Written By, Mike McGlothlin
Before the CAT C15 engine, there was the iconic CAT 3406 engine. Offered in A, B, C, and E models, each being more technologically advanced than the other, the 3406 enjoyed a very lengthy production run. The 3406E is perhaps the best-remembered version, namely due to its electronic control enabling end users to unlock huge horsepower increases without having to turn a wrench. Beyond its performance potential, the 3406 was highly durable, in every rendition. However, it’s an engine notorious for leaking oil, suffers from blown head gaskets due to sunken liners, and it also experiences rampant spacer plate issues. Its rarer setbacks include broken rocker arms, and broken exhaust manifold studs and thermostat housing bolts—along with injector cup seals sometimes being a worry. We’ll cover all of those problems and more below.
HIGHLIGHTS
Most Common Problems:
*Oil Leaks
*Failed Oil Cooler
*Blown Head Gasket
*Broken Exhaust Manifold Studs
*Leaking Spacer Plate
Most Expensive Problem:
*Blown Head Gasket
*Injector Cup Seals
*Pricy Replacement Parts
*Leaking Spacer Plate
Less-Frequent Headaches:
*Broken Rocker Arms
*Broken Thermostat Housing Bolts
*Liner Seal Failure
Problem #1: Oil Leaks
Engine Oil Pan Leak Caterpillar Class 8 Truck Application
It’s a fact that oil leaks are a part of life for most diesel engines, especially those that are decades removed from the factory assembly line. Years of hard work, thousands of operating hours compiled, and constant exposure to NVH have a way of taking their toll on gaskets, seals, and O-rings. For the CAT 3406, oil leaks are especially common. In fact, it’s arguably more frequent on the 3406 than any other comparable engine in its class. Oil loss was so common that Caterpillar redesigned much of the C15’s sealing points and methods to eliminate leaks.
Why (And Where) It Happens
Puzzle Gasket Oil Leak Cat 3406 Diesel Engine Failure
In many cases, oil leaks develop at the 3406’s oil pan. Not only does the oil pan employ a block strengthener plate, which requires two separate sealing gaskets, but it utilizes complex gaskets known as “puzzle” gaskets. The combination of the puzzle gaskets and multiple mating surfaces increase potential leak points—and that’s exactly what happens. Other common leaks tend to develop behind the timing cover (primarily due to a lack of fasteners), from the mechanical injection pump and oil supply line O-rings (namely on the 3406B), at the front structure backing plate, and from the rear structure.
Solution(s)
Cat 3406 Class 8 Truck Diesel Engine Front Structure Oil Leak
The fix for oil leaks on the 3406 are often highly involved, especially when they exist at the front or rear structure, or at the oil pan. The latter component will call for fresh puzzle gaskets while further digging is required to address leaks at the front and/or rear structure. Depending on the repair, labor can be extensive (and expensive), which forces many owners to put off performing the fix until a complete reseal (or even overhaul) can be performed. No matter what, genuine Caterpillar gaskets, seals, and O-rings should be sourced for any repair, or OEM-equivalent, high-quality components from a reputable aftermarket manufacturer.
Problem #2: Failed Oil Cooler
Cat 3406e Diesel Engine Oil Cooler Failure Heat Exchanger Plugged
Not unlike any other commercial-grade diesel engine, coolant is used to keep engine oil temperature in check on the 3406. This calls for the use of an oil cooler, which on the CAT 3406 is located midway up the passenger side of the block. In a lot of engines, this vital fluid-to-fluid heat exchanger can last as long as the rotating assembly before requiring replacement. Unfortunately, that’s not the case with the CAT 3406. Its oil cooler is prone to premature failure, and when it lets go it usually leads to a mess of fluid contamination.
What Happens
Diesel Engine Failure Oil In Coolant Antifreeze Cat 3406 Issue
In many instances, the oil cooler’s internal core cracks. In other cases, the core completely ruptures. When either scenario unfolds, engine oil infiltrates the cooling system. It’s a failure that’s most often discovered by finding oil in the engine coolant (i.e. darker than normal coolant, overflowing coolant, etc.). Why does engine oil contaminate the coolant circuit and coolant not contaminate the engine oil instead? Pressure. The engine’s oil pressure is far greater than the operating pressure of the cooling system. This means engine oil forces its way into the cooling system, rather than the other way around.
