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History of The ISL Cummins—And Its Natural Gas Variant, The ISL G

Written By, Mike McGlothlin


Creating the perfect medium-duty diesel engine is an impossible task, but that didn’t keep Cummins from attempting it with the ISL. Building off of what it had learned with the L10 (which eventually morphed into the M11) and essentially basing it on the 8.3L ISC platform, Cummins developed the ISL to replace the L10 while also outperforming it in terms of efficiency, packageability and durability. And thanks to the flexibility which was built into its design, the ISL Cummins was able to meet the ever-tightening diesel emission regulations that were enacted throughout the beginning of the 21st century. As a result, the ISL (and its subsequent variants) have remained in production for more than 20 years.

Alternative fuel engines have long been part of Cummins history and their overall business model, and the ISL G was developed (in conjunction with Westport Innovations) in 2008 to run on natural gas. Improved tailpipe emissions, higher fuel efficiency, increased reliability and lower overall maintenance costs all stemmed from the ISL G program. For more than a decade, both the ISL and ISL G have been mainstay engines in buses and refuse trucks across North America. This is the story of the Cummins power plants that not only replaced the L10 and M11 workhorses, but that also provided more muscle than the company’s smaller medium duty offerings could provide.


What The Cummins ISL Replaced

M11 Cummins Inline Six Diesel EngineIn the mid 1990s, Cummins was preparing to replace the aging L10, which had basically evolved into the M11—the M11 having had its production ceased in 1998. The company’s answer was the ISL. Released in 1999, the ISL was based on the 8.3L ISC platform, albeit with more displacement. It shared the same cast-iron block and 24-valve head as the ISC but boasted more cubic inches thanks to its longer stroke. In a way, the ISL mirrored what Cummins had previously done with the M11—the M11 essentially being a stroked L10.


A Stroked 8.3L ISC

Cummins ISL Front Cover BadgeAs mentioned above, the ISL offers more displacement than the ISC (541 ci vs. 505 ci), and this is achieved with the ISL possessing a longer, 5.69-inch stroke (vs. the ISC’s 5.32-inch stroke). They share the same, 4.49-inch bore. Use of the 8.3L ISC crankcase also plays a crucial role in the ISL Cummins’ lighter overall weight than the old L10. The difference in heft between the ISL and L10 is more than 300 pounds, with the ISL checking in at approximately 1,635 pounds and the L10 tipping the scales at 1,965 pounds.


Over-Engineered For Maximum Longevity

Big Bear Engine Company ISL Cummins Long BlockIn typical, tough-as-nails fashion, Cummins history has shown to design the ISL with solid bottom end components. The block—a cast-iron, deep-skirt crankcase with integrated fluid passages—houses an eight counterweight, high tensile strength, forged-steel crankshaft that’s had its journals and fillets treated to induction-hardening for optimum durability. Mid-stop (wet) liners exist in each cylinder, along with a high durability, two-piece steel articulated piston that benefits from targeted piston cooling. The connecting rods are made from forged-steel to stand up to the kind of cylinder pressure (torque) the low-rpm, inline-six ISL generates.


Cam, Head & Lubrication System

24-Valve Cylinder Head for ISL CumminsSeeing no reason to reinvent the wheel, the proven cylinder head from the ISC 8.3L was repurposed for use on the ISL. The cast-iron head boasts four valves per cylinder as well as six head bolts per cylinder (with sharing). However, camshaft improvements were implemented for the ISL application, including the use of roller cam followers similar to what is used in the ISX. Another notable enhancement the ISL received was a higher capacity lubrication system than the ISC. To reduce vibration and ultimately wear on the engine’s main bearings and geartrain, the ISL was equipped with a viscous crankshaft damper.


Smaller—Not Weaker

ISL 425 Cummins Engine Horsepower Torque CurveDon’t let the Cummins ISL’s smaller displacement as compared to the L10 and/or M11 fool you, this 8.9L power plant is highly powerful and efficient for its size. In fact, the ISL offered a much more favorable power-to-weight ratio than its predecessors did. In motorhome applications, the ISL could be rated as high as 425 hp. And being more than 300 pounds lighter than an L10 was a major point of focus for Cummins’ marketing team. After all, more power and less weight means more room for higher payloads.


