Written By, Mike McGlothlin
Just as we offer Long Block and Extended Long Block versions of the legendary 5.9L Cummins 6BT (and also the 5.9L ISB’s), we also remanufacture the 6.7L Cummins here at Big Bear Engine Company. Like the 5.9L ISB it replaced, the Cummins 6.7L brings great durability and a workhorse nature to the light and medium duty truck segments, as well as the agriculture and industrial sectors. But not unlike the 5.9L, the larger (and newer) 6.7L has a few chinks in its armor. This guide highlights the most common 6.7L Cummins problems owners face in real-world use. In fact, thanks to modern day emissions equipment, it could be argued that the 6.7L Cummins is the least reliable of the lot (6BT, 5.9L ISB 24-Valve, and 5.9L ISB Common-Rail included). We’ll call attention to all of the 6.7L’s emissions problems below, as well as its higher propensity to blow a head gasket, the rampant lifter failures occurring in the newest versions, and the dreaded grid heater bolt failure.
Problem #1: Blown Head Gasket
Experiencing a blown head gasket isn’t a guarantee with the 6.7L Cummins, but it happens
more often than it does on the 5.9L’s that preceded it. High miles, overheating, and certainly abuse can lift a cylinder head. Of course, head gasket failure occurs more frequently when engines are exposed to tens of thousands of miles with hot ECM tuning in the mix—which is the case for (arguably) more than half of all 6.7L Cummins-powered Ram pickup trucks. The telltale signs of a blown head gasket are 1) coolant boiling out of the degas bottle/overflow reservoir cap, 2) a stiff upper radiator hose on a cold engine, 3) a fluctuating water temp gauge, and 4) contaminated coolant or engine oil.
Why It Fails
Theories abound as to why the 6.7L Cummins has a higher propensity for blowing head
gaskets than the 5.9L, but the most logical reason has to do with its displacement advantage. The larger, 408 ci 6.7L Cummins produces more torque than the 5.9L. This means it sees higher cylinder pressure, which translates into a harder life for both the head gasket and the head bolts. The 6.7L’s larger cylinder bore means the seal is spread across a wider area. Then you have the 6.7L’s use of a variable geometry turbo (VGT). While a VGT might not make more peak drive pressure than a fixed geometry turbo, by design a VGT provides consistently higher drive pressure, which can take a toll on the head gasket over time.
The Solution
Aftermarket head studs in place of the factory head bolts have long been the go-to solution for avoiding a blown head gasket (shown). On top of resurfacing the cylinder head, the block’s deck surface should be checked for trueness before head studs are threaded into place. Building on the first solution, a resurfaced cylinder head that’s been machined to accept steel fire-rings adds a level of insurance on top of what head studs alone provide. Head studs offer improved head-to-block clamping force and the fire-rings better seal combustion. Last but not least, replacing the factory VGT with a fixed geometry turbo (such as an S300-based BorgWarner or a small S400 frame unit) lowers drive pressure, which reduces stress on the head gasket, head fasteners, and the engine overall.
Problem #2: VGT Actuator
The variable geometry turbocharger is a marvel of modern engineering, but it’s not
without its faults and, in particular, the VGT actuator on the 6.7L Cummins is notorious for failure. When it stops functioning properly, a loss in drivability and performance is immediately apparent. With an inoperable VGT actuator, the turbo will either spool quickly but limit the engine’s performance at mid-range and high rpm, or spool slowly and cause a significant lack of low-rpm torque output. Electronically controlled yet mechanically actuated, the 6.7L Cummins’ VGT actuator can fail both on the wiring/circuit board side or the mechanical side.
Why It Fails
A 6.7L Cummins VGT actuator can fail for several reasons, including soot and carbon
buildup or rust and corrosion limiting the travel of the vane plate arm, actuator gear issues, or a bad or corroded actuator circuit board (typically due to coolant or water infiltration). Many times, an accumulation of soot and grime occurs within the actuator housing due to a leaking or sticking EGR valve. P003A and P00AF diagnostic trouble codes are the most common DTCs associated with actuator failure. In the field, VGT actuator failure occurs most frequently on engines that spend an inordinate amount of time idling.
