Written By, Mike McGlothlin
There’s no question that the Cummins 6BT is one of the most durable and sought after diesel engines ever assembled. Its simple yet robust design makes it a million-mile contender while its performance potential makes it the hot-rod engine of choice for truck pullers, drag racers, and dyno melters. But, like any engine that appears tough-as-nails on the surface, the 5.9L 12-valve does have a few chinks in its armor. Chiefly among them, you’ll find the killer dowel pin, a leaky tappet cover, blown rear freeze plugs, and a worn-out, horsepower-robbing overflow valve on P-pumped models. There are other quirks, of course, and we’ll cover those problems in detail below—as well as offer a solution for each.
Highlights
Most Common Problems:
*Leaking Tappet Cover
*Worn Overflow Valve (P7100 models)
*Underpowered (medium duty applications)
*Transmission Failure
Most Expensive Problems:
*Blown Head Gasket
*Transmission Failure
Less-Frequent Headaches:
*Rod Bolts (high-horsepower applications)
*Poor Drivability (’91.5-’92 engines)
*EGR Issues (’96-’98 California models)
*Blown Rear Freeze Plug
Problem #1: Killer Dowel Pin (KDP)
Premature camshaft lobe wear wasn’t the entire genesis behind Cummins’ redesign of the 
Back on the assembly line, every Cummins 6BT was fitted with an 8mm diameter dowel pin in order to properly align the front gear cover to the block. The dowel pin was pressed into the block, but not secured permanently. Over time, and after thousands of heat cycles and constant exposure to vibration, the small steel dowel can work itself free from the block. And by working itself free from the block, we mean it moves forward. The dowel pin rarely ever works itself further into the block, although some do remain in place for the entire life of the engine
What Happens
When the alignment dowel pin does come loose, it becomes what’s known throughout the6BT (and 4BT) world as the killer dowel pin (or KDP). Unfortunately, and because the KDPis made of steel, it won’t simply break apart if it comes into contact with anything. If theKDP falls into the cam gear, directly beneath it piston-to-valve contact is likely to happen,and total destruction of the engine can be the result. However, the KDP can also avoid thefront geartrain altogether and end up in the oil pan. In other cases, minimal damage isincurred, such as a cracked gear housing or chipped tooth on a gear.
Solution(s)
The best way to rule out a potential KDP catastrophe is to lock the dowel pin permanently in place. Various KDP solution kits are offered in the diesel aftermarket, which serve to tab, stake, or lock the KDP within the block forever. Our advice is to purchase an all-inclusive kit that comes with a new gear case gasket and a fresh crank seal. Installation requires tearing into the front of your 6BT Cummins, as well as at least 4 to 6 hours of labor, but the peace of mind gained is 100-percent worth your time and money.
Problem #2: Overflow Valve (P7100 Models)
Injection pump overflow valve issues are reserved specifically for Cummins 6BT’s equipped with the Bosch P7100 (’94-’98 on-highway/Dodge Ram applications, for example). Located on the return side of the P7100’s fuel circuit, its failure to function properly leads to low supply pressure being delivered to the injection pump (usually somewhere around 15-psivs. the 25-30-psi you really want to see). This lack of fuel supply pressure also means lower fuel volume making it to the P7100, which equates to less cooling and lubrication on the fuel side of the pump. Of course, an ailing overflow valve will also kill the performance of your 6BT
What Fails
In most cases, overflow valve problems stem from a worn internal spring. With age and extended use, the OEM spring weakens due to stretching (but sometimes even breaks). Key symptoms of spring failure include hard starting, loss of power, a miss at low engine rpm, reduced overall drivability, a warm engine that stalls at idle, an erratic, rough idle, and even a dead-pedal feel in the accelerator. In other instances, the overflow valve’s internal seat becomes rounded off (rather than flat) and begins to leak, allowing excess diesel to backflow back to the fuel tank
Solution(s)
All of the factory Bosch P7100 overflow valve spring’s shortcomings were resolved with an aftermarket overflow valve produced by Tork Teknology. Fully adjustable (and available for roughly $50), its overflow valve features a superior quality internal spring, which the company says will never stretch. Tork Teknology also redesigned the seat and ball, making it considerably more wear-resistant (and it too will never leak). It delivers precise fuel supply (and at the exact pressure you want) to the P7100. With 25 to 30-psi of supply pressure, you get a Cummins 6BT with a smoother idle (less NVH), a 30-50 rpm increase at idle, and added off-idle responsiveness.