Solution
OEM Caterpillar Oil Cooler Heat Exchanger Replacement Part Cat 3406
The correct remedy begins with a brand-new oil cooler. Never repair or reuse an oil cooler. Its role within the engine is much too crucial to experiment with any shade-tree mechanic type fixes. In addition to (and technically well before) installing a new oil cooler, take great care to ensure the cooling system is completely flushed and free of any leftover engine oil. Loads of old-time mechanics swear by the use of water and Cascade dishwasher detergent for best results—and most admit the powder detergent works best. Last but not least, start over with Cat extended life coolant (ELC).
Problem #3: Blown Head Gasket
Caterpillar Blown Head Gasket Coolant On Piston
Head gasket failure is a problem that effects a lot of heavy-duty diesel engines at some point. And not only can a blown head gasket lead to significant downtime, but it can contribute to a whole host of other failures. Think cracked cylinder heads, dropped valves, hydro lock, and even piston damage to name a few. As for the 3406, head gasket issues are highly common. However, it’s not a gasket problem, nor is it the fault of the bolts tasked with fastening the head to the block. Rather, it’s the design of the 3406 engine that causes it.
Why It Happens
Cat 3406 Diesel Engine Head Gasket Failure Repair
Typically, high-mile engines are at the biggest risk of experiencing a head gasket failure. Sometimes, it happens after operating the engine while low on coolant. But in most cases, you lose a head gasket due to a liner protrusion issue. On CAT’s 3406, its wet liners sit on top of the cylinder block and, over time, they can gradually wear into the block’s deck surface. This is often referred to as a “sunk liner” or “sunken liner.” All it takes is a protrusion issue of a few thousandths of an inch and you’ve lost the head gasket.
Solution(s)
Top End Diesel Engine Teardown Cat 3406E
There is never a cheap fix for a blown head gasket. But not only that, it’s a fairly laborious undertaking. The head has to be broken free and removed, followed by a thorough inspection of the engine’s liner protrusion. In most cases, new counter bores will have to be cut. This is required if you want to keep the new head gasket alive. It requires special tooling to cut new counterbores—and it should only be done by a mechanic with ample experience. With a new counter bore, a fresh cylinder pack, and proper liner protrusion, the CAT 3406 head gasket will live for hundreds of thousands of miles, if not more.
Problem #4: Spacer Plate Leaks
Cat 3406e Diesel Engine Spacer Plate Cylinder Block Wet Liners Head Gasket
Building on the head gasket failures that often plague older CAT 3406 workhorses, we come to the engine’s spacer plate. Like the head gasket, it’s also located between the cylinder head and the block. Essentially a 0.014-inch thick aluminum gasket, the spacer plate’s job is to control liner protrusion by reducing stress imposed on each wet liner flange and also to facilitate an optimum seal for the head gasket. Unfortunately, the spacer plate often fails to provide the structural and sealing support it was intended for—and it’s also one of the key contributors in head gasket failure.
What Happens
Puzzle Gasket Oil Leak Cat 3406 Diesel Engine Failure
The spacer plate utilizes a shim (along with the head gasket) that is notorious for crushing. When the spacer plate crushes, it effects liner protrusion. And as we learned earlier, once liner protrusion exceeds its allowable tolerance head gasket failure is soon to follow. But on top of expediting a blown head gasket, spacer plate failure can also lead to oil leaks. This frequently shows up in the form of oil emanating (and often bubbling) from the spacer plate location between the block and the head.
Solution(s)
Cat 3406 Liner Protrusion Head Gasket Diesel Engine Repair
Obviously, when you’re addressing a spacer plate issue, you’re also tackling a head gasket and liner protrusion problem. This means it’s highly involved, laborious, and a rather costly repair. Along with proper liner protrusion being established (as well as a new head gasket and a myriad of other parts sourced), many CAT mechanics advise the use of aviation gasket maker on the new spacer plate. The general consensus here is that, while it isn’t guaranteed to keep things sealed up for a million miles, it works better than any other gasket maker (or no gasket maker).
