In a world where emissions regulations drive diesel engine technology, the venerable N14 Cummins—a power plant largely based off of the 855 Big Cam that was first introduced in 1976—didn’t stand a chance of surviving the nitrogen oxide (NOx) and particulate matter (PM) emissions clamp down that was set to take place around the turn of the century. To meet the stringent standards that were coming, Cummins developed the ISX, a clean slate engine design that incorporated some of the best aspects of the N14 but that also took future emission regulations into consideration. Among other goals, the primary objective was to ensure the manufacturer’s premium Class 8 engine option retained its reputation for million-mile durability.
Of course, fuel efficiency was a contributing factor in the ISX engine’s design, as was its power rating. In both arenas, the ISX had to match, if not outperform, the heavy hitters of the day—namely the 14.0L Series 60 from Detroit Diesel and Cat’s renowned (and infinitely coveted) 3406E. Thanks to its rugged yet lightweight construction, Cummins being one of the first manufacturer’s to market with EGR, precise fuel control, and being a long-term-intended platform, the ISX delivered on all counts. As a result, it’s enjoyed an extensive, 20-plus year production run. All of the ingredients that made the transition away from the N14 possible—and the ISX engine a success—are detailed below.
Why The Transition From N14 To ISX Occurred
While the Cummins N14 was very similar to the 855 Big Cam, the ISX represented a notable departure in both engine architecture and engine controls—and that’s because it was 100-percent necessary. The ISX was designed with future, highly stringent emissions regulations in mind—regulations that would eventually take Caterpillar out of the on-highway business. With emissions driving its design, the ISX platform was highly adaptable, with pre-EGR, EGR-only, EGR and DPF, and EGR, DPF and SCR versions all being offered throughout its extensive (and on-going) production run.
Not necessarily interested in reinventing the wheel, some aspects of the ISX are the same. Still, others (such as the ISX’s one-piece cylinder head) are not. Like the N14, the ISX boasts a deep-skirt, cast-iron block. However, and while the ISX enjoys a displacement advantage over the N14 it replaced, its bore is actually smaller (5.39-inch vs. 5.50-inch). The ISX’s stroke of 6.65-inches (vs. 6.00-inches on the N14) accounts for its additional cubic inches (912 ci vs. 855 ci) over the N14. An integral oil cooler cavity is contained within the ISX block, and all threaded ports within the crankcase are designed for metric, straight thread, O-ring fittings.
At the heart of the ISX Cummins, you’ll find a forged-steel crankshaft that tips the scales at more than 430 pounds. As mentioned above, its 6.65-inch stroke accounts for the displacement advantage the ISX enjoys over the N14—and it’s also responsible for the ISX’s abundance of low-end torque. The crankshaft was designed with no counterweights required on the number 2 and 5 crank throws. The crank gear, a 60-tooth tone wheel, and the crankshaft adapter are all press-fit onto the crankshaft. Both the tone wheel and crankshaft adapter are fully serviceable.
Connecting Rods, Pistons & Liners
Practically everything associated with an ISX rotating assembly screams heavy-duty. Its beefy connecting rods are secured to the crank via four cap screws and the rods themselves are drilled in order for engine oil to travel through them to lubricate the wrist pins. Each ISX piston is a one-piece, forged-steel monotherm unit that utilizes a full floating wrist pin. The wrist pin is held in place by way of retaining rings. A wet-sleeve engine, the ISX makes use of six mid-stop cylinder liners. Each steel liner is equipped with a thick lower O-ring seal to prevent coolant loss.
Further distancing itself from the N14, the ISX features a one-piece, 4-valve, cast-iron cylinder head (not modular cylinder heads, which was the case on the N14 and the 855 Big Cam). It’s accompanied by a one-piece, reinforced plastic resin rocker lever cover with a reusable gasket. Twenty six fasteners are employed in order to anchor the head to the block, and there are six head bolts per cylinder (with sharing) for optimum clamping force. Up until 2010 model year engines, the ISX cylinder head housed dual overhead camshafts.
