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Top ISX15 Problems

Written By, Mike McGlothlin


The ISX15 is one of the most common over-the-road engines you’ll find in North America. Both during and following its 22-year stint on Cummins’ assembly line, it enjoyed a reputation for reliability, power, and fuel efficiency. The most durable of the lot would be the pre-emissions ISX mills, released in 1999 as the replacement for the revered yet dated N14 Cummins. These early ISX power plants weren’t saddled with the complex and often problematic emissions-scrubbing equipment later engines would be, which made them million-mile contenders. Surprise surprise, when the aforementioned emissions equipment did enter the picture so did a lot of problems.


Exhaust gas recirculation (EGR), diesel particulate filter (DPF) and selective catalytic reduction (SCR) system issues plague most late-model ISX15’s, just as they do on other manufacturers’ engines. But the ISX wasn’t without its fair share of mechanical flaws and factory shortcomings, either. Dual overhead cam models were prone to excessive lobe wear, early high-pressure common-rail fuel pumps could self-destruct, and later ISX15’s were hindered by poor OEM counterbore machining. From emissions-related failures to head gasket headaches to fuel system catastrophes, we’re bringing the ISX15’s biggest problems to light this time—along with the proper solutions.


Early ISX Reliability

DOHC Dual Overhead Cam ISX15 CumminsGenerally speaking, the ISX15 Cummins engines built between ’99 and ’02 are considered to be the most reliable versions ever produced. These early models, denoted via their ECM code of CM570, were void of the exhaust gas recirculation system that would be introduced midway through 2002 model year production (on CM870 models). However, any ISX15 built between ’99 and ’09 that’s been treated to favorable maintenance is considered to have the best shot of hitting the million-mile mark. This is because Cummins developed the ISX15 (in the mid-to-late 1990s) with tightening NOx and particulate matter (PM) emissions regulations in mind.


Common EGR Problems

EGR Cooler Cummins ISX15 Diesel Engine PartsMake no mistake about early ISX15 reliability, these pre-DPF engines still regularly experience various EGR-related issues. As with all diesel power plants in the modern era, EGR cooler, valves and pressure differential sensors all fail at some point. With any EGR system, problems usually surface via the cooling system. Discolored coolant and especially coolant loss are telltale signs of a cracked EGR cooler. For a DPF-equipped (or DPF and SCR-equipped) engine, burning coolant and sending those gases through the particulate filter is never healthy for the longevity of the aftertreatment system.


DPF—An Inevitable Failure Point

Diesel Particulate Filter ISX15 CumminsThe addition of the DPF to ISX engine added another level of complexity to the exhaust system—and one that inevitably requires cleaning and eventually complete replacement. Within the DPF system, a large filter is employed to trap the particulate matter in the exhaust stream (i.e. soot), keeping it from exiting the tailpipe. Eventually, the DPF fills with soot and requires either an active or passive regeneration event to essentially turn PM into a fine ash. Regular ash cleanings should be carried out every 250,000 to 400,000 miles on an ISX15, with eventual DPF replacement being necessary at some point in the engine’s lifetime. Along with the DPF, the various sensors (namely the pressure differential variety) associated with the system are prone to failure with both use and age.


Top Problems For SCR Engines

SCR Emissions System Cummins ISX15When selective catalytic reduction (SCR) entered the picture in 2009, additional sensors were added, more parameters had to be monitored, and extra NOx-curbing tasks needed to be carried out. This was accomplished through the use of injecting diesel exhaust fluid (DEF) upstream of an SCR catalyst, and the doser valves responsible for handling the job are prone to sticking and leaking. Other common issues with SCR ISX engines are DPF pressure differential sensor failure (which it shares in frequency with the pre-SCR versions discussed above), SCR NOx sensor failure, and various DPF and NOx catalyst faults, many of which are believed to be due to excessive idling or PTO use.


