Written By, Mike McGlothlin
Outside of displacement, the CAT C12 is basically the same as a Cat C10. Like the C10, it has three valve covers, its intake manifold located on the same side as the exhaust manifold, an external fuel rail and external oil pump, and is easy to work on. Both engines share the same block and head, but thanks to the C12’s longer stroke and additional cubic inches, it doesn’t get the “underpowered” rap most C10’s do (especially in Class 8 tractor trailer applications). It’s also highly reliable—save for a few costly failure points, common weak links, and minor quirks. From camshaft and valvetrain issues to over heating, and oil leaks to O-ring failures, we’ll shed light on all the Cat C12’s problems and shortcomings below.
HIGHLIGHTS
Most Common Problems:
*Overheating
*Cracked Cylinder Head
Most Expensive Problem:
*Aftercooler Failure (Marine Applications)
*Cracked Cylinder Head
Less-Frequent Headaches:
*Oil Leaks
*Contaminated Engine Oil
*Injector O-ring Failure
*Broken Coolant Manifold Bolts
*Engine Overspeed
*Liner Seal Failure
Problem #1: Excessive Camshaft Wear
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Like the CAT C10, camshaft damage and failure is fairly common on the C12. Luckily, mostproperly maintained C12’s go between 700,000 to 1-million miles before any valvetrain-related issues surface—which by that time usually warrants a complete engine overhaulanyway. However, sometimes excessive camshaft wear becomes apparent and undeniableearlier on in the engine’s tenure, and it must be addressed immediately. Failure to do so canmean a new cylinder head, lifters, pushrods, and a decent amount of labor cost(s) will beincurred.
What Causes It
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A C12 camshaft typically suffers damage when a camshaft roller seizes. Even if a failurehasn’t yet unfolded, with higher miles and/or operating hours, it’s common to find acamshaft or injector lifter roller with extreme wear. A roller that no longer spins freelycreates friction, friction that can begin to degrade both the material on the camshaft lobe itrides on and the roller itself. Unfortunately, roller failure isn’t specific to any certain type inthe C12 engine. The intake, exhaust,andinjector rollers can all fail.
Solution(s)
Whether or not the problem is noticed during a routine valve adjustment or when anaudible ticking becomes evident, the cylinder head has to be pulled to replace the lifters ona CAT C12. This is why it pays to replace everything all at once. And, as mentioned above, ifthe engine is within the realm of requiring a full-on overhaul it’s an opportune time to startfresh from the bottom up. Whatever you and your mechanic decide, just make sure yousource a remanufactured OEM camshaft or a proven reman unit from a reputableaftermarket supplier.
Problem #2: Overheating
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Elevated coolant temperature is always a worry when pulling a grade, working in low-speed, dirty environments, or when ambient conditions make everything else run warmer. Overheating remains one of the most reported C12 Cat engine problems across every application. Similar to its smaller sibling (again, the C10), the C12 doesn’t take overheating well. In fact, even if the engine appears to survive an overheat, a deeper problem could’ve been created. Think cylinder head warpage and (worse) cracks. Believe it or not, head gasket failure is highly rare on the CAT C12. Unfortunately, overheating isn’t.
Common Causes
Whether it’s due to a lack of maintenance or attentiveness, airflow blockages cause moreinstances of overheating than anything else. Accumulated debris that keeps air fromflowing through a radiator or an intercooler is a common cause, with stuck thermostats,low coolant level, and failed water pumps also being key contributors. Combine either ofthe above issues with an engine that’s being worked hard in elevated ambient airtemperature, or that is subjected to excessive exhaust gas temperature, and it’s common tosee coolant temp rise.
Solution(s)
The best solution for overheating is avoidance. 1) Keep the cooling stack in front of theengine clean, 2) make sure the engine fan is operating as it should, 3) stay on top of coolingsystem maintenance, and 4) run the correct antifreeze. Also replace thermostats as neededor at the proper interval and keep tabs on your water temp gauge. Neglecting engine coolant temperature or ignoring it altogether can potentially lead to serious engine damage,not to mention a big repair bill. The C12 is a very reliable engine—just don’t let it overheat.