DOHC TO SOHC
Cummins’ dual overhead cam (DOHC) design was in operation from 1998-2009, when the ISX utilized a cam-driven injection system. These versions of the engine featured a camshaft that was solely dedicated to actuating the fuel injectors. The other cam was tasked with operating the valves and Jake Brakes. Beginning on ’10 model ISX engines, and along with the changeover to high-pressure common-rail injection, the ISX was of a single overhead cam (SOHC) design. The transformation to SOHC simplified the valvetrain, and ultimately led to reduced wear and tear on the cam lobes.
The Injector Cam, The Valve Cam, And Common DOHC Issues
In DOHC versions of the ISX Cummins, the injector-actuating camshaft (also called the injector cam) features a larger diameter than the valve cam. It’s located on the fuel pump side of the engine. The valve (and Jake Brake) cam is located on the exhaust side of the ISX. If camshaft lobe wear becomes excessive on DOHC ISX engines, correct lift can no longer be achieved. But camshaft-related issues can run much deeper than that on the ’98-’09 ISX. It’s not uncommon for the cams, which are press-fit onto their respective gears, to spin within the cam gears in higher-mile applications. This throws off the timing of the engine.
On the DOHC ISX, the injector cam drives a set of hydro-mechanical, open-nozzle style fuel injectors. Within the cylinder head, there are timing, metering, and drain rifles that control the supply and return fuel coming and going to the injectors. And while much of the cam-driven injection system is mechanical, it’s important to remember that even the early ISX power plants were highly electronically controlled. Case in point, the ECM controls those timing and metering actuators. Essentially, that means no fuel enters the injectors through these—and no injection event takes place—without the ECM saying so.
Things get interesting on the ISX when you look at its injector sleeves. Each sleeve depends on pressure from the fuel injector in order to maintain its seal against the cylinder head. More importantly, without the injector installed engine coolant will leak past the sleeve and into the cylinder. This is why the engine coolant has to be drained, at least to a level that is below the location of the injectors within the head, prior to removing the injectors. A second option requires the use of the coolant dam tool, an asset that’s commonly found in a Cummins tech’s toolbox.
Injector Removal & Running The Overhead
Predictably, valvetrain components are tied in with the fuel injectors on the DOHC ISX Cummins. When more than one injector requires removal (and we’ll note that a special injector puller tool is recommended for the particular task.), the valve rocker lever assemblies have to be removed also. Upon reinstallation, the valves call for adjustment after the injectors go in. The rocker lever adjusting screws feature nose pads that are swaged into the adjusting screws. Excessive force is known to pull the pad from the adjusting screw. As you can imagine, air tools are highly discouraged.
The entirety of the DOHC ISX fuel system revolves around the IFSM, or Integrated Fuel System Module. The IFSM was present on all ’98-’09 model year ISX engines. It bolts to the intake side of the cylinder head (and is mounted there using two dedicated dowel pins). There are fueling and timing actuators near the IFSM, as well as the fuel shutoff valve and lift pump. The lift pump feeds the gear pump, but only initially. After the first two minutes of engine operation, the lift pump shuts down, leaving the gear pump to pull fuel from the tank(s).
EGR Debuted Ahead Of Schedule
Although original versions of the ISX were free of the kind of emissions control equipment we’re familiar with today, the engine was designed with exhaust gas recirculation (EGR) in mind. Due to an EGR system’s ability to reduce peak cylinder combustion temperatures—the key to controlling nitrogen oxide (NOx) pollutants—it was the most logical method for meeting the more stringent emissions standard coming in 2004. And as part of the consent decree of 1998 (a settlement reached between the EPA, CARB, Department of Justice, and various engine manufacturers regarding NOx software defeat devices), Cummins integrated EGR on its ISX engine 15 months ahead of schedule, in 2002.
