plan ahead in the form of using head studs as opposed to head bolts (for improved clamping strength) and/or cylinder head O-rings (for better combustion sealing) can result in the head lifting off the block. Overheating, excessive cylinder pressure, and elevated boost and drive pressure are all common contributors to a blown head gasket on the 4BT. With a few mechanical fueling tweaks, significant gains in horsepower and torque can be realized on this engine—but without the proper supporting modifications in place the added power can take a toll on the head gasket.
The Solution
For unmodified or slightly modified Cummins 4BT’s, the key to avoiding a blown head gasket is to keep tabs on coolant, engine oil, and even exhaust gas temperature (EGT). Excessive engine oil temp and EGT can contribute to elevated coolant temps, and once things boil over the cylinder head can warp, leading to an eventual head gasket issue. For highly modified engines, all of the above applies—as well as the addition of head studs in place of head bolts to keep the head securely fastened to the block under heightened boost, drive, and cylinder pressure. In cases of extreme horsepower, an O-ringed or fire-ringed cylinder head should be considered.
Problem #3: Cracked Cylinder Head
What Is It?
It might come as a surprise to some, but finding a crack in a cylinder head isn’t exactly
uncommon, especially in the cast-iron units of old. Most begin as a hairline crack and, depending on where the crack is located, can either go unnoticed for tens of thousands of miles or become immediately apparent. And when a crack spreads into a valve seat, it can spell disaster—especially if it leads to a dropped valve seat scenario. Cracks can also lead to coolant loss (with no visible external leaks), loss of compression, coolant mixing with engine oil, and white smoke leaving the tailpipe.
Why It Happens
Age, overheating, and abuse can all contribute to a cracked cylinder head. Over time,
thousands upon thousands of heat cycles can fatigue the factory casting. Add elevated heat (in the form of high EGT due to performance mods) and you increase the possibility of cracking a head. Modified or not, cracks occur in roughly 3-percent of all 4BT (and 6BT) cylinder heads, according to our numbers. In most cases, the same symptoms of a blown head gasket are present. With either failure (head gasket or cracked head), the cylinder head has to be removed, so there is no added labor there. However, the cost difference between buying a new head vs. resurfacing the original can be substantial.
The Solution
The only real fix for a cracked 4BT cylinder head is to start with a fresh one. OEM, remanufactured OEM, and aftermarket versions are available, with some of the latter heads being manufactured with thicker castings. Once installed, the best way to avoid cracks is through monitoring coolant temp and EGT—and ensuring that neither one of them ever get out of hand. Never allow coolant to exceed 220 degrees F, and don’t let EGT live at 1,200 degrees F sustained. Also refrain from lugging the engine at low rpm for extended periods of time, where poor coolant flow and reduced airflow can get both parameters on the warm side.
Problem #4: Cracked Valve Seats
What Are They?
Cracked valve seats often lead to dropped valve seats, which can mean a bent or broken
valve, piston and head damage, and even cylinder wall gouging. As mentioned, our records show that approximately 3-percent of all 4BT cylinder heads crack. And because not all cracks occur in the valve seat area, that means a far smaller percentage of heads suffer from cracked valve seats. However, it can and does happen in some 4BT heads, and the results can be devastating as they can culminate in a dropped valve seat. Unfortunately, instantaneous damage is done when a valve seat comes apart.
How They Fail
When the pressed-in valve seats are exposed to extreme EGT on a regular basis, as well as
overheating coolant, the seats can become fatigued, more brittle, and begin to crack. Operating at high EGT levels for sustained intervals is the most common culprit, and the elevated temperature the valve seats are exposed to can even allow them to move within the head itself. Other leading causes are high engine rpm, the size and material of the seat (especially if aftermarket), and the amount of crush on the valve seats—which is often just 0.002 to 0.003-inches in an OEM cylinder head.
The Solution
While there is no way to know if an assembled engine is harboring cracked valve seats,
there are ways to ensure they don’t become damaged. The biggest key here is to avoid excessive EGT and engine coolant temp at all costs. As for remanufactured heads, oversized valve seats are a great way to guard against dropping. But more than that, the oversize seats should be made of an OEM-equivalent (or better) material, be heat-treated to resist cracking, radiused on the inboard side, and pressed in deeper than the factory valve seats were.
Problem #5: Oil Pump & Oil Cooler
What Is It?
This issue primarily only effects modified 4BT Cummins engines, but because most 4BT’s
aren’t left at the factory power level it’s worth diving into. And no, the factory oil pump and oil cooler don’t outright fail. They’re simply inadequate in higher horsepower applications. In plain English, both OEM components are undersized in engines where the stock horsepower rating has been doubled—and especially when it’s been tripled. Trust us, a Cummins 4BT that’s been fitted with a P7100, larger injectors, and compound turbos better have an oil system upgrade of some kind, too.
