Written By, Mike McGlothlin
CAT’s C15 is hated by some and lauded by others, depending on which specific version you’re referring to. Designed to replace the outgoing (and highly coveted) 3406E Cat engine, the C-15 (emphasis on the dash) was initially available as a single turbo power plant (with “6NZ” models being highly sought after), morphed into a “bridge engine” to fill the gap between the C-15 and the more emissions-friendly (and sequential turbocharged) C15 ACERT, and then finally became the C15 ACERT regen. While some common problems exist across all iterations, each C15 has its own set of weak links. These issues run the gamut, from sunk liners and rocker failures to sticking IVAs and bad ARD heads—and we’re covering all of them below.
HIGHLIGHTS:
Most Common Problems
*Sunk Liners
*ARD Head
*Oil Leaks
*Rocker Failure
*Intake Valve Actuators
Most Expensive Problems
*Sunk Liners
*Cracked Cylinder Head
*Diesel Particulate Filter
*Rocker Failure
Less-Frequent Headaches
*Sequential Turbo Issues
*Cracked Precooler
*Overheating
Problem #1: Liner Protrusion
Cylinder Liner Protrusion Caterpillar C15 Class 8 Diesel Engine Repair
Like other engines in Caterpillar’s heavy-duty lineup, the C15 boasts a wet liner in each cylinder. By having coolant circulate around the outside of the cylinder, the cylinder itself remains perfectly round for hundreds of thousands of miles—and many times for more than a million. In a C15 application, the liners are even triple O-ringed (as well as filler-band equipped) for maximum longevity. However, the C15 has one design flaw that often compromises durability. The wet liners rest on the engine block’s deck surface and, over time, can begin to “sink” into the top of the crankcase.
What Happens
C15 Cat Sunk Liner Diesel Engine Sleeve Head Gasket Failure
As the miles rack up, the portion of the liner that sits on the block’s deck surface begins to wear into the block. This is often called a “sunk liner.” And because the liner doesn’t sink at the same rate all the way around its circumference, the liner tends to “tilt” to one side within the block. Typically, the situation culminates in a blown head gasket, where the fire-rings blow out, allowing combustion to contaminate the cooling system. The first sign of a blown head gasket usually surfaces in the form of pressurized coolant being noticed by the operator.
Solution(s)
Class 8 Diesel Engine Cat C15 Liner Protrusion Check
For both liner protrusion and the blown head gasket it caused, the only fix is to tear the top of the C15 apart. Once torn down, it’s extremely vital that new counterbores be cut to ensure the problem is eliminated for good. In fact, a blown head gasket scenario is precisely why a liner protrusion check should always be performed on a C15. A liner protrusion test should also be undertaken any time the engine receives an overhaul, or any other in-depth hard-part-related repair for that matter. Incorrect liner protrusion is one of the leading causes of blown head gaskets in C15 engines.
Problem #2: Intake Valve Actuators
Cat C15 IVA Intake Valve Actuators Diesel Engine Parts
Every CAT C11, C13, and C15 produced during the emissions era came equipped with intake valve actuators. Known as IVAs for short, they’re designed to decrease cylinder temperature during combustion, which in turn decreases harmful nitrogen oxide (NOx) emissions. By holding the intake valves open longer than they traditionally would be, IVAs allow intake air to enter the cylinders during the combustion stroke, dropping combustion temperature enough to keep NOx production below its allowable limit. It’s not quite EGR (exhaust gas recirculation), but this variable valve actuation tactic is an efficient method of lowering NOx.
What Happens When They Fail
Cat C15 ACERT Intake Valve Actuator
IVAs are especially problematic on sequential turbo (i.e. compound turbo) C15 ACERT engines. Oftentimes, they stick in the open position due to a failed solenoid. Every IVA uses a solenoid, and they are notorious for becoming weak and sticking over time. When an IVA solenoid fails and its respective IVA remains open, the operator usually notices an audible miss in the engine or an obvious visual indication via excessive white smoke out the exhaust. A bad IVA solenoid can be diagnosed by performing an ohm test to narrow down which one is the culprit. Most failed solenoids trigger low current or high current codes.
Solution(s)
Intake Valve Actuator Solenoid Caterpillar C15 Diesel Engine Parts
A while back, Caterpillar released updated solenoids to replace the C15’s problematic factory units. Sold in kits (two per box), you’ll need three for an entire engine. Located directly under the valve cover, IVA solenoids are straightforward and fairly easy to replace—and they’re relative inexpensive to buy. Another manufacturer-backed remedy came in the form of CAT making software improvements, which most mechanics agree helped tremendously in eliminating stuck IVAs. Last but not least, an updated intake valve actuation oil pressure sensor complemented the factory available fixes.