One key component in any EGR system is the EGR valve. The EGR valve controls the amount of oxygen-divested exhaust gases that are allowed to reenter the intake (charge air) stream. It is controlled by way of an actuator with a high-speed electric motor. To maintain durability and operability in all conditions aboard the ISX Cummins, the EGR valve actuator is water-cooled. Fully serviceable, the EGR valve is also cleanable and should be regularly cleaned in order to maintain precise functionality. Despite its reputation for failure, a properly maintained EGR valve can last 500,000 to 600,000 miles on the ISX.
Shedding more than 600 to 700 degrees F worth of heat off of exhaust gases can only be accomplished using a properly sized EGR cooler, and the unit employed on the ISX Cummins is massive. An incredible 78-percent of all water pump outlet flow goes to the EGR cooler. The remaining 22-percent of coolant flow leaving the water pump goes to the engine block and cylinder head for traditional cooling. For maximum cooling efficiency, engine coolant flows parallel with exhaust flow through the EGR cooler.
Diesel Particulate Filter
Beginning on January 1, 2007, allowable PM limits became a fraction of what they formerly were, and the OEM’s responded by equipping their diesel engines with diesel particulate filters. The job of the DPF is to trap the particulate matter (i.e. soot) produced by the engine’s combustion, but storing PM requires eventual cleaning of the DPF. This is handled in two ways: 1) the DPF is removed and cleaned, or replaced altogether, or 2) using regeneration events to convert the trapped PM into fine ash. Regeneration intervals are part of the engine’s normal operation and (typically) require no input from the operator.
Regeneration of the DPF is carried out by using diesel fuel, and it can be done in two different ways: 1) injecting excess fuel on the engine’s exhaust stroke, allowing the fuel to enter the exhaust aftertreatment system to fuel the regeneration event, or 2) inject fuel downstream of the engine through the use of an additional fuel injector. The latter arrangement is employed on the ISX Cummins and is more advantageous than the former for several reasons. First, because excess fuel isn’t washing down a cylinder wall, but also because the percent fuel content in the engine oil doesn’t increase. The regeneration injector (which benefits from water cooling just like the injectors in the engine), is pulse width modulated via the ECM.
Selective Catalytic Reduction
Beginning in 2007, a gradual phase-in to meet the 0.20 g/bhp-hr NOx emission standard was permitted for engine manufacturers. By 2010, 100-percent compliance was required. In order to get there (and like most engine makers), Cummins turned to selective catalytic reduction (SCR). The technology was introduced on CM2250 model ISX engines—the first engines with the new SOHC design and XPI (high-pressure common-rail injection). Also known as urea injection, where diesel exhaust fluid (or DEF) is injected upwind of an SCR catalyst, SCR effectively converts NOx into atmosphere-safe nitrogen.
Making The Switch To XPI
Although Cummins’ cam-driven injection system had advanced exponentially since its early days of implementation, it was clear that meeting the 2007 standard of 0.01 g/bhp-hr and then looking ahead to the future PM standard of 0.005 g/bhp-hr was going to call for more precise control over the injection event, along with higher injection pressures. High-pressure common-rail injection (or XPI, according to Cummins) was the answer. In its light-duty applications such as the 5.9L and 6.7L Cummins found in ’03-later Dodge Ram 2500 and larger pickup trucks, common-rail technology had proven itself from a performance, cleanliness and durability standpoint.
XPI: The Key To Curbing PM Emissions
Extreme injection pressures created with a high-pressure common-rail fuel system represent its biggest selling point—because it’s the most effective way to curb PM emissions. The highly pressurized fuel optimizes in-cylinder atomization, making for a comprehensive and efficient burn. The icing on the cake with high-pressure common-rail injection is its ability to meet horsepower and torque goals without sacrificing emissions. At the heart of the ISX system, a high-pressure fuel pump (shown), operating independently of engine speed, converts low-pressure fuel supply at its inlet to high-pressure fuel at its outlet (as much as 35,000 psi). This highly pressurized fuel is stored in the fuel rail until the injectors call for it.