Production Plant(s)

Cummins Rocky Mount Engine Plant ISL AssemblyProduction of the ISL (and later variants, such as the ISL9 and L9) has been split between Cummins’ Rocky Mount Engine Plant (RMEP) in Whitaker, North Carolina and Darlington Engine Plant in the United Kingdom. At present, RMEP produces the L9 and B6.7 engines, with one of the latter engines representing the 5-millionth engine assembled at this facility. RMEP has been open since 1981, consumes more than 1.2 million square feet and employs more than 1,800 people. The L9 plant over in the U.K. has been in operation since 1965, employs roughly 1,500 workers and produced its first natural gas L9N in 2023.


Emissions Equipment: From ISL To ISL9 To L9

Exhaust Aftertreatment System for Cummins ISL9From its debut in 1999 until 2007, the ISL Cummins was virtually free of what would be considered modern age emissions control equipment. For example, 2003 was the year that nearly every engine manufacturer was essentially forced to implement exhaust gas recirculation (EGR)—but Cummins was able to hold off until 2007. By 2010, selective catalytic reduction (SCR) became part of the ISL (which had been renamed the ISL9) aftertreatment system, where it joined forces with the diesel particulate filter (DPF). Renamed again in 2017, the L9 (as it was now known) featured a more compact aftertreatment system with a high-efficiency SCR, a more durable DEF dosing unit and a DPF combined into a single, flow-through unit.


Fuel Systems: CAPS To HPCR

Comparison of ISL Cummins CAPS Pump and Common Rail High Pressure Fuel PumpAlong with a lack of emissions control devices on early ISL Cummins engines, the injection system was fairly low-tech, too. 1999 to 2003 model year engines were equipped with the Cummins accumulator pump system (CAPS) fueling arrangement. Beginning in 2003, CAPS was replaced with high-pressure common-rail (HPCR) injection to better curb particulate matter (PM) emissions at the tailpipe. The common-rail system featured full electronic control rather than the CAPS electronic (pump) over mechanical (injector) configuration, bringing finer atomization and significantly higher injection pressures into the equation. The HPCR system consists of the high-pressure fuel pump, fuel rail, high-pressure lines and solenoid-equipped injectors.


ISL400 (CM850): The Emissions-Free, Common-Rail ISL

Cummins ISL400 Diesel EngineOf course, there are always outliers in changeover years—and in this case some ISL’s built before January 1, 2007 featured common-rail injection, a variable geometry turbocharger and no EGR or DPF system. These 400 hp “unicorn” engines, ISL400’s with the CM850 ECM designation, are few and far between but remain highly sought after in the motorhome segment. They were commonly used in firetrucks, where their lack of a potentially problematic EGR system ensured optimal reliability in emergency situations. Full disclosure, some ISL’s produced during this timeframe were prone to a specific type of connecting rod failure. More on that in a bit.


Maintenance Highlights (EPA 2007 Engines)

EPA 2007 ISL Cummins Common Rail Diesel EngineDespite the use of EGR and DPF on EPA 2007 ISL engines, Cummins offered exceptional service intervals. Engine oil and filter changes were recommended every 20,000 miles (up to 49,700 miles on Euro 4 ISL engines), 500 hours or 6 months, and the same interval was suggested for the primary fuel filter. The secondary fuel filter change was required every 40,000 miles, 1,000 hours or 12 months, while the coalescing filter interval checked in at 80,000 miles, 2,000 hours, or 24 months. Additionally, overhead adjustment on the ISL wasn’t due until the 150,000-mile mark, 5,000 hours on the meter or 48 months had transpired.