Problem #3: ARD Head
Cat C15 Diesel Engine Emissions ARD Head Exhaust Aftertreatment Component
The aftertreatment regeneration device (ARD) head is one of the most notorious failure points on the CAT C15 ACERT regen engine. Its purpose is to initiate the regeneration process that’s required to convert soot accumulated in the diesel particulate filter (DPF) into a fine, more easily storable, ash. When it fails to function properly, power derate, limp mode, and eventually engine inoperability is the result. Unfortunately, the ARD head fails regularly on these engines, as well as on the C13. Complex in nature, the ARD head features two separate fuel lines, coolant lines, a spark plug to ignite the fuel-air mixture during regen, a secondary air port, an ARD flame temperature port, and a heated nozzle port.
What Happens
ARD Head Used Diesel Emissions Parts Cat C15
As a critical piece in the C15 ACERT’s PM-curbing, exhaust aftertreatment system, a failing ARD head usually means you’re left with a truck or piece of heavy equipment that’s locked in derate or limp mode. And, as the weakest link in the CAT C15’s DPF system, the ARD head unfortunately sidelines a lot of vehicles and machinery. Typically, the ARD head fails to initiate the ignition event that begins the regeneration process and, adding insult to injury, the ARD head is all but unserviceable, with even the fuel injector being part of the overall assembly.
Solution(s)
Diesel Engine Exhaust Aftertreatment Parts Caterpillar C15 ARD Head
Complete replacement is the only real solution for ARD head failure. In fact, the only serviceable component in the entire assembly is the spark plug. And, even if an owner-operator or fleet mechanic replaces the ARD head independently, the engine will still require help from the local CAT dealership in order to clear the fault codes that were tripped when the previous ARD head failed. That’s right, in addition to physical replacement, you’ll have to have a laptop give the all-clear to the ECM. Annual ARD head replacement isn’t atypical on the CAT C15—nor is a bill of $2,000 or more.
Problem #4: Diesel Particulate Filter
Caterpillar C15 Acert Diesel Emissions System
While all C15 ACERT models marked the end of CAT’s on-highway run, C15 ACERT regen versions essentially represent the final days of yellow engines powering Class 8 trucks in North America. The C15 ACERT regen engines all came equipped with a diesel particulate filter (DPF), along with the intake valve actuator technology and ARD head mentioned above. And like those emissions-related components, the DPF is also highly problematic. The DPF, which became standard for diesel engine manufacturers across the board after 2006, was part of the solution for solving the new, ultra-stringent 0.01 g/bhp·hr PM regulation that went into effect on January 1, 2007.
What Happens
Diesel Particulate Filter DPF Cat C15 Class 8 Truck Exhaust Emissions Equipment
First and foremost, with regular servicing (cleaning) and uninterrupted regeneration intervals, the DPF can last several hundred thousand miles behind a C15 ACERT. However, there is no getting around one inevitability: the DPF will have to be replaced at some point. Beyond that, ruptures or cracks in the DPF’s internal core, either due to age or a manufacturer defect, can force replacement. Then come the sensor failures, which are rampant on any early DPF system. As for the DPF’s effect on engine life, most well-maintained, DPF-equipped SDP code C15 ACERT’s go 700,000 to 800,000 miles between overhauls. By comparison, pre-DPF engines typically go at least 1 million miles before any major engine work is required.
Solution(s)
PDI Tuner Caterpillar C15 Acert Big Boss Monitor Screen
Emissions compliance is a no-compromise proposition for over-the-road heavy-duty vehicles, which means DPF replacement is the only solution for on-highway applications powered by the C15 ACERT regen. Unfortunately, DPF replacement comes at a heavy cost for a truck owner and can run $15,000 or more. In some off-highway applications or geographical locations where adherence to the EPA’s 0.01 g/bhp·hr PM regulation isn’t required, the DPF can be removed. However, to keep the engine operating with the exhaust aftertreatment system no longer in the mix requires aftermarket ECM calibrating, which carries its own cost(s).
Problem #5: Oil Leaks
Cat C15 Diesel Engine Oil Leak Block Spacer Plate Seal Failure
When Caterpillar developed the C15, one of its objectives was to make the engine less leak-prone than its predecessor, the 3406E (the 3406E having been notorious for oil leaks developing at the front and rear structure). For the most part, CAT achieved its objective, thanks in part to higher quality and better-designed seals. As a result, the C15 is way less likely to spring an oil leak than a 3406E. However, that doesn’t mean the C15 has no chinks in its well-sealed armor. In fact, a common point of oil seepage on the 3406E was carried over to the C15, and it originates at the spacer plate.
What Went Wrong
Caterpillar C15 Class 8 Truck Diesel Engine Repair Spacer Plate Parts
On CAT’s C15, the spacer plate is located between the engine block and the cylinder head. For reference, the head gasket is located above the spacer plate. A very thin piece of steel, the spacer plate isn’t technically a gasket but, ironically enough, through the use of the spacer plate gasket above it, it is supposed to seal and keep oil from escaping. It’s common for an operator or mechanic to notice oil bubbling out of the point where the block and spacer plate meet while the engine is idling.