Potentially Catastrophic Fuel System Failure On Early Common-Rails

High-Pressure Fuel Pump Failure Cummins ISX15When Cummins released the high-pressure common-rail ISX15 in 2010, it did so with a potentially fatal flaw present in its high-pressure fuel pump. Until 2017, all ISX15 pumps were equipped with ceramic fuel plungers, which were prone to premature failure. But not only that, they often failed in catastrophic fashion, breaking apart or outright exploding and sending shrapnel through the rest of the engine by way of contaminating the engine oil. Steel plungers replaced the ceramic units within the HPFP beginning in 2017, which solved the issue. Since that time, countless early pumps (’10-‘16) have been retrofitted with the updated steel plungers.


Crooked Counterbores And Head Gasket Issues

Block Counterbores ISX CumminsFor whatever reason, many 2010-later ISX15’s left the factory with poor counterbore machine work. The improper cylinder liner protrusion that results from this leads to head gasket failures very early on in the engine’s projected life cycle. The asymmetrical counterbores also cause the head to unevenly press down on the liners, even wearing them into the block. After any head gasket failure, the counterbores need to be checked for proper depth. This is paramount in both determining the exact cause of the failure but also to ensure it doesn’t happen again. The words “quick” and “repair” never belong in the same sentence as “blown head gasket”—and especially not when you’re dealing with an ISX15 Cummins.


Top VGT Issues

Holset HE451VE Turbocharger Cummins ISX15Variable geometry turbo (VGT) technology, with its ability to provide instantaneous response at virtually any engine speed, is a marvel of modern engineering. However, the exhaust side of these turbochargers—where all the magic happens—has its work cut out for it. Soot buildup, rust, and other corrosion often gums up the works in the variable geometry equation. On every ISX15 built after ’02, sliding nozzle seizure is rampant whether it be the Holset HE551V, HE561VE, or HE451VE. This failure often leaves you with an engine that’s laggy down low and strong up top, or very responsive at low rpm but choked off and restricted at higher engine speed. Additionally, VGT actuator failure is common—and in many cases will occur before an issue arises with the turbo itself.


Camshaft Problems In Unit Injector Engines

Camshaft Wear ISX15 CumminsFrom 1999 to 2009 the ISX15 utilized dual overhead cams (DOHC), with one camshaft operating the valvetrain and the other actuating the fuel injectors for the unit injection system. The injector camshaft is highly susceptible to internal debris buildup (the cam is hollow), which overtime can clog oil passages and take out cam bearings. Excessive roller and rocker lever wear is also very common on DOHC ISX15’s. Pitted rollers and extreme camshaft lobe wear (often coined “flattening out the cam”) are typical finds when these engines are torn down. Neglected oil changes, extensive oil change intervals, and oil that’s been contaminated by engine coolant frequently contribute to ISX camshaft issues.


Engine Oil Pump Failures

Oil Lubrication Pump Cummins ISX15Adequate oil pressure is the lifeblood of any engine’s hard parts, so any issue that leads to low (or no) oil pressure is about as serious as it gets. Within the ISX15’s oil pump, the oil pressure relief valve used to be notorious for sticking. Luckily, Cummins updated this years ago. However, that doesn’t mean oil pump failure is non-existent. Most oil pump failures on the ISX occur from improper installation. Remember, these pumps must be aptly shimmed in order for the gears to have the proper backlash they need to operate effectively and reliably for hundreds of thousands of miles.


The Best Way(s) To Avoid Problems With Your ISX15

ISX15 Cummins Class 8 Diesel EngineSometimes engine, fuel system, or emissions equipment failure is inevitable, but by observing a sound maintenance regimen you give your ISX the best possible chance of going the distance. This means regular oil changes at the proper interval (no extended schedules) and using both the engine oil and filter specified by Cummins (no cheap knock-offs). In the modern era, where EGR systems constantly deposit soot into the engine oil, keeping clean oil in your Cummins is more vital than it’s ever been. Where applicable, it can also pay big dividends to observe regular emission system cleanings (namely for EGR and DPF).

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