Turbos, Through The Years
Much like the emissions equipment changes it’s received throughout its production run, a handful of turbochargers have been present on the ISX. On ’98-’02 engines, the fixed geometry Holset HX55W or the HX60W were employed. But because a quicker-reacting turbocharger means less PM out the tailpipe, variable geometry turbocharging entered the picture in 2003. A more advanced version of the HE551V (’03-‘06) called the HE561VE debuted in ’07 and was in use through ’09. Then, starting in ’10, the Holset HE451VE arrived. Each generation Holset VGT built off the last one, with responsiveness being a very important goal, as time spent “under” the turbo means more soot accumulating in the DPF (and more soot being rerouted back into the intake, courtesy of EGR).
N14 Common Problems
As the memory of the N14 continues to fade, many forget that it too had its fair share of problems. Injector and especially injector harness issues were common (primarily on early N14’s), as were problematic fuel solenoids. Failure of the latter component due to shorting out could even damage the N14 ECM to the point where the engine’s timing was drastically thrown off. Leaky injector O-rings were an N14 regularity, as well as instances of the injector wiring harnesses shorting out. On modern electronically controlled engines, various stages of protection are in place to protect the ECM from frying in the event of a wiring issue surfacing. The N14 didn’t have that luxury.
Because the N14 was never equipped with the modern emissions equipment the ISX is, it stands to reason that ISX engines experience more emissions-related failures. Among the most frequent issues encountered by ’03-newer ISX owners is EGR valve, EGR cooler, and EGR sensor failure. Not unlike most EGR valves, it’s prone to sticking and/or leaking over time, and EGR pressure differential sensor failure is a highly familiar occurrence. A ruptured EGR cooler is also a well-documented pattern problem on the ISX, with discolored engine coolant and coolant loss being the primary indicators of this costly failure.
ISX Mechanical Issues
The ISX isn’t notorious for mechanical issues but suffering a blown head gasket (especially in ’10-newer/CM2250, CM2350 engines) is never out of the question. Sub-standard counterbore machine work performed at the factory level leads to improper liner protrusion, which eventually takes out the head gasket. These crooked counterbores also contribute to liner failure, highlighted by finding coolant in the engine oil. At higher mileage and operating hours, dropped valve seats can occur, and the camshafts in DOHC engines are known for “flattening out” due to improper oil flow through the rocker arms.
ISX XPI Issues
As we’ve alluded to so far, Cummins’ XPI common-rail injection system is better in nearly every way when compared to the original, cam-driven injection system present on early ISX engines. However, all ’10-’16 XPI engines were equipped with a high-pressure fuel pump that was akin to a ticking time bomb. The HPFP’s fuel plungers were ceramic (as opposed to steel) and quickly became infamous for failing in catastrophic fashion. Sometimes the plunger(s) would break apart, and in other instances they would completely explode, allowing shrapnel to contaminate the rest of the injection system. The factory fix was a steel plunger HPFP, which Cummins introduced in 2017.
Like all modern turbo technology, VGT’s have come a long way in the reliability department. However, their long-term durability still pales in comparison to the kind of reliability seen in fixed geometry units. The complexity of the Holset VGT is to blame here, with moving components often becoming immobile due to rust, corrosion, or soot and carbon buildup. Be it due to damage of the pinion gear teeth, wear on the teeth, or a seized pinion gear, actuator failure leads a long list of VGT-related issues on the ’03-newer ISX. Within the VGT itself, sliding nozzle seizure is commonplace, often requiring disassembly and cleaning, if not complete replacement.
Fuel Economy That Maximizes Profit(s)
Ever since the 1973 oil crisis (which directly impacted Cummins and led to the development of the 855 Big Cam), fuel economy has been a huge selling point for Class 8 engine manufacturers. And from the moment the ISX was contrived, Cummins had your bottom line in mind. Better yet, throughout the ISX’s production run (including when it became the ISX15 and now the X15), fuel economy continued to improve. In the hands of a veteran driver, it’s not uncommon for an ISX to average 6.5 to 7-mpg (or more) in a fully loaded, over-the-road Class 8 application.