Why The Under-Deliver
For starters, at higher horsepower the stock 4BT oil cooler is too small. The 5-plate design
of this heat exchanger isn’t enough to shed the kind of heat that’s created by elevated cylinder pressure, EGT, boost, and every other additional stress a modified engine produces. Super-heated engine oil changes the oil’s viscosity and, in extreme cases, it results in smoked rod and main bearings. As for the oil pump, a factory 4BT unit flows between 12 and 14 gpm, depending on the model. However, higher horsepower requires more than that if any sort of longevity is expected.
The Solution(s)
Luckily, the added oil flow and cooling needs of a modified 4BT can be met by raiding the 6BT Cummins’ parts shelf. The 7-plate oil cooler employed on the 4BT’s larger, six cylinder brother does a much better job of reducing oil temperature and, thanks to their similar engine designs, is a bolt-in proposition for the 4BT. Beyond that, remote, externally located liquid-to-air oil coolers have been tried with great success. As for the oil pump, it’s back to the 6BT parts bin once more as its OEM unit flows a little more than 16-gpm and swaps directly in place of the factory 4BT pump.
Problem #6: Cracked Injection Pump Gear
What Is It?
The fact that the 4BT Cummins is gear-driven is one of its biggest advantages. However,
the rare problem of a cracked injection pump drive gear does occur from time to time. A hairline crack often develops at the keyway and spreads outward, eventually passing through the thickest portion of the gear and ultimately out to the teeth. Thankfully, it’s a failure that is often caught before the gear itself has a chance to break. But occasionally one does come apart, and steel-on-steel carnage is the result.
Why It Fails And What It Leads To
Age and fatigue, exposure to excessive heat, or an over-torqued pump drive gear bolt can
all cause the pump gear to crack—and once it cracks there is no remedy other than complete replacement (be it a brand-new or used gear). Worse yet, a cracked injection pump gear can alter the injection timing within the engine. Symptoms of a cracked injection pump gear include poor performance and a loss of power, additional smoke out the tailpipe, and decreased all-around drivability.
The Solution
Sourcing an OEM injection pump gear is the only fix for this particular failure. Due to the gear’s vital role (as well as its location) within the 4BT engine, it’s not worth chancing it by attempting to weld the crack. A brand-new gear isn’t cheap (think upward of $200) but used units can be sourced with a bit of digging. As for preventing a crack from forming, it’s important to never over-tighten the pump gear nut. Additionally, closely inspect the keyway to ensure no metal burring is present prior to installing the injection pump gear.
Problem #7: NVH
What Is It?
Take our word for it, shake, rattle, and roll isn’t as fun as it sounds… NVH stands for
Noise, Vibration, and Harshness, and the 4BT Cummins has plenty of it. The engine is notably loud, especially at idle, but is also notorious for its drone at cruising speeds and higher rpm. Highly vibratory, the 4BT typically gets a bad rap from folks who swapped the highly sought after Cummins 3.9L into place without performing the proper research. However, even seasoned diesel enthusiasts have been known to complain about this engine’s higher-than-average NVH. Don’t worry, there are a few ways to help quell it…
Why It’s So Primitive
Not unlike other inline-four cylinder engines, the 4BT Cummins has inherent vertical
shake due to its design, where a secondary imbalance creates a noticeable harshness when operating. There are also no balancer shafts on the 4BT (something most modern 2.0L and larger I-4 engines tend to have) or harmonic balancer from the factory. Combine that with its gear-driven design, thin front timing cover and oil pan, and direct injection, single shot injectors and you end up with some serious NVH. In some cases, vibration in high-mile and high operating hour engines has even been blamed for cracked timing gear covers…
The Solution
While the 4BT can’t be completely tamed, its inherent NVH can be made much more tolerable with a few tricks. First and foremost, high-quality motor mounts and correct motor mount location are crucial. Hydraulic mounts work wonders in helping dampen things down as well as isolate the engine from the chassis, and the mounts’ location(s) should always be above the center line of the crankshaft and sit at an approximate 45-degree angle. The motor mounts should also be centrally located on the engine block. Last but not least, backing off the injection timing of the 4BT (i.e. retarding it) can serve to quiet the engine some.
Problem #8: NVH (They Leak!)
What Is It?
It’s that three letter acronym again. NVH can (and often does) take its toll on an aging 4BT
Cummins, oftentimes causing it to lose some of its fluids. This isn’t a failure that will sideline the engine from duty, but it can make for one heck of an eyesore on your driveway, cause you to have to top off more often, or even create fumes that make their way into the cockpit. Valve covers, front timing cover, tappet cover, oil pan, oil feed and drain lines, fuel lines, vacuum lines, and even radiator hoses can all spring leaks with age—and all are common problem areas.
