Top 10 5.9L Cummins Problems

Written By, Mike McGlothlin

We work with many renditions of the 5.9L Cummins here at Big Bear Engine Company. And while we believe it’s one of the best light to medium-duty diesels ever produced, there is no denying that—like any engine—it has its share of shortcomings. In fact, a few of its flaws can even spell disaster for its owners. But luckily, even the 5.9L Cummins’ most severe pitfalls can be addressed before catastrophe strikes. From cylinder head issues like dropped valve seats or blown out freeze plugs to cracked ISB blocks and the infamous killer dowel pin, we’ll cover every major 5.9L weak link in this article—and then we’ll provide a permanent solution.

Problem #1: Killer Dowel Pin (KDP) What Is It? We’ve covered the killer dowel pin issue before, but that doesn’t make this failure any less important to address. In fact, if this message reaches only one 5.9L 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 employed at the factory level to mount the front gear cover to the block, and it earned its nickname for the carnage it causes if it works itself free from the block. The KDP affects all 5.9L Cummins engines built for 1989 to 2002 model year Dodge Rams. This includes both the 6BT (12-valve) and ISB 5.9L (24-valve).

How It Fails There is no specific timeline for when the KDP will strike, but until it is formally ruled out as a threat the potential for its failure (and all that that entails) is always on the table. Be it due to age, miles, operating hours, vibration, or a combination off all of the above, the steel dowel pin can work itself out of its bore. When this happens, it can drop into the engine’s front gear train (namely the cam gear), potentially leading to piston-to-valve contact, a ruined front gear cover, and shrapnel everywhere inside the engine. In some applications, the owner lucks out in that the KDP manages to avoid the gear train fiasco by making its way into the oil pan. Still, in other engines the KDP remains securely in place within the block.

The Solution Although ruling out the KDP is fairly labor intensive, it’s 100-percent worth the peace of mind it brings a 5.9L owner. Staking the pin in place (or covering it in a way that permanently keeps it in the block) are more popular than KDP removal, but either method has proven itself an effective solution. The job calls for a new crank seal and front cover gasket, along with what essentially amounts to tearing into the front of the engine. The fan, fan shroud, and front gear cover all have to be pulled. It’s a job that normally takes a veteran mechanic half a day to complete. NOTE: All engines coming from Big Bear Engine Company have the KDP issue fixed in production, no need to worry about it on your end.

Problem #2: Cracked Block What Is It? Shortly after Cummins changed over to a new block for the ISB 5.9L engine (i.e. the 24-valve), casting was outsourced to Brazilian manufacturer, TUPY, a familiar name in the world of cast-iron engine components. These blocks, distinguishable by the “53” cast into the driver side (and sometimes the passenger side) of the crankcase, were later found to be prone to cracking. Roughly 100,000 blocks were produced by TUPY for Cummins, with Chrysler ultimately eating the costs associated with full-on engine replacement as it pertained to affected ’99-‘01 Dodge Ram 2500 and 3500 trucks that were still under warranty.

Why (and Where) It Fails When the 53 block cracking issue became rampant, teardown analysis determined that TUPY had implemented a thinner casting around the water jackets. The failure typically surfaces on the passenger side of the ISB block, and in engines that spend much of their time under extreme load and/or that have been modified to produce higher horsepower and torque. One theory suggests that thermal expansion (during the course of warmup) is to blame, and not excessive load or added performance. Either way, a cracked block—the block which, of course, houses the crankshaft, rods and pistons—means you’re looking at a major, costly engine repair, or all-out replacement.

The Solution Though Chrysler replaced countless effected ISB 5.9L Cummins engines back when the ’99-’01 Rams were under warranty, owners no longer have that option. In these cases, many opt for the lock-n-stitch method of attempting to quell coolant leaks and stop the block from splitting any further. In most cases, the lock-n-stitch method doesn’t solve the issue. The ultimate solution is to source a late production block, namely one cast by Teskid of Mexico. These later blocks were cast with significantly more material around the water jackets and, 20 years on, have never been associated with cracking.