Cummins History: Useful Life

EGR Cooler Repair for ISL CumminsProperly maintaining an ISL Cummins goes a long way in reaching the 500,000-mile mark, the average distance covered before an overhaul is required. This is especially true on ’07 and newer models, where emissions aftertreatment systems contaminate engine oil substantially more than on pre-’07 engines (soot from EGR, and percent fuel content via regeneration to name a few examples). But don’t let the fact that most ISL’s fall short of the million-mile club confuse you, these engines are generally placed in service in motorhomes, refuse trucks and transit buses—vehicles which generally don’t accumulate miles like Class 8 power plants do.


The Motorhome Favorite—The ISL425

Gulf Stream Motorhome with ISL425 CumminsIn the world of motorhomes, namely the diesel pusher category, the ISL Cummins has achieved near-legend status over the years. Naturally, the highest available horsepower rating (425 hp) is the favorite here, which also happened to turn out 1,200 lb-ft of torque at 1,300 rpm. According to countless RV owners, the ISL425—and specifically the pre-emission version—is the best engine option ever offered. At the time the ISL425 could be optioned it was the most powerful engine available in this class of vehicles, soundly outperforming the Caterpillar competition.


Medium-Duty Applications

Freightliner Dump Truck with ISL Cummins EngineIn the medium-duty segment the ISL Cummins didn’t enjoy class-dominating power ratings, but the engine did prove itself a reliable workhorse in demanding applications. Here, power ratings varied from 345 hp to 365 hp while peak torque ranged from 1,150 lb-ft (at 1,300 rpm) to 1,250 lb-ft (available at 1,400 rpm). In the waste management industry, the ISL Cummins is everywhere. But that’s not all. The engine is a hit in the tandem and triple-axle dump truck segment as well, not to mention them being fairly common finds in concrete mixers.


Transit Bus Applications

Transit Bus with ISL Cummins EngineThe ISL also found a comfortable home within the commuter bus realm. Here, lower 250 hp to 330 hp ratings are common (and torque outputs between 730 lb-ft and 1,100 lb-ft, respectively) with the primary goal being fuel efficiency over anything else. Its expansive horsepower ratings allowed municipalities to spec out the perfect buses for their needs, be they 30-foot units or 60-foot articulated models. Cummins furthered the attractiveness of the ISL by making it available in both conventional driveline and hybrid-ready configurations.


ISL Hard-Part & Lift Pump Problems

Wrist Pin Bushing Failure in Cummins ISL EngineThough the ISL and its subsequent variants have built and enjoyed a reputation for reliability over the years, one major weak link was encountered in 2006. A machining defect in the wrist pin bushings of the connecting rods was leading to wrist pin seizure and, in some cases, windowing engine blocks. The failures were limited in scope to ISL CM850 engines produced between October 17, 2005 and April 18, 2006, but Cummins issued a voluntary recall to address the matter. Another widespread problem, although much less likely to contribute to a vehicle crash or catastrophic engine damage, is lift pump failure. It's most common on ISL’s equipped with the CAPS fuel system (’99-‘03) in motorhome applications.


EGR Issues

EGR Cooler Failure in ISL Cummins EngineThis type of failure won’t surprise anyone, but the ISL Cummins wasn’t immune from exhaust gas recirculation problems. When the system was introduced, there were some growing pains associated with it (which arguably could be said of every engine manufacturer’s early EGR systems). Perhaps the biggest issue—both then and now—is EGR cooler failure. A common symptom of the failure for ISL owners is coolant consumption. Oftentimes, in Cummins history, when the internal heat exchanger portion of the EGR cooler is ruptured, coolant leaks into the exhaust tract where it is burned off. It’s not atypical for an ISL application suffering from a cracked EGR cooler to be two to three quarts low on coolant every 200 to 300 miles.


Additional EGR Headaches

Cummins ISL EGR Differential Pressure SensorFurther EGR-related issues include EGR valve failure, which is sometimes caused by a leaking EGR cooler, and failure of the EGR differential pressure sensor (pictured). When enough soot, carbon and grime builds up in the EGR differential pressure sensor ports the EGR system’s flow will be hindered. This triggers a fault code (DTC, CEL). Removing these sensors and periodically cleaning them can pay big dividends. In fact, regular EGR system cleaning intervals should be adhered to in order to avoid downtime. Many of the EGR cleaning products and procedures on the market today don’t even require any component removal to perform the job.


