Written By, Mike McGlothlin
Highlights:
Most Common Problems:
*Lift pump failure
*VP44 injection pump failure
*Accelerator pedal position sensor (APPS) failure
Most Expensive Problem:
*Cracked block
Most Potentially Damaging Problems:
*Killer dowel pin
*Cracked valve seats
Less-Frequent Headaches:
*ECM failure
*Blown freeze plugs
*Cracked exhaust manifold
*Oil leaks
Improved horsepower and torque, reduced emissions, the introduction of an electronically controlled injection pump, and a new, 24-valve cylinder head all helped to make the 5.9L Cummins ISB 24-valve an avant-garde engine in the late 1990s. It was designed to meet the more stringent diesel emissions standards that were on the way, and it delivered. However, the early ISB 5.9L Cummins (often called the 24-valve) also brought a host of problems with it. To be sure, it’s no lemon—but its weak links are more worrisome than the few shortcomings associated with its predecessor, the 6BT. Common issues run the gamut in severity, with the killer dowel pin and cracked blocks epitomizing the worst of the worst, lift pump and VP44 failure representing a few of its textbook (and costly) headaches, and a bad APPS sensor or cracked exhaust manifold typifying some of its drivability problems. We’ll cover all the above and more here—as well as provide a permanent remedy for each issue.
Problem #1: Cracked Block
Finding a cracked block is perhaps the worst failure an ISB 24-Valve 5.9L Cummins owner
can face. Coolant loss, overheating, and a problem with no easy fix sum up this issue—and thousands of 1999 to 2001 model year engines (most of which can be found in Dodge Ram 2500 and 3500 series pickup trucks) were built using this crankcase. The problematic blocks feature a “53” cast into them, numbers which can be found directly under the oil cooler, along the passenger side of the crankcase, and just above the oil pan. In some cases, the “53” is present on the driver side of the block.
Why It Fails
Cummins’ Brazilian block supplier, TUPY, originally cast the gray iron “53” model
crankcases, but they were produced with too little material present around the water jackets. Over time, and especially in vehicles that are worked hard or that have been treated to horsepower upgrades, the block is known to crack in the water jacket areas. Thermal expansion (such as during engine warm-up) has also been blamed for the formation of cracks, but this theory has never been substantiated. Back when affected ’99-’01 Dodge Rams were still under warranty, Chrysler carried out thousands of complete ISB 24-Valve 5.9L Cummins engine replacements due to the 53 block problem.
Solution(s)
Solutions and potential solutions for cracked 53 blocks range from shade-tree mechanic to full-on block replacement. A lock-in-stitch method, where steel inserts are used in an attempt to hold the block together, can keep the crack from spreading—but doesn’t always work (or last). That makes the best solution block replacement—namely with a 54 series or higher numeric version. Later blocks, cast by Mexico’s Teskid, were cast with notably more material in the water jacket regions. These blocks are much, much stronger and have never been reported to crack.
Problem #2: Lift Pump Failure
While not all diesel engines require a fuel supply pump, many do need a lift pump or
pusher pump. Such is the case with the Cummins ISB 24-Valve 5.9L. From the factory, the engine gets its fuel supply from a self-priming, electric Carter lift pump. This low-pressure pump is vital in supplying adequate fuel pressure and ample volume to the VP44 injection pump. As you’ll read in Problem #3, without sufficient fuel pressure and volume, the VP44 isn’t properly lubricated or cooled. The problem with the lift pump is that it’s mounted on the engine, as opposed to being located along the chassis.
Why It Fails
Perpetual exposure to NVH thanks to the highly vibrant 24-valve engine, combined with 
age and countless heat cycles, can kill an engine-mounted lift pump relatively quickly. Unfortunately, in many cases the lift pump dies a slow death—which is to say it gradually starves the VP44 injection pump of its lifeblood. Unless an owner has a means of monitoring low-side fuel supply pressure, there is often no way to know if the lift pump is on its way out. In a lot of cases, the lift pump is supplying less than 5-psi, when at least 12 to 15-psi is required.
Solution(s)
The old-school, in-tank fix from Chrysler was an improvement, but at this point most of those setups are gone. Today’s more common solution for ISB 24-Valve lift pump failure is the use of an aftermarket, chassis-mounted lift pump such as a FASS or AirDog system along the frame rail. Another sound solution is the PowerFlo in-tank lift pump offered by Fleece Performance Engineering (pictured), and it’s a remedy that eliminates the quarter-tank/low-on-fuel issues associated with the aforementioned FASS and AirDog systems equipped with draw straws. All of the above systems feature an electric lift pump, but a fuel pressure gauge is also recommended to ensure 15-psi is always on tap for the VP44.