Problem #5: Liner Seal Failure
Replaceable Wet Liner Caterpillar Diesel Engine Parts Cat 3406e
As with many heavy-duty diesel power plants, CAT’s 3406 benefits from the use of wet cylinder liners (also known as wet sleeves). These replaceable liners not only make rebuilding each cylinder possible individually, but thanks to having coolant circulating around the outside of them they offer the kind of exceptional heat transfer that allows each cylinder to remain perfectly round for hundreds of thousands of miles. And because they can be replaced one at a time, they make in-field repair much more affordable for the average 3406 owner. However, wet liners aren’t without their own sets of occasional issues.
What Can Cause It
Cat 3406e Wet Liner Seal Failure Diesel Engine Block Parts
Liner seal failure is another issue that can often be traced back to improper liner protrusion. However, it can also be caused by poorly maintained coolant, low coolant volume, and the incorrect coolant being run through the engine. Of course, with age, hundreds of thousands of miles or tens of thousands of operating hours, exposure to excessive coolant temperature, or prolonged instances of extreme exhaust gas temperature, the liner seals don’t maintain their structural integrity forever. Lack of maintenance and operational abuse will eventually take a toll on these vital seals.
Solution(s)
Cat 3406 Diesel Engine Rebuild Liner Protrusion Dial Indicator Measurement
It goes without saying that a fresh liner is in store when the original seals give way, but the story doesn’t end there. With liner protrusion being a common source of liner seal failure, inspection for proper liner protrusion should always be performed. And, if liner protrusion is indeed the cause, it must be dealt with by cutting new counter bores—not to mention the fact that the job of rebuilding a single cylinder gets much more involved at this point. Following the repair, always ensure the proper type (and volume) of approved coolant is being run.
Problem #6: Broken Exhaust Manifold Studs
Caterpillar 3406 Diesel Engine Exhaust Manifold Studs Nuts
Father Time, Mother Nature, and man-made corrosion (think road salt and other ice-melting treatments) not only take their toll on Class 8 trucks equipped with CAT 3406 engines, but external parts of the engine as well. Take for example the 3406’s exhaust manifold. Thankfully, the manifold is made with an extremely thick casting. However, the studs that attach the exhaust manifold to the cylinder head are highly susceptible to rust and corrosion. Over time, the corroded manifold studs become weakened, and they’re notorious for seizing in place or snapping off completely.
What Happens
Cat 3406e Broken Exhaust Manifold Stud Cylinder Head Repair Removal
As the integrity of the exhaust manifold studs break down, rust can effectively lock them in place within the cylinder head. This often causes the studs to snap when attempting to remove them. Typically, the manifold studs break off flush with the cylinder head. When this happens, a routine exhaust manifold removal can quickly snowball into a several hour (or an all-day) ordeal. Retrieving the remainder of the manifold studs from the cylinder head can be time-consuming, but if done right (and performed with patience) the head won’t incur any damage.
Solution(s)
Engine Block Cylinder Head Valvetrain Exhaust Manifold Removal Cat 3406
First and foremost, always hit the exhaust manifold studs with a penetrating oil long before attempting to break them loose (hours ahead of time, if not letting them soak overnight). When it comes time to remove them, heat from a torch can help free them up, but doesn’t always work. Drilling out the original studs and using HeliCoils works, but if a manifold bolt snaps off in the head, the best way to extract the remaining stud is to break out the welder. First, weld a washer to the broken stud, followed by welding a nut to the washer. Then use the appropriate socket to zip it out.
Problem #7: Broken Thermostat Housing Bolts
Cat 3406 Thermostat Housing Cooling System Water Pump
On the subject of rusting and corroded fasteners, it’s important to take a hard look at the bolts that attach the thermostat housing to the 3406’s cylinder head. This is one of those components that must be pulled during several different types of repairs. Unfortunately, it has a way of slowing down—and even putting the job at a temporary standstill—when it strikes. It’s also the kind of failure that brings added costs and increased labor time into the equation when working on the 3406—especially older versions.
What Happens
Cat 3406 Thermostat Housing Bolt Extraction Drilling HeliCoil
The thermostat housing bolts run through the thermostat housing itself and into the cylinder head on the 3406. The bolts are made of aluminum and, over time and due to corrosion, are prone to seizing within the cast-iron head. They can be a bear to remove and oftentimes every fastener requires considerable finagling before finally breaking loose. Occasionally, it’s even more involved than that, with the bolts breaking off in the housing. Worse yet, it’s not uncommon for a bolt to break off in the cylinder head.