Solution(s)
Cylinder Head Torque Sequence Cat C15 Diesel Engine Overhaul
As a known problem that was brought to the attention of Caterpillar engineers time and again, a host of updates were released over the years to try to address these oil leaks. Though none of them are a guaranteed fix, two updates tend to work well. The first update from CAT entailed the use of aviation gasket sealant on the spacer plate gasket during engine assembly. The second solution called for the employment of a revised head bolt torque sequence and no aviation sealant. Both remedies tend to hold off oil seepage from this trouble area longer than any other methods.
Problem #6: Rocker Arms (Single Turbo, C-15 Engines)
CAT C-15 Diesel Engine Class 8 Truck On Highway
Valvetrain failures of any kind can spell disaster for any engine. Whether it’s the valves themselves, the valve seats, guides, stems, keepers, seals, or the lifters, pushrods, rocker arms or even the camshaft, the carnage can quickly turn catastrophic. A minor yet vital component within the early, single turbo, CAT C-15 engines that is notorious for failure is its rocker arms. When it happens, valve timing is immediately thrown off, a notable (and audible) tick develops, and continuous operation of the engine can eventually lead to internal mayhem.
Why They Fail
C15 Cat Rocker Arm Assemblies Diesel Engine Valvetrain
Rocker failure on the early C-15s has been linked back to poor design and construction of the rocker arms. More specifically, the rocker arms aren’t strong enough for the valvetrain they’re designed to operate in. They are thinly cast pieces, with very little meat where it’s needed most. This thin casting is simply not up to the challenge of handling the constant stresses imposed on it by the valvetrain. As a result, the rocker arms often crack, usually between the camshaft and the valve bridge. In some cases, the rockers break apart completely.
Solution(s)
Cat C15 Diesel Engine Repair New Rocker Arms
There is no tangible remedy for the rocker arm issue, at least not on early C-15 power plants. But CAT did come up with a factory fix later on. Recognizing the problem, Caterpillar addressed its weak rocker arms with the release of the sequential turbo ACERT models. Starting with these compound turbo engines, the C15 was equipped with thicker, stronger rocker arms. But while this seemed to solve the problem of cracking and ultimately breaking, it led to the creation of a much bigger headache, which we’ll cover next.
Problem #7: Failed Rocker Studs (ACERT engines)
Cat C15 Acert Overhead Valvetrain Rocker Arms Diesel Engine
It’s always a shame when an engine manufacturer solves a problem in one of its most popular power plants only to create a different issue somewhere else. That’s exactly what happened with the emissions-equipped versions of the C15. Beginning with the sequential turbo C15 ACERT, CAT solved its rocker arm breakage issues by casting thicker, stronger units. Unfortunately, at the same time Caterpillar replaced the rocker arm shaft bolts with rocker arm shaft studs—studs that would prove very problematic. While the redesigned rocker arm fasteners are significantly stronger, they can cause several thousand dollars’ worth of damage.
What Happens
C15 Cat Rocker Arm Stud Failure Cylinder Head Damage
Over time, the rocker arm shaft (which resides in a provision that’s cast into the cylinder head) will wear into the cylinder head. Eventually, this wear creates a miniscule gap between the rocker shaft and where it sits in the head, which allows the nut to loosen. Once the rocker arm shaft studs are loose, the rocker arms (and Jake brake and IVA housings) are no longer secured. This often results in broken Jake and IVA housings, sheared rocker arm shafts, fasteners that break off in the cylinder head, and it can even damage the head enough to render it unusable.
Solution(s)
Caterpillar OEM C15 Diesel Engine Replacement Rocker Stud
It’s unfortunate, but there is no permanent remedy for rocker stud failure in a C15 ACERT. CAT did release an updated bolt design, and one that features an integral relief area, but these revised fasteners are still known to break with age. On top of that, Caterpillar’s unique rocker fasteners are expensive to replace, and typically run $60 or more apiece. When you add the cost of replacement rocker fasteners in with broken Jake and IVA housings, you could easily be looking at a several thousand-dollar repair (if not more).
Problem #8: Cracked Cylinder Head
C15 Caterpillar SOHC Cylinder Head
Cylinder head damage can range from minor to extreme on the CAT C15, where non-performance hindering cracks go completely unnoticed and others immediately sideline a truck. In general, cylinder head cracking isn’t as common on the C15 as it is with smaller Caterpillar engines of the same era. As for the CAT C15, its cylinder head is much less likely to crack from an overheat than the more thinly cast heads employed on the C13 and C11. However, C15 heads do crack on occasion. It’s just usually for a different reason.