Problem #3: VP44 Failure
A much more advanced piece than the P7100 that preceded it (on the 1994-1998 6BT), the
Bosch VP44 is an electronically controlled, distributor style injection pump that came standard on all Cummins ISB 24-Valve 5.9L’s prior to the 2003 model year. It features three pumping plungers (vs. just one in the mechanical Bosch VE employed on 1989-1993 6BT engines) and got the call on the ISB platform thanks to its ability to vary injection timing independent of engine speed. However, in addition to timing, the VP44 is also tasked with controlling injection pressure and volume.
What Fails
Without question, the most problematic component on the VP44 is its PSG, the computer
married to the top of the pump. Surprising to some, the biggest killer of the PSG is heat (even more so than age or NVH). Vehicles that make frequent stops and that perform a lot of hot restarts are especially susceptible to killing the VP44. Typically, the solder melts within the internal circuit board. Telltale signs of a dead or damaged PSG are long crank/no start scenarios, erratic engine idle, and rough engine operation. Other, lesser frequent failure points associated with the VP44 pump entail a seized rotor, a failed distributor, or a bad diaphragm.
Solution(s)
VP44 fixes begin with avoidance, and the best way to keep this injection pump alive is to install a reputable, reliable electric lift pump to ensure adequate fuel pressure (and volume) is always being supplied to the VP44. An in-cab fuel pressure gauge is the best way to keep tabs on the health and functionality of the lift pump. As for complete pump replacement, only remanufactured VP44’s are available at this point. Do your homework and source a rebuilt unit from a reputable remanufacturer. The VP44 can also be completely replaced via a P7100 conversion (pictured) but going this route isn’t cheap, is time-consuming, and can be extremely labor intensive for the novice mechanic.
Problem #4: APPS (Throttle Position Sensor)
Accelerator pedal position sensor (APPS) failure is highly common on the Cummins ISB
24-Valve 5.9L platform. It’s a problem that, sooner or later, effects virtually every 1998.5-2002 model year Dodge Ram equipped with this engine. Also known as the throttle position sensor, the APPS is integrated with the bell crank assembly on the engine and is part of a prehistoric version of fly-by-wire, with a cable running from the bell crank (and sensor) to the accelerator pedal. The APPS provides the ECM a DC voltage signal, which varies with throttle position (typically between 0.5 volts and 4 volts).
Why It Fails
Problems begin when contact between the APPS’s internal fingers and the resistance track
they’re designed to ride in is lost. This causes rapidly changing voltage to occur, and drivability issues are immediately noticeable. Many symptoms are associated with APPS failure, but a telltale sign is “dead pedal.” When dead pedal happens, the engine idles but is totally unresponsive to throttle input. As you can imagine, a dead pedal scenario taking place at highway speed (and/or with a heavy trailer behind you) can make for a scary, white-knuckle situation. Other symptoms include rough or erratic idle, surging rpm, random engine hesitation, and a host of other drivability issues.
Solution(s)
A good first step in addressing APPS-related issues calls for resetting the APPS (no tools required here), followed by breaking out a few simple hand tools and a voltmeter to recalibrate the sensor. If those measures don’t do the trick, it’s time for a new APPS. But don’t spring for the most affordable option on the market. Cheap, knock-off versions of the OEM APPS often don’t last, and many times don’t even function correctly right out of the box (often due to their inability to remain calibrated). Buy a reputable aftermarket APPS for roughly $200 or an OEM Cummins replacement (which can retail for $400 or more).
Problem #5: Killer Dowel Pin
Contrary to what many believe (or want to believe), the killer dowel pin (KDP) wasn’t
solely used in Cummins 6BT production (1989-1998). The KDP is an 8mm diameter, press-in steel pin installed in the block to help align the front gear cover during assembly. It was still in use on Cummins’ production line for the ISB 24-Valve 5.9L and when it strikes it can have catastrophic consequences. In some engines, the KDP never shows its face. In other cases, it causes cracked timing gear covers, destroyed front geartrains, and even engine-killing piston-to-valve damage.