The Solution
In some cases, the VGT actuator simply needs to be recalibrated. The latter process calls for a scan tool, entails pre-aligning the actuator gear, and calibrating the start and stop points of the actuator (to allow for full vane plate travel). In other instances, a thorough cleaning of the actuator and exhaust side of the turbo can free things up enough to restore functionality. When both of the above options don’t solve the problem or can’t be performed, a replacement actuator becomes one of two remaining solutions. The final remedy entails replacing the factory VGT with a fixed geometry turbo. This calls for aftermarket ECM calibrating and can render the engine incapable of meeting emissions standards.
Problem #3: EGR Valve
Diesel engines produced in the early to late 2000s and beyond are all equipped with
exhaust gas recirculation systems (EGR). The 6.7L Cummins is no different. Its system consists of an EGR valve that routes a small percentage of spent exhaust gases back into the engine’s intake tract, and an EGR cooler which uses engine coolant to decrease exhaust gas temperatures before they’re allowed to reenter the intake. The former component, the EGR valve assembly, features an electric motor and a spring-loaded, solenoid-activated, sliding valve. Over time, the EGR valve succumbs to the environment it lives in.
Why It Fails
A combination of soot, carbon, and oil vapor gradually accumulates within the EGR valve
(not to mention the EGR cooler, intake, and even the cylinder head). Eventually, this accumulation hinders the full functionality of the sliding valve and it can begin to stick. Diagnostic trouble codes P0403, P0405, P0401, and P0402 can all result from EGR valve issues. Codes P0401 and P0402 in particular are thrown when insufficient (or excessive) exhaust gas flow is detected, and they’re highly common when the sliding valve begins to stick or seize up in the open or closed position.
The Solution
The best way to avoid EGR valve issues with the 6.7L Cummins is to subject the system to regular cleanings. For 2007.5 to 2018 model year engines, the EGR valve, EGR cooler, and their respective piping should be cleaned every 67,500 miles. On 2019 to 2024 engines, the EGR cleaning interval jumps to 75,000 miles. Between cleaning intervals, minimizing the 6.7L Cummins’ idle time can significantly reduce soot and carbon buildup inside the EGR valve. Of course, disabling or removing the EGR system is another solution—but it will void your engine’s warranty, may increase its NOx emissions and, in most applications, renders the engine illegal for on-highway use.
Problem #4: VGT
We’ve detailed the issues associated with the VGT actuator, but now it’s time to delve into
the turbo itself. The Holset VGT used on 6.7L Cummins engines, not unlike the VGT’s employed on comparable diesel engines in this segment, is prone to experiencing turbine (exhaust) side issues—namely from the sliding nozzle mechanism. We’ll also note here that any time a VGT actuator is replaced, the turbo itself should be removed for disassembly, thorough inspection, and a good cleaning of all internal components (unison ring, nozzle cage, turbine housing, turbine wheel, thrust washer, shaft, journal bearings, compressor wheel, etc.).
Why It Fails
In most cases, the VGT aboard the 6.7L Cummins becomes problematic due to the soot,
carbon, oil vapor buildup we mentioned earlier, as well as rust and corrosion being common contributors. Any accumulation that hinders the sliding nozzle ring’s travel can result in poor performance, reduced fuel economy and, depending on the position it’s stuck in, can bring about extreme turbo lag or virtually no top-end power. Sticking VGT issues can create overboost and/or under boost scenarios (which can force the ECM to throw DTCs). Instances of overboost increase stress on the turbo, but also the engine (think head gasket).
