Cummins, Duramax, Power Stroke —They All Break

As much as some of us hate to admit it, diesel engines do fail. Pushed too far, there is only so much stress an OEM rotating assembly or valvetrain will tolerate. And sometimes even balanced, blueprinted, and beefed up power plants succumb to excessive boost, cylinder pressure, or even a fluke hard-part failure. Whatever the case may be, no engine is immune to catastrophic damage if it’s leaned on hard enough. This month, you’ll find a full-on carnage clinic, with various failures—from all three brands—on full display. Bent and broken rods, dropped valves, cracked pistons, grenaded blocks, exploded CP4’s, smoked rod bearings, snapped crankshafts, and sheared transmission shafts. It’s all here for your viewing pleasure.

First-generation 6.7L Power Stroke rods are known to bend around the 650-rwhp mark, but in the early days of tuning the 6.7L it was common for it to happen sooner than that. This rod was pulled from an ’11-’16 Ford with an S400 and added fuel that came into LinCo Diesel Performance with a loss in compression… Thanks to better tuning, factory 6.7L Power Stroke rod failure isn’t very common at this point, and the larger OEM rods that entered production beginning with Job 2 ’17 model engines certainly helped, too.
If the big ends of these rod caps look like they got a little toasty (notice the bluing), it’s because they did. Lack of sufficient oil pressure at full load and high rpm thanks to an inconsistent oil pressure regulator smoked this 6.7L Power Stroke’s rod bearings. Low oil pressure is a common problem in the 6.7L Power Stroke, and it has the potential to become a huge problem once you double the factory horsepower.