Problem #3: Lift Pump Failure What Is It? On ’98.5-’02 model year Dodge Ram 2500 and 3500 trucks, trucks which were offered with the ISB 5.9L Cummins, the lift pump responsible for feeding adequate low-pressure fuel supply to the injection pump is notorious for failure. And as many of you already know, a lack of sufficient fuel supply (less than 5-psi) sent to the expensive electronic Bosch VP44 injection pump will kill it as well. Unfortunately, a failed lift pump, failed injection pump scenario is a chain reaction failure that many 24-valve Cummins owners know all too well.

How It Fails From the factory, ISB 5.9L Cummins-equipped Dodge Rams were fitted with an engine mounted lift pump. And while the Carter lift pump was electric and self-priming (both of which were steps in the right direction), its location along the engine block subjected it to constant vibration. The perpetual pulsing from the 24-valve Cummins often rattled this little fuel pump to death and did so in relative short order. Once upon a time, Chrysler’s under-warranty fix was to replace the block-mounted lift pump with an in-tank unit. However, this still failed to provide the type of fuel system longevity Cummins owners were after.

The Solution Enter the diesel aftermarket, where Cummins is king in The Big Three pickup truck segment. Several independent companies developed all-inclusive fuel supply systems to replace the failure-prone factory arrangement (FASS and AirDog being the biggest names in town). These systems mount a high-flow electric lift pump along the chassis and come with improved fuel filtration and water separation. In recent years, Fleece Performance Engineering released an in-tank replacement pump (pictured) that offers quiet operation, higher flow, and factory-like fill buckets (no “low-fuel in tank” issues). In either case (in-tank or frame-rail mount), steady fuel supply pressure (and volume) from a more reliable electric lift pump ensures both consistent performance and that the VP44 injection pump lives a healthy life.

Problem #4: ECM Failure What Is It? Once again, the ISB 5.9L Cummins found in ’98.5-’02 Dodge Rams is on the list, and this time it’s for electronic control module (ECM) failure. As these engines and their electronic control systems age, dead ECM’s are becoming more and more common. When an ECM is dying in a 24-valve Cummins application, a myriad of symptoms can occur, but most notably owners will experience intermittent free-revving (or dead pedal) of the engine without driver input, a delay in the “wait to start” light illuminating after turning on the ignition switch, and/or the dreaded P0606 fault code.

Why It Fails Similar to the factory lift pump situation we covered earlier; vibration is a major contributor to ISB 5.9L ECM failure. However, vibration isn’t the only factor. Age, use and exposure to the elements can all play a part in the demise of its internals. After all, a 25-year run is actually a pretty good run for a module in the modern automotive era. To a lesser extent, 24-valve ECM’s fail due to water and moisture infiltration.

The Solution Although an OEM ECM can be sourced directly from Cummins, it will likely cost you a pretty penny. A more cost-effective alternative to this is to have your original ECM sent in for repair or to buy a remanufactured unit. The biggest thing to remember here is that ECM’s have to be programmed for your specific vehicle in order for the engine to operate correctly. Providing your VIN number will allow any reputable supplier the ability to properly calibrate an ECM for your specific truck, making the repair a simple, plug-and-play task.

Problem #5: P7100 Overflow Valve Failure What Is It? The Bosch P7100 (i.e. “P-pump”) injection pump isn’t exactly known for its mechanical shortcomings. Rather, it’s renowned for its simplicity and million-mile durability, not to mention the fact that it’s synonymous with performance in the diesel aftermarket. However, the coveted pump (commonly found on ’94-’98 Dodge Rams equipped with the 6BT 5.9L Cummins) does have a few flaws that surface from time-to-time—and arguably the biggest one revolves around what is known as the overflow valve. Hard-starting, reduced power and increased smoke are all telltale signs of an ailing overflow valve. It’s the result of poor low-pressure fuel delivery to the P7100.

Why It Fails Within the overflow valve, an improperly designed ball and seat places added stress on the factory Bosch spring. Inevitably, this culminates in a broken spring, which significantly reduces pressure within the P7100 pump and the resulting reduced power, additional smoke, and hard-starting that comes with it. Further, erosion of the seat (due to age and use) leads to over-extension of the spring—another major contributor to low pressure. And on top of all of that, the factory Bosch overflow valve leaks—even brand-new units.