DPF Issues

NOx Sensor Failure in ISL Cummins EngineEvery diesel particulate filter has a lifespan before requiring cleaning and/or complete replacement, but over the years this interval has continuously been extended thanks to breakthroughs in technology. However, while the DPF itself has been made to last, its surrounding and supporting components don’t always live up to their end of the deal. For example, pressure differential sensors, temperature sensors and especially NOx sensors are frequent points of failure—and not just on the ISL Cummins but for every engine in the on-highway market. SCR issues with components like dosing modules, dosing valves and SCR catalyst failures are never out of the question, either.


VGT Issues

Holset VGT Actuator for ISL Cummins EngineAlong with EGR, DPF and SCR dilemmas come the problems associated with variable geometry turbochargers—and the ISL is no stranger to experiencing its fair share of VGT-related fault codes and failures. Perhaps the biggest problems stem from the VGT actuator and not the VGT itself. A bound-up turbo shaft sector, coolant leaks, fluid intrusion or even a damaged wiring harness can disturb the seamless operation of the actuator. Above all else, soot accumulation within the VGT actuator is the common culprit. In most cases, the actuator will cause drivability issues long before the turbo will.


Why The ISL Works So Well In Refuse (And Other Vocational) Applications

Refuse Recycle Truck with ISL Cummins EngineAt home and abroad, the Cummins ISL is an ideal midrange engine for a variety of vocational applications. One segment where the ISL tends to dominate is in the business of refuse. Because most refuse applications call for operation in urban, stop-and-go environments, the engine powering these trucks has to be as clean and fuel efficient as possible. This is where the ISL platform’s advantageous power to weight ratio came into play, which carried over to the emissions-friendly ISL9 and L9, as well as the natural gas-burning ISL G.



ISL-G Cummins Diesel EngineAs the big diesel emissions crunch began to take hold around 2007, Cummins released the ISL G. Based on the 8.9L ISL, the alternative fueled “G” was engineered to run on natural gas as opposed to diesel. With the primary goal of the ISL G being to reduce tailpipe emissions over its diesel counterparts, several added benefits came with it. The payoff in burning natural gas was lower maintenance costs and higher fuel efficiency while retaining the ISL engine’s proven reliability. Among other things, the ISL G Cummins was treated to a new ECM, a redesigned fuel module and air intake system, a maintenance-free aftertreatment catalyst and a different turbocharger.


ISL G Ignition System

ISL-G Cummins Spark Plug Ignition SystemThe ISL G’s use of stoichiometric EGR combustion technology with spark ignition produces oxygen-free exhaust, which enables the use of a simplified (and maintenance-free), three-way aftertreatment catalyst. Six high-voltage ignition coils, working in conjunction with a control module and spark plugs, carry out the combustion process. The precise burn that results from this means that all fuel and oxygen is consumed during combustion. Ultimately, it means no unburned fuel or oxygen exists to exit through the exhaust system. The ISL G can be run on either compressed natural gas (CNG) or liquified natural gas (LNG).


ISL Vs. ISL G: Mechanical Similarities & Differences

Comparison between ISL and ISL-G Cummins Diesel EnginesThanks to sharing much of its mechanical makeup with the ISL, Cummins’ production of the ISL G was a fairly straightforward undertaking. In fact, both engines were produced on the same assembly line. However, a few key mechanical differences between the two exist in the turbocharger and water pump. The ISL G is equipped with a smaller frame turbocharger equipped with an electronically controlled wastegate and that benefits from a water-cooled bearing cartridge. To meet its increased cooling needs over the ISL, the ISL G was fitted with a higher flow water pump.