The Solution
To help prevent sticking VGT issues from starting, it pays to work the 6.7L Cummins hard. Heavy towing, full-throttle operation doesn’t have to be an everyday occurrence, but the more rpm, load, and heat the engine sees the better. Any time the variable nozzle side of the VGT sees full range of motion (i.e. hard acceleration), it will serve to keep the turbo in good working order. Leaving the exhaust/turbo brake on and in continuous use will also help limit accumulation. To rule the 6.7L Cummins’ VGT issues out for good, it can be replaced with a fixed geometry turbo. It’s a swap that will sacrifice some of the engine’s low-end torque, but a fixed geometry unit will outlast two or three factory VGT’s before requiring an overhaul.
Problem #5: Diesel Particulate Filter (DPF)
When diesel engine manufacturers were forced to meet a 90-percent reduction in
particulate matter emissions between 2006 and 2007, most of them turned to the diesel particulate filter to get the job done. And the old saying “It’s not a matter of if, but when” has long-applied to the guaranteed failure rate of the emissions-curbing equipment employed on modern day diesel engines—especially the DPF. It’s a cliché that most defnitely applies to DPF failure on the 6.7L Cummins. Sooner or later, the sensors scattered throughout the exhaust aftertreatment system or the filter itself will have issues.
What Fails
Inevitably, the DPF fills up with soot and self-cleans via “regeneration,” transforming the
trapped diesel particulate matter into a fine, storable ash. However, eventually an accumulation of ash renders the DPF unusable until it’s either physically removed and cleaned or replaced. Outright failure of the DPF usually comes in the form of a cracked element, which can allow soot to exit out the tailpipe and (typically) trigger a check engine light. Then come the various sensors. Pressure differential sensors, NOx sensors, and EGT sensors. Any one of them can cease to function properly at any time, prompt a CEL, and even derate engine power.
The Solution
While there is no end-all, be-all solution for DPF failure, observing regular cleaning intervals can help preserve its life and ensure it remains in good working order. No DPF cleaning interval exists for the 6.7L Cummins, but literature for its bigger brother, the ISX, calls for a fresh start every 200,000 miles. Given the smaller size of the 6.7L’s particulate filter, we would opt for 100,000-mile cleanings. Driving habits that include allowing the active regeneration process to complete (i.e. not turning the engine off until it’s finished) and avoiding excessive idle time (less heat means higher levels of PM are produced) will also help to ensure a long service life for the DPF.
Problem #6: Lifters
For the first time in B series and ISB series engine history, Cummins switched from flat
tappets to hydraulic lifters beginning in 2019. The self-adjusting, hydraulic units rule out the need for periodic valve lash adjustments while simultaneously serving to dampen noise within the valvetrain. Unfortunately, things haven’t exactly panned out in the durability department. In as little as 10,000 miles some ’19-newer 6.7L Cummins owners begin noticing a top-end “tick” coming from the engine. As the miles rack up, the lifter noise increases, and so does the risk of them wiping out the camshaft and, in a worst-case scenario, even the engine.
Why They Fail
A flawed design is to blame for the 6.7L Cummins’ failed hydraulic lifters, and it all starts
with lubrication. Microscopic passageways within the roller assembly are believed to limit the engine oil’s ability to adequately lubricate, and ultimately cool, the component. And because such tight internal tolerances are in play, running a thicker engine oil (such as 15W-40) exacerbates the inadequacy. Additionally, and in direct contrast to what is offered in the hydraulic lifters used in the 6.7L Cummins’ direct competitor, the 6.7L Power Stroke V-8, there are no needle bearings present in the roller. Eventually, the roller begins to seize, becomes blued due to super-heating, and the lifter can collapse.
The Solution
Other than starting anew with fresh OEM lifters and (in most cases) a factory camshaft, the hydraulic lifter issue is best solved by turning to the diesel aftermarket. The first fix is offered by Wagler Competition Products (shown), and the company’s solid roller conversion kit converts the factory hydraulic lifters to solid versions using spacers. The package also includes appropriate length replacement pushrods and OEM adjustable rockers. For higher horsepower applications, Wagler also offers a Jesel lifter upgrade and a performance camshaft option. Hamilton Cams’ flat tappet conversion kit offers 6.7L Cummins owners another sound solution. The company’s all-inclusive system returns the ’19-newer engine back to the tried-and-true flat tappet arrangement Cummins used from 1989-2018.