How It Fails
Due to only being pressed into the block, the KDP isn’t permanently held in place. Time,
age, and especially NVH can cause the pin to work itself out of its bore. Once free, the KDP can fall into the cam gear, potentially also getting into the injection pump gear, oil pump gear and crank gear. When the latter chain of events unfolds, the aforementioned piston-to-valve contact is inevitable—as is major carnage. Under luckier circumstances, the KDP can avoid falling into the front geartrain altogether and land safely in the oil pan.
Solution(s)
There is no quick-fix for the KDP, but the solution does bring peace of mind for ISB 24-Valve 5.9L Cummins owners. By installing a retaining tab over the KDP using an existing bolt located right next to it, the pin is permanently secured (i.e. tabbed, or staked) in place within the block. The job of tabbing the KDP calls for gaining access to the front of the ISB engine, removing the front gear cover, and roughly half a days’ work for an experienced mechanic. During reassembly, a new front gear cover seal and crank seal are required. There are various KDP “repair” kits available in the diesel aftermarket, some of which are more comprehensive than others.
Problem #6: Blown Freeze Plugs
Freeze plugs have long been an effective way to plug holes left behind in a crankcase or
cylinder head after being cast. The primary purpose of freeze plugs is to seal off coolant passages, but it’s also to allow for expansion as the block and head inevitably experience regular swelling (heat) and contraction (cooling). On the ISB 5.9L 24-valve Cummins, various freeze plugs are employed in the block and head. In fact, there are more than a dozen freeze plugs in the ISB long block. That means more than a dozen potential failure points.
Why (and When) They Fail
Over time, freeze plugs can fail due to high miles, elevated operating hours, and age.
However, in over-pressurization or excessive rpm scenarios—often the case with engines that’ve been treated to additional horsepower— these expansion plugs can give way in violent fashion. A blown head gasket, where the cooling system is over-pressurized, can contribute to a blown freeze plug, oftentimes within the cylinder head. The key indicator of a blown freeze plug is rampant coolant loss. Antifreeze can disappear so quickly that an internal freeze plug failure is sometimes misdiagnosed as a cracked block.
Solution(s)
Freeze plug failures can be addressed by starting over with fresh units, complete with Loctite retaining compound (or equivalent), or opting for more permanent solutions. These include threaded (screw-in) freeze plugs in the cylinder head. It’s a method that requires machine work, followed by the installation of pipe plugs (torqued somewhere in the neighborhood of 100 ft-lbs), but you’ll never have a freeze plug issue again. As for the block, various aftermarket freeze plug kits allow you to install billet versions, most of which utilize a threaded tie-bar and O-ring to thoroughly seal them.
Problem #7: ECM Failure
As Cummins ISB 5.9L 24-valve power plants continue to age, failures of the electronic
control module—the central brain of the engine—are becoming more and more commonplace. To say the ECM is vital for optimum engine operation would be an understatement, as it’s tied in with all of the vehicle’s control systems, as well as the ISB’s VP44-based injection system. When it heads south so does the performance, drivability, and even the overall functionality of the engine. Unfortunately, ECM issues have been on the rise over the past decade, as the ISB engines near or surpass a quarter century of use.
Why It Fails (And Key Symptoms)
Located on the driver side of the engine block, the ISB 5.9L’s ECM is exposed to constant
NVH, heat cycles, and exposure to the elements. After years of trouble-free service, failure within its internal circuitry and soldering can spark a myriad of issues out of the blue. Leading signs of ECM failure often include sudden free-revving of the engine and the dreaded dead-pedal phenomenon. Other oddities that can accompany ECM failure entail an abnormally long wait-to-start prompt on the dash prior to engine startup, or a P0606 fault code being stored.
Solution(s)
ECM repair is the most cost-effective means of solving this type of highly bothersome electronic gremlin, and various aftermarket companies can perform the work. A second fix entails the use of a salvaged ECM (from a wrecked Dodge Ram 2500 or 3500, for example). Just take note that the ECM must be programmed for your specific engine and vehicle application in order to work seamlessly. The third option is to buy a new ECM. To be clear, OEM ECM’s aren’t being produced any longer, so a remanufactured version is the only way to start fresh. Again, remember that the ECM will have to be programmed for your specific application.
Problem #8: Cracked Valve Seats
It’s fairly common for cast-iron cylinder heads to develop hairline cracks over the course of
a diesel engine’s lifetime. For the most part, these small cracks are benign. However, if cracks develop in a head’s valve seats (or spread into the valve seat area) it can spell disaster at some point. The Cummins ISB 5.9L 24-valve is equipped with press-in valve seats from the factory. With age, high miles, elevated operating hours, extreme exhaust gas temperature (EGT) or a combination of all of the above, cracks can begin to form on them.