What Happens
C15 Cat Cylinder Head Failure Leak Test Diesel Engine Repair
Often, a cracked C15 cylinder head results from improper liner protrusion. Because the liner doesn’t sit true within the cylinder, but rather tilted, it damages the block’s deck surface, allowing head gasket failure but also extreme heat from combustion to super heat the head. Of course, as with the C13, C12, and C11, overheating (i.e. thermal stress) can lead to hairline cracks developing in the head’s casting. The most common causes behind an overheat include low coolant, a failing water pump, stuck thermostats, or a plugged cooling stack. As with any damaged head, cracks that protrude into the valve seat regions can eventually become catastrophic.
Solution(s)
C15 Caterpillar Class 8 Diesel Engine Cylinder Head Replacement
In some cases, the C15’s cylinder head can be repaired. In others, especially when the original is irreparable due to warpage or deep cracks, a brand-new or remanufactured head is the only feasible (and worthwhile) solution. Whether a new, reman, or aftermarket head is used, one should only be obtained from a reputable source. Perhaps most importantly, to avoid any cylinder head sealing issues from reoccurring (possibly leading to another cracked head scenario), liner protrusion must be checked and verified prior to reassembling anything. In most cases, new counterbores will need to be cut.
Problem #9: Precooler Failure
Precooler Caterpillar C15 Acert Diesel Engine Turbo System Part
Like the C13, compound turbo CAT C15 ACERT’s make use of a precooler, a coolant-fed, liquid-to-air heat exchanger positioned in the intake tract. Because the sequential turbo arrangement on these engines compresses the intake air twice, exceptional heat is produced. Before this hot intake air passes through the intercooler (i.e. charge air cooler)—and long before it reaches the engine—it’s forced into the precooler. Often exposed to temperature in excess of 700 degrees F, the precooler leads a tough life on the C15 and, unfortunately, it’s prone to failure.
What Happens
C15 Caterpillar Diesel Engine Turbo Precooler Failure
The precooler is most notorious for cracking internally, which permits coolant to infiltrate the intake path to the intercooler and ultimately the cylinder head. Mixing with boosted intake air, some of the coolant coats the intercooler core while the rest of it makes it in-cylinder, where it is burned during combustion. This culminates in white smoke leaving the exhaust system and, because some coolant slips past piston rings, the engine oil level rising on the dipstick. A restricted intercooler and rampant coolant loss is inevitable, with many engines losing several gallons of antifreeze in a matter of only a few miles.
Solution(s)
Single Turbo Conversion Cat C15 Diesel Kenworth W900L Class 8 Truck
The most common fix is to replace the precooler and, depending on how catastrophic the failure was and what the engine owner wants to do, either clean or replace the intercooler. Purchased at a CAT dealership, you could be forced to pay $2,000 or more for an OEM precooler, while a used unit can usually be sourced for $600 to $1,000. But for C15 owners who’ve experienced multiple precooler failures, a single turbo conversion often gets the call. By scrapping the factory sequential turbo system, the precooler is eliminated completely, and the owner is left with a simpler (usually more reliable) single turbo arrangement.
Problem #10: Sequential Turbos
Sequential Turbo Cat C15 Acert Diesel Engine On Highway
CAT’s sequential turbo system was only offered on C15 ACERT engines. The two-stage arrangement entails a larger, low-pressure (atmospheric) turbo, which is the first to ingest intake air, feeding a second and smaller, high-pressure (manifold) turbo. On paper, it’s very appealing. Not only do sequential (also known as compound) turbos provide quick spool up and optimum transient response at speed, but they also force more airflow into an engine, thereby getting more performance out of any given displacement. In the case of the C15, CAT used all of the benefits above to both meet emissions regulations and horsepower expectations.
What Happens
Compound Turbo System Cat C15 Acert
Sadly, in factory form the sequential turbo arrangement never really impressed operators of the C15 ACERT. Spool up wasn’t as lightning-quick as advertised, nor was transient response, and the setup proved unreliable in some applications. And not only is a compound turbo system harder on an engine (added boost and drive pressure take an eventual toll on things like a head gasket and even hard parts), but the presence of two turbos as opposed to one, along with all the corresponding piping, made repairs more complex and expensive. And while the turbos themselves were fairly reliable, when they did leg go they were costly to replace.
Solution(s)
Cat C15 Diesel Engine Repair Single Turbo Swap
To simplify the turbo system, make the engine more accessible for repair, and add reliability to the CAT C15, many owners convert to a single turbo configuration. The upfront cost can be a bit steep, but for operators that have experienced factory turbo or precooler failures, a single turbo swap pays for itself rather quickly. For other C15 owners, the performance of the factory sequential turbo arrangement can be improved through aftermarket ECM calibrating. In these instances, the C15 can be brought to life in more ways than one: quick spool up, better mid-range pulling power, and improved top-end horsepower.