Performance Improvements Over The ISL

Torque Curve Dyno Graph for ISL-G Cummins EngineCummins’ advanced stoichiometric combustion technology, combined with its smaller frame turbocharger, makes significant low-rpm torque output possible. Overall, in Cummins history, the ISL G’s peak torque output is generally lower than the ISL, but it produces a much broader torque curve—a torque curve that essentially begins right off idle. The abundance of low-end torque makes for superior transient response throughout the ISL G’s power band, which affords the ISL G exceptional drivability. Another advantage the ISL G has over its ISL counterpart is in the form of decibels. The ISL G boasts noticeably quieter operation.


Transit Bus Applications

Mass Transit Bus with Cummins Compressed Natural Gas ISL-G EngineIn North America the ISL G found a very comfortable home in transit buses. In these applications there were three basic power ratings. For smaller, lower GVW vehicles, the 250 hp at 2,200 rpm and 730 lb-ft of torque at 1,300 rpm option was typically preferred. A 280 hp version that turned out 900 lb-ft represented the mid-level power option, while a 320 hp and 1,000 lb-ft version was the most powerful. Along with achieving its peak horsepower at 2,200 rpm, the ISL G was also electronically governed at that same engine speed.


Medium-Duty Truck Applications

CNG Garbage Truck with ISL-G Cummins EngineFor refuse trucks, dump trucks, concrete mixers and other medium-duty on-highway vehicles, the ISL G was offered with five different power ratings. Entry level power ratings could be had in the form of 250 hp at 2,200 rpm and 730 lb-ft, or 260 hp at 2,200 rpm and 660 lb-ft. A 280hp (at 2,000 rpm) mid-grade option was available, which turned out 900 lb-ft of torque, and a 300hp version that turned out 860 lb-ft was the next step up. The premium engine option produced 320 hp at 2,000 rpm and 1,000 lb-ft of torque. All ISL G engines produced peak torque at just 1,300 rpm.


Maintenance Highlights (EPA 2007 Engines For Bus And Refuse Applications)

Spark Plug for Cummins ISL-G EngineAlthough some maintenance intervals are shared between the ISL G and ISL, oil changes, secondary fuel filter replacement and coolant flushes must be conducted sooner. Perhaps more importantly, the ISL G’s spark plug changeout interval occurs every 22,500 miles, 1,500 hours or 18 months—whichever comes first. Additionally, the nature of the ISL G’s operation requires overhead adjustment much more frequently. According to Cummins, the valves on the ISL G should be run every 30,000 miles, 2,000 hours, or 24 months. And that’s after an initial adjustment required at 1,000 miles.


Most Notable ISL G Problem

Cracked Cylinder Head in Cummins EngineA cracked cylinder head is never good for business. However, the formation of hairline cracks in the ISL G’s head are quite common—and far more common than in ISL applications, even though they share the same basic cast-iron piece. The ISL G’s tendency to crack cylinder heads is believed to be caused (at least in part) by the engine’s lean-burning nature. Of course, high air-fuel ratios, extreme cylinder pressures, timing and compression ratio can’t be ruled out as contributing factors as well. A quality aftermarket replacement head can cost as much as $3,000.



Near Zero Emissions Cummins ISL-G NZ EngineIn an effort to meet the “near zero” NOx emissions standard (0.02 g/bhp-hr) set forth by the EPA and CARB, Cummins developed the ISL G NZ in collaboration with Westport Innovations. This evolved version of the ISL G, introduced in 2016, was the first mid-range engine in North America to meet Near Zero NOx emissions standards for refuse, urban bus, medium-duty, and school bus applications. The ISL G NZ runs 100-percent on natural gas, which can be carried onboard with the vehicle in either compressed (CNG) or liquified (LNG) form.


Further Versatility Of The ISL G NZ

L9N Cummins CNG EngineAn added benefit with the Cummins ISL G NZ is that it can also be run on renewable natural gas (RNG). RNG can be produced from a variety of sources (including livestock waste, food waste and even landfill waste) and is a major selling point for companies looking for environmentally friendly fleets. The ISL G NZ was replaced by the L9N in 2018. Cummins’ L9N is yet another spin-off of the original ISL engine design, but this version effectively represents a new-and-improved version of the ISL G NZ. Like its predecessor, the L9N (shown) is compatible with CNG, LNG and RNG.

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