What Fails
Aside from abuse due to excessive heat, the ISB’s 24-valve cylinder head design itself may
contribute to the cracked valve seat issue. With two exhaust valves positioned relatively close to one another above each cylinder (as opposed to one per cylinder on the 12-valve 6BT Cummins), the heat they produce is believed to take a toll on the valve seats over time. In other failures, a high-mile, well-aged ISB engine that’s been asked to handle the additional stresses (i.e. heat and cylinder pressure) that come with increased horsepower is often to blame. No matter the reason for the failure, the cracks start out relatively small but can develop into larger separations and begin to spread. In a worst-case scenario, they lead to a dropped valve seat.
Solution(s)
While cracked valve seats may be unavoidable in the original ISB engine, ensuring the engine doesn’t see exhaust gas temperature higher than 1,300-degree (F) for anything longer than short (15-30-second) bursts, or 1,200 degrees for sustained periods of time can help prolong the integrity of the valve seats. When it is time to address the OEM valve seats, oversize valve seats, still of the press-in variety but larger in diameter, should get the call. Additionally, hardened replacement valve seats (often heat-treated to help resist cracking) should be the order of the day from any reputable engine builder.
Problem #9: Cracked Exhaust Manifold
The exhaust manifold is a vital piece in the Cummins ISB 5.9L 24-valve engine’s ability to
run efficiently and perform as advertised. One small leak in this crucial component and the exhaust gases required to drive the turbocharger—which produces boost to feed compressed air into the engine—are reduced (i.e. bled off), creating performance and drivability issues. On top of an immediately noticeable loss in power, a cracked exhaust manifold will result in a mess of soot along the passenger side of the engine, the firewall, or the underside of the hood.
What Fails
A cracked exhaust manifold is typically the result of old age, where years (even decades) of
heat cycles and regular exposure to the elements have taken their accumulative toll. When it happens, a trail of soot—often along the cylinder head, the passenger side of the block, the firewall and transmission tunnel—will lead you back to the crack’s point of origin. A failed exhaust manifold gasket is another common failure point on the ISB engine. And while one failure is surely more expensive to address than the other, either one will always lower boost pressure (and horsepower output), decrease fuel economy, hurt drivability, and soil the engine bay.
Solution(s)
In some cases, a cracked exhaust manifold can be welded and repaired, but this is usually a temporary stopgap (as the component itself is often corroded, weakened, and prone to further cracking). Most ISB owners start over with a new manifold. Several aftermarket versions feature thicker castings than OEM, which can potentially hold up to the elements for a longer period of time. Some budget-minded owners source a factory take-off manifold in good condition. Regardless which replacement option you choose, always make sure to install fresh exhaust manifold gaskets. And, if you have the funds, it pays to start with new exhaust manifold bolts, too.
Problem #10: Oil Leaks
As with any aging diesel engine, oil leaks tend to develop with time. Due to the highly
vibrant nature of the 24-valve Cummins, oil leaks are extremely common—not to mention the fact that the original engines to bear the ISB name aren’t getting any younger. More than 20 years removed from their original production date(s), corroded and shrunken seals, O-rings, and gaskets—combined with the ISB arguably producing the highest NVH levels of any 5.9L ever produced—are bound to allow seepage between two mating surfaces at some point.
Where Oil Leaks Occur
The Cummins ISB 5.9L 24-valve can spring a leak in a lot of areas, but the tappet cover is
one of the most notorious spots to find oil. As the years go by, this gasket tends to dry out, losing its ability to fully seal the tappet cover to the block. The tappet cover is located along the driver side of the block which, ironically enough, is the side of the engine that yields the most oil leaks. Further highly frequent points of oil seepage can be found at the vacuum pump seal (pictured), the timing gear cover, and the CCV filter. Other problem areas include the oil pan and both the front and rear main seals.
Solution(s)
Solutions vary depending on the leak and its location, but the common tappet cover issue can be resolved either by replacing the bad gasket with an OEM or factory-equivalent replacement, or installing an aftermarket (and often billet) tappet cover that integrates a quality O-ring seal. With an aftermarket tappet cover, it pays to make sure it’s internally baffled. Why? Because the OEM cover has no baffling provision for the crankcase breather ports. This results in excessive oil discharge from the crankcase. A built-in baffle system drops oil discharge considerably (up to 60-percent with the billet Power Driven Diesel tappet cover pictured here).