The Solution Designing its own overflow valve, aftermarket company Tork Teknology hit a homerun for P-pump 12-valve Cummins owners. Not only is its overflow valve adjustable (and adjustable without stressing the spring), but a superior spring is employed that won’t stretch or lose its spring force over time. Additionally, its overflow valve is engineered with a redesigned valve seat—and one that ensures the valve seat is exposed to as little stress from the ball as possible. And unlike the factory Bosch overflow valve, Tork Teknology’s is leak-proof thanks to the super-finishing it performs on the valve seat.

Problem #6: Coolant Flow to Number 6 What Is It? While the 5.9L Cummins’ inline-six design is ideal for producing low-rpm torque, its downfall is poor coolant flow to the rearmost cylinders. In hard-working applications (or those turning out additional horsepower), the number five and especially six cylinders always run warmer. This means the rearmost cylinders are consistently under more stress than the others. Over time, this can culminate in overheating and even a blown head gasket. On a related note, excessive pressure can also contribute to blown freeze plugs.

What Fails Along with excess coolant temperature comes elevated coolant pressure. So, in addition to the number six cylinder never quite being able to operate at a preferable temperature, under heavy workloads the over-pressurization can lead to blown freeze plugs. Typically, an external freeze plug will blow out of the block, but it’s not unheard of for the smaller diameter freeze plugs (under the valve cover) to blow out as well. Of course, the aforementioned overheating issue that stems from elevated rear cylinder coolant temperature can also provoke a head gasket failure.

The Solution To keep coolant temp in check across all cylinders, coolant bypass systems have been implemented. However, the most effective coolant bypass systems work based on temperature, not pressure. In the diesel aftermarket, Fleece Performance Engineering developed such a system—and it works flawlessly. Its system adds a second thermostat to ensure consistent temperature exists in every cylinder. The secondary thermostat installs within its own housing in the rear of the Cummins’ block.

Problem #7: Dropped Valve Seats What Is It? Aside from a rotating assembly failure (something that’s rarely experienced with a 5.9L Cummins), dropped valve seats are about as destructive as it gets. Unfortunately, this type of failure is fairly common on all Cummins cylinder heads, and Phase 2 Automotive of Silsbee, Texas (the provider of this photo), encounters it regularly. As the head ages, the press-in OEM valve seats become more and more susceptible to cracking. Over time, the crack(s) spread and can eventually allow the valve seat to break off and fall into the cylinder below it. You can imagine what happens after that: pure, metal-on-metal mayhem.

Why They Fail The most common cause for dropped valve seats boils down to age (i.e. mileage and/or operating hours), but when you combine the latter with additional abuse the process is accelerated. Many Dodge Ram owners, for instance, drop a valve seat after purchasing a truck with a higher-mile engine and then treating it to additional horsepower. The added exhaust gas temperature (EGT) that comes with higher performance can be the nail in the coffin for aged factory valve seats.

The Solution Many independent engine rebuilders have improved upon Cummins’ valve seat issue. Among the list of improvements made to ensure the failure doesn’t occur again are oversized valve seats, as well as installing hardened (or heat-treated) valve seats to resist cracking. Further durability is gained when the heat-treated valve seats are radius’d on the side that mates to the cylinder head. Another key insurance measure entails pressing the new valve seats deeper into the head than the factory units.

Problem #8: Blown Cylinder Head Freeze Plugs What Is It? There are 11 internal freeze plugs pressed into a factory 5.9L Cummins cylinder head. Unfortunately, from time to time they blow out—and when that happens coolant loss is rampant. But in addition to that, coolant is allowed to mix with engine oil, and contaminated oil is never welcomed by rod and main bearings. A blown internal freeze plug in the cylinder head is sometimes even misdiagnosed as a cracked block, even though that particular form of failure is extremely rare for a 5.9L Cummins.