Problem #3: Cracked Cylinder Head
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Unfortunately, when overheating gets out of hand with a C12, it takes a heavy toll on the cast-iron cylinder head. As soon as engine coolant temperature (ECT) reaches 230 degrees F, the head is in jeopardy of developing hairline cracks on the combustion deck surface. Normally, a cracked cylinder head is the result of experiencing extreme heat for extended periods of time. Multiple overheats will almost guarantee the formation of cracks in the C12’s cylinder head. This is easily one of the most costly common Cat C12 problems on this list. Hence our advice of keeping close tabs on the water temp gauge and properly maintaining the engine’s cooling system.
Why It Happens
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With a thinner casting than what you’ll find on a C15 cylinder head, the lighter-duty headon the C12 doesn’t dissipate heat as effectively as its big block counterpart. This fragilitymakes it more susceptible to cracks. Oftentimes, a C12 head will develop cracks betweenvalves, which can prove devastating if the crack(s) spreads into the seat region. From thatpoint on, you have the potential of the valve seat dropping out, damaging the head, thepiston, and even wrecking the entire engine. In most instances, a cracked cylinder headgoes unnoticed until an overhaul or related repair is performed on the engine.
Solution(s)
As with overheating, the best way to avoid a cracked cylinder head with a C12 is byensuring the engine never sees extreme coolant temps. It also pays to avoid elevated EGT, asprolonged exposure to elevated exhaust gas temperature inevitably causes coolant temp toincrease. In most cases, a cracked cylinder head is irreparable and must be replaced. Onlyan OEM or OEM-quality aftermarket head should be sourced here, along with brand-newhead bolts. Of course, measuring for proper liner protrusion and following the correcttorquing sequence should be adhered to as well.
Problem #4: Broken Coolant Manifold Bolts
Every engine has its fair share of head-scratching quirks, and the coolant manifold takes the cake on the C12 platform. Among all the Cat C12 problems on this list, this one catches the most owners off guard. Also, this is one of those headache problems that forces a mechanic into a much deeper repair than he or she anticipated. The coolant manifold is made of aluminum, while the bolts that fasten it to the cylinder head are made of steel. Not only do the two dissimilar metals expand and contract at different rates during normal engine operation, but they also corrode and rust at different rates.
What Happens
Long story short, the mounting bolts are known to corrode and seize within the coolantmanifold’s aluminum housing. This can make servicing the coolant manifold or workingaround it for a different repair difficult. During an attempted removal, the coolant manifoldcan rarely be broken free without it taking on damage, and the bolts are infamous forsnapping off. It’s an issue that can turn a minor coolant leak repair into a major repair inshort order. Of course, it goes from a quick fix to a more labor intensive (and exponentiallymore costly) repair at the same time.
Solution(s)
Because you can rarely get the coolant manifold off without damaging it, complete replacement of the coolant manifold is often the only option. Brand-new, it can run $700 or more at a CAT dealership. Secondhand, you’ll still likely have to pay $300. Other than shopping around, there may not be a “cheap” path to sourcing a genuine Caterpillar replacement. Of course, sourcing more affordable, aftermarket parts is always an option—so long as they meet (or exceed) OEM quality. Just make sure you source fresh mounting bolts, too.
Problem #5: Bad Liner Seal
A failing wet liner seal can allow coolant to mix with engine oil—and few fluids are as damaging to main and rod bearings than antifreeze. Liner seal failure ranks among the more serious Caterpillar C12 problems since it directly threatens the bottom end. A bad liner seal in a CAT C12 is often noticed by the operator when coolant begins disappearing from the system—and it’s usually confirmed by pulling the engine oil dipstick. Finding a milky discoloration on the dipstick is a telltale sign that coolant contamination is taking place. Once discovered, it’s vital that the engine no longer be run.
What Happens
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Liner seal failure is sometimes caused by improper liner installation. Other times, highmiles are to blame. However, liner seal failure can frequently be traced back to the incorrectantifreeze being used in the engine’s cooling system. Just like the C10, CAT’s C12 requires the use of a coolant that’s free of nitrites, as nitrites contribute significantly to electrolysis. Also, like the C10 the wet liners employed in the C12 only utilize a single seal. In comparison, the liners used in the C15 engine incorporate three seals.