The twin-piston Bosch CP4.2 high-pressure fuel pump has been used on the 6.7L Power Stroke since it debuted in ’11 and is a fairly neat, cost-effective piece. However, its tendency to self-destruct and contaminate the engine’s high-pressure, return, and low-pressure fuel circuits (usually due to air infiltration, believe it or not) is worrisome. A typical failure entails the roller tappet present in the bottom of the piston (shown) rotating within its bore, where its relationship with the camshaft it’s supposed to ride on changes drastically. From there, the roller tappet begins digging into the cam lobe, sending metal shavings through the rails, injectors, and even back to the tank.
Here you can see the result of the CP4.2’s roller tappet digging into the camshaft. Although this type of failure occurs on 6.7L Power Strokes (and every engine equipped with a CP4.2, for that matter), it’s noticeably more common in the LML Duramax. More on that and why it’s so much more frequent on GM’s ’11-’16 6.6L workhorse later.
Occasionally, a CP4.2 failure yields a few fireworks. This pump exploded and made a huge mess of fuel in the valley before killing the engine and getting towed in to Zeigler Diesel Performance. As previously mentioned, air infiltration is the biggest cause of CP4.2 failure (many times due to an improperly installed fuel filter, not priming the low-pressure fuel system after a fuel filter change, or running the fuel tank empty), but contaminated fuel or a lack of fuel system maintenance can do the trick just the same.
Slipping the crank gear is a problem that surfaces when the 6.7L Power Stroke’s factory rotating assembly is tasked with handling hundreds of extra horsepower. And because the camshaft and CP4.2 are both driven off of the crankshaft, for obvious reasons this can spell disaster. To avoid it, many pull the front cover and TIG weld the crank gear to the crankshaft. For added insurance, some even weld the cam gear to the camshaft while they’re in here.
On the 6.4L Power Stroke, cracked pistons are a major concern. The failure is common in both high-horsepower and completely stock engines, but is most frequent in higher mileage candidates. It’s believed that the thin lip on the piston’s fuel bowl is to blame, as pistons employed in Navistar’s MaxxForce 7 engine (the commercial 6.4L), which feature a thicker lip, seem to hold up better to excessive heat and cylinder pressure.
Eventually, the fatigued, heat-abused cast-aluminum 6.4L piston cracks, and the crack almost always spreads along the center line of the wrist pin from there. In nearly every scenario, the crack also begins at the lip of the fuel bowl. While the 6.4L can make nearly 600-rwhp in tune-only form, this particularly common failure understandably makes many ’08-’10 Ford owners nervous. It doesn’t help that the 6.4L is one of the most expensive diesel engines to rebuild.
This problem doesn’t result from horsepower, but it can happen in any engine that’s neglected. Active EGR, an active DPF and regen system, plus extended and possibly forgotten oil changes ended up bringing this 6.4L into JH Diesel and 4×4 with a major sludge problem. It’s said to be more prevalent in service or fleet trucks that spend inordinate amounts of time idling. To that we say remember this: idling is equivalent to traveling 25 mph…so make sure to factor that into your service intervals.
The 5R110W TorqShift enjoys a mostly positive reputation for being tough as nails in factory form, especially in the version used behind the 6.4L Power Stroke. However, RCD Performance was quick to show us what an aggressive 3-to-5 shift can do to the factory intermediate shaft. The splined end of the shaft was completely snapped off when the harsh gear change took place.
In the 7.3L world, stacking power on top of a powdered metal rod bottom end is a big no-no. The PMR’s (as they’re commonly referred) were phased into the 7.3L in late 2000 (which means ’01 model year trucks) and eventually replaced the forged-steel units, which came standard in all ’94.5-’00 engines. When this ’02 PMR-equipped 7.3L was saddled with larger injectors and tuning that called for aggressive low-rpm fueling (i.e. big torque), one of its eight powdered metal rods failed, breaking along the lower portion of the beam. The general consensus in the 7.3L aftermarket is to draw the line for a PMR engine at 400 to 450-rwhp, or 800 to 850 lb-ft of torque.
Depending on how much abuse they see, the 7.3L’s factory forged-steel connecting rods can handle 500 to 600-rwhp reliably, and sometimes even 650-rwhp. Eventually however, they will buckle under the strain of high boost and elevated cylinder pressure. Fortunately, forged-steel rods typically bend rather than break like the powdered metal units do, meaning you’re not searching for a fresh block when one fails. Here, a bent forged-steel factory rod out of a 700-rwhp 7.3L is pictured next to an aftermarket 4340 forged, I-beam rod from Manley.
The easiest way to identify a forged-steel rod from a powdered metal unit is by the rod cap fastener. Forged-steel rods secure the cap via studs, while PMR’s utilize bolts. Neat tidbit courtesy of Hypermax Engineering: the cut-off from 7.3L’s with forged-steel rods starts from beginning of production through engine serial number 1425746, and then 1440713 through 1498318 (when Ford/Navistar reverted back to the forged-steel units in order to deplete its remaining inventory). Powdered metal rods infiltrated the 7.3L beginning with serial number 1425747, through 1440712. Following the reversion back to forged-steel rods, serial number 1498319 to final production were assembled with powdered metal rods.
Built engines break, too. In this case, a forged R&R rod residing in Unlimited Diesel Performance’s P-pumped 7.3L buckled while making more than 2,000 hp on the engine dyno. Admittedly, after two full seasons of truck pulling and dozens of dyno pulls, they knew the R&R units owed them nothing. Prior to the failure, UDP’s wild mechanical 7.3L—equipped with a 15mm Pump Doctor injection pump, Scheid billet triple-feed injectors, and a 4.4-inch Wimer turbo—produced an incredible 2,180 hp and 2,821 lb-ft of torque on Marlatt Competition Engines’ dyno.
Twice the factory boost, stock valve springs, and a tachometer that read more than 4,000 rpm culminated in some pretty destructive valve float in this 7.3L. What’s left of one of the valves is embedded in the cylinder head. Further down the line, the powdered metal connecting rod also gave way during the melee. It’s proof that stacking even mild power adders on top of a mostly stock engine, combined with aggressive driving, can kill any reputable engine.