Why They Fail High mileage and age can take its toll on the factory, 13/16-inch diameter, press-in freeze plugs utilized within the cylinder head, but most of the time elevated engine rpm is the cause of their failure. With added horsepower comes additional heat and pressure throughout the engine, and the cylinder head isn’t exempt from these conditions. Other contributors include combustion leaks due to a blown head gasket and corrosion or rusting of the OEM freeze plugs.

The Solution The ultimate remedy for blown out freeze plugs is replacing them with screw-in type replacements, such as pipe plugs. Many machine shops offer this service (and some have even made this upgrade standard when rebuilding Cummins cylinder heads), and do-it-yourself types can even tackle the job themselves thanks to aftermarket kits. Screw-in type freeze plug kits retail anywhere from $65 to $160 depending on the contents, with all-inclusive packages including the pipe plugs, red Loctite, and the required M24 x 1.5 bottoming tap needed to thread the freeze plug bores.

Problem #9: Injector Failure What Is It? Though injector failure does occur on aged ’89-’98 and ’98.5-’02 6BT and ISB Cummins engines, the electronic common-rail versions employed in ’03-’07 5.9L’s are much more prone to failure. The reason for this is twofold: 1) common-rail injectors don’t tolerate any amount of contamination within the fuel system, and 2) there are more moving parts and more injection events being carried out per combustion event with a common-rail injector. Long story short, whether you properly maintain your fuel system or not, common-rail injectors wear out sooner. By the 200,000-mile mark, the units in your ’03-’07 5.9L Cummins will likely be ready for an overhaul.

How They Fail By far, the biggest contributor to common-rail injector fail is a contaminated fuel system. With debris present in the high-pressure common-rail system, worn injector ball seats, high balance rates, damaged nozzle needle seat and incorrect injector control capability are par for the course, as these injectors are highly intolerant of even microscopic contaminants (1 to 3 microns in size). Common-rail injector failure has also been linked to low sulfur fuel (ULSD in the U.S.), abrasion from fuel, and fuel deposits.

Solution(s) With the price of a single replacement injector well over $400, it pays to take care of your Cummins’ fuel system. It all starts with a proper maintenance schedule. This means fuel filter (and water separator) changes at the correct interval, if not before, and the use of high-quality filters. On ’03-’07 Dodge Rams equipped with the 5.9L common-rail engine, it pays to improve upon the OEM fuel filter’s 7-micron rating with one that possesses a 2 to 4-micron rating. The last order of business boils down to where you fill up. Knowing that diesel fuel begins to break down within weeks after being produced, topping off at high-traffic fueling stations should be your goal. With quick fuel turnover rates, these high usage pumps will always dispense the freshest fuel available.

Problem #10: Accelerator Pedal Position Sensor Failure What Is It? APPS or Accelerator Pedal Position Sensor problems are reserved for ISB 5.9L Cummins engines and early common-rail owners (’98.5-‘04), and often begin with intermittent failure. The infamous “dead pedal” sensation is a textbook example of a malfunctioning APPS. During a dead pedal situation, the engine is unresponsive to throttle input and/or changes in accelerator pedal position, and at least one or more diagnostic trouble code will be thrown. Early stages of APPS failure typically entail engine surging, rather than complete loss of pedal input.

How It Fails Failure of the APPS stems from a gradual loss in contact between its internal rotating fingers and the resistance track they ride in. The surging, erratic engine issue mentioned above occurs when the APPS’s electric signal comes and goes in a rapid manner. When complete loss occurs, you get a dead pedal event. All symptoms of a failing (or ailing) APPS are highly noticeable, and will disrupt the driving experience. APPS failure has been a frequent occurrence, especially in ’98.5-’02 Dodge Rams, for two decades.

Solution(s) Before purchasing a new APPS, two possible fixes can be attempted. These potential remedies include the APPS reset procedure, which recalibrates the accelerator pedal with the sensor, and recalibrating the sensor to the computer (performed by removing the sensor, obtaining the number off of its ID tag, and reverting back to the OEM setting). Luckily, the permanent solution—i.e. buying a new APPS—isn’t the end of the world, as the price has come down on this part over the years. Currently, a replacement APPS can be obtained for $150 or less.