Problem #7: Selective Catalytic Reduction (SCR) System
Although the introduction of selective catalytic reduction (SCR) for the 2013 model year
improved the longevity of the engine’s EGR system, it brought added complexity and introduced its own unique sets of issues to the 6.7L Cummins platform. The process of using diesel exhaust fluid (DEF)—a mixture of 32.5-percent pure urea and 67.5-percent deionized water—to convert harmful NOx pollutants into harmless nitrogen requires a handful of components, sensors, and fluid all working in perfect unison. When one piece of the puzzle fails, you can get a slew of DTCs, an engine with de-rated power, and even a truck parked on the side of the road.
What Fails
Diagnostic trouble codes calling attention to DEF quality are highly common on 2013-
newer 6.7L Cummins SCR systems. These issues typically come from poor quality, expired, or contaminated DEF in the DEF tank (yes, DEF has an expiration date), and many times they’re solved by cleaning or replacing the DEF injector (also known as the doser). A dead DEF tank module—typified by the DEF level reading low and triggering limp mode/power derate—is another frequent (and costly) SCR-related failure. Then come the sensors, with the system’s NOx sensors leading the charge. Without them functioning properly, the efficiency of the SCR catalyst is compromised and you often get a CEL, increased DEF consumption, and usually a drop in fuel economy.
The Solution
Sorry, other than removing the SCR system in strictly off-road applications there is no permanent solution for SCR-related problems. However, cleaning the system’s sensors (especially the NOx sensors) at regular intervals can help keep them reading accurately for longer periods of time. System uptime can also be maximized by keeping spare sensors onboard the vehicle and ensuring that only high-quality, good condition diesel exhaust fluid is present in the DEF tank. And finally, always address any check engine light (CEL) or DTC related to the SCR system as soon as possible to avoid a possible engine derate or limp mode scenario.
Problem #8: Grid Heater Bolt Failure
It’s a failure that can take years to strike, if it happens at all, but once it does, metal-on-
metal mayhem ensues and you can be out a cylinder head, a cylinder, or even an engine. Grid heater bolt failure is a growing problem on high-mile ’07.5-’18 6.7L Cummins mills. The condition of the fastener (which is technically a stud, not a bolt) is often overlooked due to its location on the underside of the grid heater. However, its condition should be inspected any time the intake elbow or grid heater is removed, or every two to three years.
What Fails
The fastener and 10mm nut that ties in with the grid heater’s 12-volt lead is exposed to
thousands of heat cycles. Over time, it becomes fatigued, gets brittle, and can eventually deteriorate to the point where the stud breaks. From there, it falls into the intake runner, rolls to the back of the intake shelf and, due to gravity, can fall into the number 6 cylinder through the intake port on the head. Valve and other cylinder head damage, a destroyed piston, and a beat-up cylinder wall are often the result. In extreme cases, a complete replacement long block is required.
The Solution
First, what’s known as the “jiggle test” should be performed. This entails wiggling the grid heater’s 12-volt post from the topside and stopping if you feel any noticeable amount of movement. Movement means the grid heater stud could fail at any time. This early detection affords you the ability to replace the stud and nut with a new version or rule out the failure completely. To eliminate the grid heater stud for good, Banks’ Monster-Ram intake system is a popular solution. Its high-flow intake elbow system includes a billet intake plate that replaces the grid heater (and the problematic fastener) altogether. In place of the grid heater, a coil heater provides equal (if not better) cold-start performance.