Problem #3: Blown Freeze Plug
Blown freeze plugs can be a concern in both high-mile and high-horsepower 6BT applications. Typically, the rear freeze plug is the one that let’s go, but the freeze plugs in the side of the block can also fail, as well as the plugs present in the cylinder head. When the rear freeze plug in the block fails, you get a mess of coolant all over the bellhousing of the transmission—as well as zero coolant within a two-minute window… Driving down the highway, you could overheat in short order. If an internal plug within the cylinder head fails (rarer), you will notice coolant disappearing in rapid fashion, and this is sometimes misdiagnosed as a cracked block.
Why They Fail
In stock 6BT applications, where the factory power rating is still in play, freeze plugs usually fail due to age (especially the rubber expansion style versions). In performance applications, excessive engine rpm can force the water pump to over-pressurize the coolant, and a blown freeze plug is always a worry. At any horsepower rating, a stuck thermostat can also contribute to a blown freeze plug. But why is the rear freeze plug in the block the common culprit? Likely because it’s subjected to the highest pressure from the cooling system. In virtually every inline-six engine design, the number 6 cylinder is the last hole to receive proper cooling (coolant and airflow).
Solution(s)
Rear freeze plug replacement is best performed with the transmission removed—although trained hands can pull it off with it still in place. As for a permanent solution, various billet freeze plugs are available in the Cummins 6BT aftermarket, and they secure via bolts using existing holes in the block. These bolt-in freeze plugs can’t blow out without taking the fasteners or the block with it… A great way to avoid over-pressurization of the cooling system (again, a common culprit in freeze plug failure) was developed by Fleece Performance Engineering. Its coolant bypass system (shown) relieves the kind of elevated temperatures that cause excess pressure in the cooling system in the first place.
Problem #4: Leaking Tappet Cover
The tappet cover is one of the most notorious spots to find oil on a 5.9L Cummins (ISB versions included). And while it’s not detrimental to performance nor will it hurt anything, it will lead to a buildup of debris along the driver side of the block, and eventually oil spots underneath the truck. Like all diesel engines (and especially older, pre-common rail versions), the 6BT is constantly exposed to harsh NVH, be it at idle, cruising speed, or high rpm. On top of that, these engines aren’t getting any younger. In fact, many 12-valves are more than 35 years old at this point.
Why It Leaks
Speaking of age and NVH, oil leaks are inevitable on the Cummins 6BT. Corroded, shrunken seals, gaskets, and O-rings are par for the course for any diesel power plant in its fourth decade of service. Once the gasket that seals the tappet cover in the crankcase dries out, it becomes brittle and eventually allows oil to escape. A replaced tappet cover gasket can also begin to leak ahead of its time if improperly installed. To a lesser degree, tappet bolts that’ve backed out over time can also initiate a leak.
Solution(s)
A billet aluminum tappet cover, available in the thriving 6BT aftermarket, is a highly common solution for a leaky factory cover. However, not all billet tappet covers are created equal. Not only should you choose one that comes with a high-quality O-ring seal (for long-term, leak-free service) but also one that is internally baffled. The OEM tappet cover has no baffling provision for the crankcase’s breather ports, which results in excessive oil discharge from the crankcase. A built-in baffle system drops oil discharge significantly in the 6BT Cummins.
Problem #5: EGR ('96-'98 CA engines)
Unknown to many, California model Cummins 6BT engines were equipped with exhaust gas recirculation (EGR). In order to meet California’s more stringent NOx emission standards in the mid 1990s, EGR became mandatory for ’96-’98 model year engines. The NOx-fighting equipment is immediately noticeable when you pop the hood of a 1996-1998 California model Dodge Ram, shown. These engines feature a different style intake manifold with a provision for EGR to reenter the intake tract, an exhaust manifold that accommodates an EGR valve, and a crossover tube. They are are notorious for airflow restrictions and lack of power issues due to EGR-related carbon buildup.
What Happens
Like any diesel power plant retrofitted with modern emissions control equipment, these EGR-equipped 6BT’s are infamous for airflow restrictions and performance loss due to carbon buildup. A common diagnostic trouble code for this is the dreaded P0401. EGR-laced 5.9L’s are also known for experiencing sticking or completely failed EGR valves. Key symptoms associated with EGR on a California model 12-valve Cummins are: lack of power, poor acceleration and overall drivability, and reduced fuel economy. An EGR valve stuck in the open position means less exhaust gases are available to drive the turbo, hence the common complaint for poor acceleration.