Solution(s)
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With a wet liner’s key function being to provide million-mile durability, it’s important to remember that a simple liner reseal won’t suffice—the entire liner should always bere placed. In every wet sleeve diesel engine with significant run time, subtle pitting and scarring will be present on the liner’s exterior. It’s best to start over with a clean slate here. And regardless of whether the job is being performed as an in-frame on the jobsite or indoors, cleanliness is key when handling and installing new cylinder liners and seals. Always make sure the working area is as clean as possible.
Problem #6: Contaminated Oil
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Building on the cracked cylinder head and failed liner seal issues discussed above, it’s time to talk about engine oil contamination. Oil contamination rounds out some of the more interconnected C12 cat engine problems covered so far. While not an everyday or every-engine occurrence on the CAT C12, the latter problems can allow coolant to mix with engine oil—and when they do it can be devastating for vital components like crankshaft main and connecting rod bearings. One saving grace is that the C12 utilizes sizeable main and rod bearings, so if the contamination is noticed in a timely manner, it usually doesn’t cause extensive damage.
What Happens
The most common cause of coolant in engine oil contamination comes from a failing O-ringon a liner. Once an aged, corroded, or damaged O-ring fails, the problem can only growworse as time goes on. To a lesser extent, a cracked cylinder head can allow coolant and oilto mix. Again, the C12’s propensity to develop hairline cracks in the thinly-cast cylinderhead strikes again. Another potential source of contamination comes from the water pump(pictured). If the water pump’s weep hole becomes blocked or plugged up, it can vent intothe engine’s crankcase.
Solution(s)
Once antifreeze infiltrates the crankcase, the remedy is going to be laborious. There is noway around that. However, preventative maintenance will ensure your chances of coolant-contaminated engine oil remain low. For maximum liner O-ring life, always remember torun the correct, CAT-specified coolant in your C12. Never allow the engine to overheat (i.e.avoids the dreaded cracked cylinder head problem). Routinely clean the engine and enginebay (think water pump weep hole here, as well as the ability to spot the source of a fluidleak). Finally, keep regular tabs on engine coolant level.
Problem #7: Injector O-ring Failure
Each fuel injector in the CAT C12 engine is sealed via three separate O-rings. The injectors themselves are known to be quite reliable, but occasionally an O-ring failure surfaces, and usually does so long before the injectors are due for a rebuild. As the miles rack up, the lower, mid, and upper O-rings can begin to wear, eventually even breaking down. Full disclosure: injector O-rings are sometimes damaged due to improper installation. A damaged O-ring—be it torn or rolled—will be immediately apparent upon startup.
What Happens
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Common symptoms of a failed injector O-ring begin with the discovery of “extra” oil on the dipstick. In addition to the heightened oil level, a discoloration of the engine oil itself will be a giveaway. Left unaddressed, any fuel-in-oil issue can hurt the engine due to a lack of lubricity. In cases of extreme neglect, the engine can even hydrolock. Deciphering which injector is the culprit can be achieved by removing the rocker covers and pumping the handprimer for fuel. Doing this, an alert mechanic may be able to spot a noticeable leak at one of the injectors.
Solution(s)
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The only fix for aged, rolled, or torn injector O-rings (or nozzle gasket) is new, high-quality O-rings. And while resealing a set of injectors on a C12 is painless, the overhead will have to be run at the conclusion of the process—not to mention the fact that the injector height will have to be set. During the process, always make sure the injector bores are clean and free of any carbon buildup, and that each rubber O-ring is treated to a coat of fresh engine oil prior to being installed on the injector body.
Problem #8: Aftercooler Failure (Marine Applications)
CAT’s C12 wasn’t only popular in the on-highway segment. In fact, it was (and still is) widely used in countless boat applications. For owners running the engine at sea, this is one of the most common Cat C12 problems they'll ever face. Without question, the biggest weak link in these C12-powered, sea-going vessels is the aftercooler. Prone to leaking, they can pass water on to the engine—and they’re notorious for trashing cylinder heads. Needless to say, it’s a failure than can cost you a five-digit dollar figure to repair. Several different versions of aftercoolers were offered for the C12, but the units found on pre-2005 marine engines seem to last the longest.