There is abusive driving, excessive stress for stock parts to have to cope with, and then there are fluke failures that happen once in a blue moon. When this bone-stock 6.7L Cummins dropped a valve seat at 17,000 miles all hell broke loose in the cylinder. Of course, the failure was covered under warranty, but the Ram’s owner was out his workhorse—and a means to make a living—for nearly a month.
If an engine has a weak link, a shop like Hardway Performance can find it. An OEM 6.7L Cummins rod enters the danger zone around the 1,800 lb-ft of torque mark (roughly 900 hp), but of course any hard part can fail under a heavy right foot—and at any power level.
While we all love the quality of the products in the diesel aftermarket, there are certain areas where OEM parts should be adhered to. This is especially true in the maintenance department. Believe it or not, an off-brand oil filter came apart on this low-mile Ram 2500, clogged a couple piston cooling nozzles, and starved the pistons of oil in two cylinders.
Although you can get away with making four-digit horsepower and (more importantly) 2,000 lb-ft on a stock bottom end common-rail Cummins for a little while, the factory rotating assembly won’t last forever—especially if it’s abused. Such was the case for this engine, the recipient of a windowed block. The owner was running big compounds, dual stroker CP3’s and sizable injectors, but also had a built backup power plant in the works. Needless to say he was fully prepared when this day arrived.
The right horsepower combination, good tuning, and a bit of luck kept Steven Giordano’s 5.9L common-rail alive for countless dyno pulls and bottom 7-second eighth-miles. Then a factory rod called it quits and exited the block in spectacular fashion. Prior to the ’06 Cummins’ abrupt end of life, Steven was able to squeeze as much as 1,370-rwhp and 1,900 lb-ft out of it. Impressive to say the least.
Don’t worry, plenty of Power Stroke and Duramax mills chuck rods, too, but being that the Cummins is the most commonly modified engine in the diesel segment lends itself to the majority of the crankcase failures we see. Here, another catastrophic connecting rod failure is on display, courtesy of Redline Diesel Power.


In the Duramax camp, it doesn’t get any more serious than a broken crankshaft. Even though the advent of billet cranks and alternate firing order camshafts has reduced the amount of high-horsepower 6.6L’s that experience this fatal failure, it does still occur. Typically, the factory crankshaft will break at the number one rod journal, usually as a result of excessive rpm and the large external counterweight. A standard firing order camshaft is also believed to do a number on the front portion of the factory crank as well.
CP4.2 failure on LML Duramax’s is nothing new, but occasionally we come across a cataclysmic case where the pump literally blows apart. This casualty was discovered and dealt with at Lead Foot Diesel Performance. After the cam seized, the CP4.2’s aluminum case exploded. Air entering the fuel system, maintenance neglect, lack of a factory low-pressure fuel supply pump, and contaminated fuel are the primary culprits behind CP4.2 failure on the LML.
Each generation Duramax has its share of shortcomings. On the LB7 and LLY, the connecting rods were the weak link. On the LBZ and LMM engines the rods are stronger, but the pistons are on borrowed time around the 650-rwhp mark. Like most piston failures, the cast-aluminum factory units found in the LBZ and LMM crack along the center line of the wrist pin. A word of advice: if you’re pursing 600-plus horsepower with your ’06-’10 Duramax, stick with a single turbo. The added drive pressure that compounds add can kill stock pistons in very short order.
As proof that the previous two CP4.2 explosions in this article aren’t flukes, here is yet another example. This one is brought to you courtesy of Isaac Griffith, owner of Peak Combustion in Home, Pennsylvania. It’s no wonder CP3 conversions are so popular.
As far as turbo trouble is concerned, only the IHI unit—the fixed geometry turbo employed on the LB7 Duramax—is known to fail catastrophically. Most failures come courtesy of an overspeed scenario after the wastegate has been disabled. Show an RHG6 IHI more than 30-psi of boost (and who knows how much drive pressure) and you find out the hard way that squeezing more than 500-rwhp out of the factory turbo wasn’t such a good idea.


Flynn’s Shop

Hardway Performance

Hypermax Engineering

JH Diesel And 4×4

Lead Foot Diesel Performance

LinCo Diesel Performance

Peak Combustion

RCD Performance

Redline Diesel Power

Unlimited Diesel Performance

Zeigler Diesel Performance


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