Solution(s)
Step one is to avoid EGR issues by treating the system to regular cleanings. This entails removing carbon buildup in the EGR valve, the crossover tube, the intake manifold, and even the grid heater. For engines that must remain 100-percent emissions compliant, routine cleanings are vital in prolonging the EGR valve’s service life, as a replacement valve from Cummins can cost north of $1,700. If local emissions laws permit it (or if the engine is being used in an off-highway or off-road application), it may be possible for the EGR system to be removed. Block-off plates are common here, as is sourcing a higher-flowing, 49-state intake manifold.
Problem #6: Factory Rod Bolts (Performance Applications)
This problem primarily applies to Cummins 6BT engines producing more than 700 hp (and 1,400 lb-ft of torque) which, believe it or not, isn’t difficult to do with the 5.9L platform—especially one fitted with the Bosch P7100. But because the 6BT’s factory forged-steel connecting rods and crankshaft can handle that power level with ease, many forget the OEM rod bolts can’t (or can’t for long). Even the factory cast-aluminum pistons can withstand the abuse of added cylinder pressure, so long as EGT isn’t allowed to get out of hand. The weak link here is unequivocally the factory 7/16-inch rod bolts.
Why They Fail
At elevated horsepower and torque levels, the factory connecting rod bolts stretch. Most often, these fasteners become elongated as a result of excessive rpm and cylinder pressure. Once stretched, the rod bolts become fatigued and can fail—and when one lets loose catastrophic engine failure is par for the course. When the rod cap separates from the big end of the rod, the internal mayhem can even become external, with a rod windowing the crankcase. Again, rod bolt stretch and failure doesn’t become a concern until power levels exceed 700 hp, and certainly once they crest 800 hp.
Solution(s)
A proven, affordable upgrade for factory rod bolts are the direct replacement bolts offered by ARP. Made of the company’s L19 material, its 6BT rod bolts are 23-percent stronger than the stock fasteners. An added benefit is the ARP units can be installed without pulling the engine in most applications. Simply pull the oil pan and turn the engine over by hand to access each rod cap and its bolts. Once ARP bolts are in place and torqued to spec, an OEM Cummins 6BT short block can usually handle 1,100 to 1,200 hp without issue, so long as peak torque is kept below 1,800 lb-ft.
Problem #7: Head Gasket (Performance Applications)
As with any traditional internal combustion engine, where a cylinder head is fastened to the crankcase, head gasket issues can eventually surface. And although every rendition of the 5.9L Cummins is renowned for its lack of head gasket problems, especially in comparison to its 6.7L successor, when excessive boost and drive pressure are forced on an aging engine a blown head gasket is well within the realm of possibility. In some cases, completely stock 6BT’s can even pop the head gasket due to a combination of high miles, old age, and being worked extremely hard.
Why It Fails
Factory cylinder head bolts—and in particular bolts that haven’t been checked for proper torque but have been subjected to substantial increases in horsepower—can eventually fail to hold down the fort. Stretched head bolts will allow the head to break its perfect seal with the block, leaking combustion and often pressurizing the cooling system. On top of boost pressure created by the turbo often being to blame for a blown head gasket, drive pressure (on the exhaust side) is arguably harder on a head gasket. This is why proper turbo sizing is so vital in a performance Cummins 6BT application.
Solution(s)
While a blown head gasket may be unavoidable on an older 6BT, ensuring the factory head bolts are still torqued to spec can go a long way in preventing it. OEM head bolts should sit at 150 ft-lbs. Additionally, they can be pulled, one at a time, and checked for bolt stretch. Once the head gasket is already gone, resurfacing the head, inspecting the block’s deck surface for trueness, and installing head studs along with a fresh OEM gasket is recommended. For extreme performance applications, a fire-ringed cylinder head, a custom head gasket, and larger diameter head studs can be considered.
Problem #8: Underpowered (Medium Duty Applications)
Though universally known as a highly desired powerhouse with exceptional durability, the 5.9L Cummins is underpowered in most medium duty applications it’s found in. After all, it was often a base engine option for small dump trucks, RVs, and school buses—and you got what you paid for here… Smaller displacement means less torque output than larger engine options, which requires the 6BT to live at higher rpm to get the same amount of work done. Then there is the matter of the 6BT’s non-serviceable parent bore block, an undesirable feature in the medium duty segment.
Why It's Gutless Here
Because it’s smaller than the premium workhorse variants such as the Cummins 6CT (8.3L) and International DT466 (both of which boast wet sleeve designs), more effort is required of the 5.9L to perform the same job. And, embellishing on the 6BT’s lack of torque output in comparison to its larger counterparts, a common torque rating of 440 lb-ft pales in comparison to the 630 lb-ft to 890 lb-ft ratings available with the 6CT and DT466. This lack of low-rpm grunt manifests itself in slow acceleration, but also subpar fuel economy given how hard the 6BT has to work.