Why They Fail
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Within the aftercooler housing (which is coupled to the cylinder head), the core is infamousfor rupturing and allowing water to be passed into the head. While inferior materials areoften blamed for the core’s propensity to develop cracks, owners who neglect to performtimely cleaning and servicing intervals are the leading cause of aftercooler failure.Caterpillar even recommends complete replacement every six years if the aftercooler is inuse in saltwater applications. A 1,000-hour service schedule is also recommended, butmany proactive captains opt to clean the aftercooler every 500 hours.
Solution(s)
Outside of observing regular cleaning intervals, the aftercooler should be inspected andpressure tested each time it’s removed for cleaning. Equally important, it pays to pull,inspect, and clean (or replace) the condensate drain valve at routine intervals. Thecondensate drain valve can indicate that rust and corrosion is forming within theaftercooler. Any time rust is found, complete aftercooler replacement is mandatory. Inextreme cases, a failed aftercooler can damage the cylinder headandsometimes even passwater into the cylinders. Stay on top of your aftercooler maintenance and your C12 will behighly reliable at sea.
Problem #9: Leaking Front Cover
Sharing a very similar design to the C10, the C12 also tends to spring leaks in the same locations. Reminiscent of what happens on occasion with the C10, the C12’s most common leak point originates at the front cover. Not the cover plate, but rather the aluminum housing behind it that fastens to the block—and that requires extensive labor to access. Oilspots or an accumulation of debris at the front of the engine are indicative of a front cover oil leak, and pressure washing is a proven method of tracing the leak back to its point of origin.
What Happens
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Front cover leaks on the C12 also share the same causes found on the C10. In applications where an air compressor is utilized, the compressor’s rear support bracket often isn’t installed, is installed incorrectly, or was removed during a prior repair and not reinstalled. This can put extreme stress on the front cover housing, pulling it away from the block, and ultimately resulting in an oil leak. In other cases, the front gear housing bolts are found backing out. This is typically due to the fasteners not being properly torqued during a previous repair.
Solution(s)
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If oil is in fact leaking from the front gear housing, a lot of labor hours are called for in order to address it. Of particular note, the camshaft will have to be pulled, which is a chore all on its own. The task of resealing it calls for a new OEM or OEM-equivalent aftermarket front cover gasket and a fresh front main seal, along with a comprehensive inspection of everything removed during teardown. Specifically, look for any cracks in the front gear housing and the front cover plate.
Problem #10: Overspeed
This problem is rare, but it’s happened enough (usually due to operator error) that it made the list. Unlike most common Cat C12 problems on this list, this one is entirely preventable with proper braking habits. First and foremost, on-highway, heavy-duty inline-six diesel engines don’t usually enjoy operating above 2,000 rpm. In fact, most are designed to operate between 1,200 and 1,600 rpm, where torque is abundant and the engine is most efficient. The CAT C12 is designed to operate in a range between 1,400 to 2,100 rpm, and in our experience, anything beyond 2,100 rpm—and especially in conjunction with the use of jake brakes—can be borderline dangerous.
What Happens
The combination of high engine speed and jake brakes has proven a toxic recipe for manyRV applications powered by the C12. The problem surfaces on steep downhill grades withthe jakes in use and the driver trying to keep the vehicle’s speed from increasing. A big spike in engine rpm is the root cause. In these overspeed situations, where 2,300 rpm ormore is seen, some C12’s have spun rod bearings. And as catastrophic as a spun rod bearingcan be, the catastrophe can be amplified further if the affected rod windows the block.
Solution(s)
Just as it doesn’t tolerate excessive coolant temperature, the C12 doesn’t handle high rpmwell either. So, in addition to ensuring engine coolant temp never exceeds 230 degrees F,also make certain you avoid over speeding the engine. Building on that point, don’t forgothe use of service brakes simply because the engine is equipped with jakes. Use both meansof braking to keep engine speed from cresting 2,100 rpm, if possible, no matter the gradeyou’re descending. Keep heat at bay, rpm in check, and stay on top of your maintenance andthe C12 will reward you with a million miles of service.