Why It Happens
Life is rough for the components sealed and bolted together on any diesel engine, but their
tenure aboard the 4BT Cummins may be as rugged as it gets. The combination of constant vibration mixed together with worn gaskets and seals creates the perfect environment for brittle or fatigued sources at the point of connection to spring a leak. Also consider the fact that the high operating pressures and temperatures the 4BT regularly sees don’t do these O-rings, seals, and gaskets any favors. Unfortunately, oil, fuel, and/or coolant leaks are an eventual part of life for this industrial grade four cylinder.
The Solution
There is no feasible or readily available fix for a 4BT that’s developed a fluid leak. However, addressing leaks, of any kind, as soon as they’re noticed is the best way to keep the problem from worsening and keep the engine in service. Leaks are a driving force behind the term “reseal.” Diesel engines that’ve racked up thousands of hours (or hundreds of thousands of miles) have long been prone to developing a drip-drip somewhere, if not at multiple locations. A reseal is an affordable way to keep a 4BT that is otherwise mechanically sound humming away on the jobsite, on the trail, or on the highway.
Problem #9: Overheating
What Is It?
This issue ties in with the blown head gasket problem outlined in number 2, as any form of
prolonged overheating can wipe out a 4BT Cummins’ head gasket. Boiling coolant is never welcome, and elevated heat here can lead to a myriad of other issues. Case in point, high coolant temp almost always means elevated engine oil temp—both of which are bad news for your engine. And when excessive coolant temps are sustained for lengthy periods of time, the cylinder head can become warped. In worst case scenarios, the warpage can even render the head unsalvageable.
Why It Happens
Overheating can occur for various reasons, but the most common causes of a 4BT
Cummins getting too hot are: a stuck thermostat, a failing water pump, a plugged radiator blocking adequate airflow, an inefficient or non-functioning engine fan, low coolant level, sustained exposure to high EGT (especially while towing), an overmatched oil cooler, or a cracked cylinder head. When overheating occurs due to excessive EGT (i.e. too much fuel, not enough air), piston damage can be the result, as can scarring and scuffing in the troubled cylinder’s bore. When it happens on account of neglect, all cooling system components should be inspected (hoses, clamps) and replaced (thermostat, water pump) or cleaned (cooling stack), if necessary.
The Solution
The solution for an overheating 4BT first has to be narrowed down to the root cause of it. However, it’s important that you never ignore elevated coolant temperature and continue to run the engine. Always rely on accurate water, engine oil, and exhaust gas temperature gauges to keep you informed, as those three parameters can all affect one another. Consistently high EGT will raise the operating temp of coolant and engine oil, and inadequate oil cooling (think high horsepower applications) will elevate coolant temperature. Again, run ECT, EOT, and EGT gauges and keep regular tabs on them.
Problem #10: Injector Issues
What Is It?
It’s not uncommon for the 4BT Cummins’ fuel injectors to last 400,000 miles. But
although these injectors are of the old-school, mechanical, pop-off style variety and are known for their reliability, they don’t last forever. Granted, they seldom fail catastrophically (unlike some of their high pressure common-rail counterparts sometimes do), but their performance does degrade over time. For example, at high mileage many 4BT injectors pop test as much as 50 to 60 bar below factory spec. That’s 580 to 725 psi lower than it should be. And even within the first 50,000 miles they can come out of spec…reason enough to have your injectors checked out any time they’re removed from the head.
What Fails
In most 4BT injectors, it’s a worn spring that kickstarts the engine’s fueling issues. A
weakened spring brings a lower pop-off pressure with it—which essentially means the engine’s injection timing has been advanced. Reduced performance and more smoke out the tailpipe are often the result. The next wear item is the plunger, which can fail to completely seal following the injection event. This allows fuel to dribble out of the nozzle long after the end of injection. Nozzle wear, where the orifices enlarge over time, leads to decreased atomization and increase smoke levels as well. Contaminated fuel, deposits within the fuel, and age all contribute to worn out 4BT injectors.
The Solution
For utmost injector longevity, only fill the tank with quality, uncontaminated diesel fuel. This is best achieved by topping off at high traffic filling stations, where the fuel turnover rate is high and fresh diesel is always on tap. Next, perform fuel filter and water separator replacements at (or before) the specified change intervals. Additionally, never allow air to infiltrate the fuel system, and if an air-in-fuel situation does occur (such as replacing injectors), make sure the system is fully primed before starting the engine. Finally, only use a reputable injection shop to rebuild, test, and install your injectors. They are simple—with internal components consisting only of the shim(s), a spring, intermediate plate, nozzle, plunger, and the injector cap—but each unit must be properly shimmed to achieve optimal performance and a long service life. Addressing these Cummins problems with the right diagnostics, maintenance practices, and quality 4BT Cummins parts will keep your Cummins 4BT Eengine working hard for years to come.