Problem #9: High-Pressure Fuel Pump (CP4.2)
When Ram introduced the high output 6.7L Cummins for the 2019 model year, the all-
new, CGI block engine came with a Bosch CP4.2 high-pressure fuel pump and not the highly revered CP3 that’d been in use since 2003. After just two model years and loads of warranty claims thanks to the CP4.2 coming apart, Ram and Cummins reverted back to the CP3 from 2021 to 2024. Just as is the case with the ’11-present 6.7L Power Stroke, the ’11-’16 LML Duramax, and the late 5.0L Cummins V-8 found in the Nissan Titan XD, the CP4.2 is prone to catastrophic self-destruction. Trust us, it’s more than your average pump failure…
How It Fails
When a CP4.2 checks out, it takes everything with it. Metal debris from the pump breaking
down internally is routed to the high-pressure lines, through the rail, and to the fuel injectors. On the return side, the lines and tank become contaminated with metal-laced fuel, as is the low-pressure supply circuit. When air or contaminants infiltrate the CP4.2 pump, the pistons in the high-pressure chambers can float and rotate within their bores. From there, the roller tappet in the bottom of the piston begins to ride perpendicular to the pump’s cam lobe. In short order, the new (incorrect) position of the two components creates excessive friction, metal-on-metal contact, and both pieces begin to break down.
The Solution
Ruling out the CP4.2’s potentially catastrophic problem entails doing exactly what Ram Trucks did: replace the pump with a CP3. Standing behind its product, Ram issued a recall for roughly 250,000 of its heavy-duty trucks to perform the CP3 swap at no cost to the customer. On the aftermarket side, CP3 conversion kits exist (most notably the kit shown from S&S Diesel Motorsport, a Columbus, Indiana area company with close connections to Cummins) that require zero calibrating, making them 50-state emissions-compliant right out of the box. It’s a high-pressure fuel pump swap that provides ultimate peace of mind for 2019 and 2020 6.7L Cummins owners.
Problem #10: Fuel Dilution
All internal combustion engines experience a small amount of fuel dilution during normal
operation (fuel slipping past piston rings, entering the crankcase, and mixing with the engine oil). However, the exhaust aftertreatment system on the 6.7L Cummins is notorious for causing elevated fuel dilution levels. During the regeneration interval—the process employed to convert trapped soot in the diesel particulate filter into a fine, storable ash—Cummins kickstarts the regen event by injecting fuel on the engine’s exhaust stroke. Inevitably, not all the fuel sprayed during this post-combustion injection event makes it downstream to the DPF—some of it makes its way into the crankcase and contaminates the oil.
What It Can Cause
Any time diesel fuel combines with engine oil it dilutes it, reducing the lubrication (and
cooling) properties of the oil—lubrication your engine’s vital hard parts, main bearings, and rod bearings need. Once fuel dilution rises higher than 5-percent, increased bearing wear can begin to occur, along with oil pressure dropping and the oil level beginning to rise on the dipstick… In time, excessive over-fueling—such as in the case of the 6.7L Cummins’ post-combustion injection event during regen—can wash out cylinder walls, removing the protective layer left behind by the engine oil. Over time, cylinder washing can remove the factory cross-hatching and even damage the cylinders enough to lower compression.
The Solution
Similar to SCR system issues, there is no clear-cut, fix-all remedy for fuel dilution in a 6.7L Cummins. Aside from off-road applications where the regeneration process can be altered or rendered inactive, the best way to minimize fuel dilution is with smart driving habits. First up, limit the engine’s idle time, and cut out all excess idling. Combustion efficiency is reduced at low engine speed, which means more soot build up within the DPF, which in turn forces more regen intervals to occur (which uses more fuel). Next, always allow any active regeneration event to continue until finished (i.e. don’t interrupt the process by turning the engine off…keep driving). And finally, never run the engine hard until it reaches normal operating temp. Following these practices is crucial for anyone concerned about long-term 6.7 Cummins reliability in daily use or heavy towing.
This collection of common 6.7L Cummins problems shows why buyers should carefully evaluate maintenance history and emissions system health when shopping for a truck or a 6.7 Cummins reman engine. For many owners, choosing a quality 6.7 Cummins reman engine from a reputable rebuilder is the most effective way to restore confidence in overall 6.7 Cummins reliability. Contact Big Bear Engine Company for more information on their 6.7 Cummins reman engine process.