Solution(s)
You know the old adage “no replacement for displacement,” but by tapping into the aftermarket a 6BT Cummins can be made to produce more low-end torque, as well as added midrange and top-end horsepower. The key here is to do it with well-placed power-adders that don’t simply throw more fuel at the engine. Sensible upgrades that keep EGT in check—such as combining a few P7100 tweaks and air (turbo) mods with fine-tuning of the AFC, for example—must be a top priority. When it comes to sourcing a commercial use vehicle for high payloads and grueling work, simply look for the larger engine option over the 5.9L.
Problem #9: Poor Drivability ('91.5-'92 On-Highway Engines)
A host of changes were made to the ’89-’93 model year on-highway engines (namely for Dodge trucks), but none of them altered the horsepower or torque rating. It remained 160 hp and 400 lb-ft throughout. However, on these earlier, VE rotary pumped, Cummins 6BTs the initial turbocharger was a fixed geometry, non-wastegated Holset H1C unit with an 18cm2 turbine housing. It spooled sufficiently and kept exhaust heat in check, even on engines that were void of an air-to-air intercooler (’89-‘91). But midway through the 1991 model year, the H1C received an update—and it wasn’t for the better.
Why Performance Suffers
A larger, 21cm2 turbine housing entered the picture on ’91.5 model year engines. It provided for higher exhaust flow and cooler EGT, but because it was a “looser” housing and therefore less restrictive than the 18cm2 turbine housing, spool up was slower. This lack of turbo response was immediately noticed by customers and Cummins 6BT powered ’91.5-’92 Dodge Rams developed a reputation for poor drivability and being “down-on-power.” Again, no horsepower or torque output changes occurred with the engine, but owners were extremely disappointed in the lack of off-idle responsiveness and felt as if the new 5.9L’s weren’t making the same power earlier models did.
Solution(s)
Surprisingly, Chrysler and Cummins solved this one themselves—and they acted on it in relative short order. Beginning with ’92.5 model year 6BTs, the 18cm2 turbine housing was back on the exhaust side of the Holset H1C turbo. Low-rpm responsiveness was restored and owners were happy again. If you find yourself with a second-hand ’91.5-’92 5.9L Cummins with the 21cm2 turbine housing, the smaller 18cm2 housing can easily be found online or in a scrapyard on the cheap. Of course, one can be sourced (or an upgrade like it) in the bustling 6BT aftermarket.
Problem #10: Automatic Transmission(s)
This problem doesn’t affect the Cummins 6BT, but it is caused by it. If you own any diesel-powered ¾-ton or larger Dodge truck equipped with a Chrysler automatic, chances are high that you will be addressing a transmission failure at some point. 46RH transmission problems are common on the A518/46RH four-speed, while 47RE transmission problems tend to surface on the later 47RH and 47RE units. Either way, the 5.9L's low-rpm torque output can't be harnessed forever. All of the automatics offered from Chrysler left the assembly line on the edge of what they could handle in terms of torque. Treat the 6BT to even a marginal amount of additional horsepower and you could get a transmission that's slipping..
Why It's The Weak Link
While the TorqueFlite platform works well behind Chrysler’s gas engine options, a deluge of weak points are exposed when one is placed in front of the Cummins 6BT. Every torque converter, although diesel-specific, is prone to excessive slippage and lockup clutch failure (A518-47RE versions had lockup, while the 727 didn’t). Many of the 46RH transmission problems mechanics see stem from valve body wear that's common across every model, and a lack of line pressure from the factory makes optimum clutch holding capacity impossible. Bands often stretch and can break, even though they don’t tend to require frequent adjustment. Perhaps most damaging, any amount of added power can snap the transmission’s input shaft.
Solution(s)
Many automatic transmission issues, including 47RE transmission problems, can be avoided by leaving the Cummins 6BT at the factory horsepower and torque rating. However, in time the off-idle grunt of the 5.9L I-6 will take a toll on the torque converter, clutches, and bands. When it is time for a transmission rebuild (or upgrade), reinforcements such as a more efficient, multi-disc torque converter, higher line pressure valve body, stronger or oversized bands, and additional clutches go a long way in improving durability. A higher quality material input shaft is also a great insurance measure. Combine all of the above and you’ll have an automatic transmission as tough as the 6BT under the hood.