Solution(s)
Cat 3406 Diesel Engine Cooling System Thermostat Housing Replacement
Remedies for this particular problem vary, depending on how the removal of the thermostat housing bolts goes. At the very least, you’ll need new thermostat housing bolts but, depending on the housing’s condition after removal, you may have to purchase a new thermostat housing as well. When a bolt (or several bolts) breaks off in the cylinder head, take care not to damage the head when attempting to free it. Here, welding a washer—followed by a nut—onto the stud is generally the best method for coaxing the broken fastener out of the head.
Problem #8: Broken Rocker Arms (3406E)
Cat 3406 Diesel Engine Cylinder Head Valvetrain Rocker Arms
This issue is infamous on E models of the 3406, but it can also inflict earlier engines. In short, the factory rocker arms aren’t up to the challenge of handling the stresses imposed on them by the valvetrain. Most attribute this valvetrain shortcoming to the rocker arms being very thinly cast pieces. When a rocker arm fails, the engine’s valve timing is instantly thrown off, a notable (and audible) tick starts, and continuous operation of the engine can eventually lead to internal top-end mayhem (with the melee even potentially damaging the bottom end).
What Happens
Broken Rocker Arm Valvetrain Failure Cat 3406e Diesel Engine Parts
The problem with a thin casting of a vital component is that very little meat exists where it’s needed most, and repeated stress can eventually damage or ruin it. In the case of the 3406E’s rocker arms, they tend to develop a crack first. However, in time that crack spreads and can eventually break the rocker. Typically, the rocker arms let go between the camshaft and the valve bridge. Collateral damage to the rest of the valvetrain, including cylinder head and even camshaft damage, can all result from a failed rocker arm.
Solution(s)
Cat 3406 Replacement Diesel Engine Parts Rocker Arm Valvetrain Components
Unfortunately, there is no real solution for the 3406E’s rocker arm issues. Later, after the 3406E morphed into the C15, CAT implemented revised (stronger) rockers on its compound turbo engines. Our best advice for 3406 owners is to make careful rocker inspection part of your regular routine when running the overhead. Beyond that, any time you’re under the valve covers it pays to take a look at the rockers. Use a flashlight and remember to utilize a telescopic mirror to get a glimpse at the underside of the rocker arms.
Problem #9: Bad Injector Cups
Fuel Injector Cup Failure Cat 3406e Diesel Engine Parts
Each of the 3406’s fuel injectors sit within an injector sleeve (or injector cup). One of the primary purposes of an injector cup is to circulate coolant around its exterior. This keeps the fuel injector from being exposed to excessive heat, heat which could lead to premature (and expensive) failure. And, because they have coolant on one side and the fuel injector on the other, the injector cups also prevent fuel and coolant from mixing. Additionally, their tight seal prevents combustion gases from contaminating the fuel system.
What Happens
Cat 3406 Fuel Injector Sleeve Removal Heavy Duty Diesel Engine Parts
Injector cups can fail in more than one way, but failure is most common at the O-rings. When these vital seals give way, it’s immediately apparent to the operator. If the top injector cup seal fails, you get diesel in the cooling system. If the bottom injector cup seal goes bad, you get a pressurized cooling system. Note: there are three O-rings in total. Of course, the injector cups themselves can become damaged when age and corrosion set in but, by and large, it’s usually the seals that fail rather than the cup itself.
Solution(s)
Injector Sleeve Removal Tools Cat 3406e Diesel Engine Repair
For those who decide to replace all six injector cups, the list of parts and tools is as follows. You’ll need new injector sleeve seals, but also new O-rings for every fuel injector (bathed in fresh engine oil prior to installation). You’ll also need retaining compound for the bottom, exterior portion of the new injector cups. Additionally, new valve cover seals will be required, with a fresh set of injector hold down bolts being highly recommended. You’ll also need specialized (or homemade) injector cup removal tools and an injector height tool.
Problem #10: Expensive To Repair
Cylinder Head Valve Cover Cat 3406 Diesel Engine Valve Bridges Injectors
The price of “yellow parts” isn’t cheap, and it never has been. This is a common gripe from CAT engine owners, namely those who source genuine parts from their local Caterpillar dealer. In comparison to other engines in its class, CAT-manufactured 3406 parts do carry a premium, but it’s not as outlandish as some would have you believe. In fact, a lot of the price you pay for OEM Caterpillar components is directly dependent on where you source your parts. Regardless, the stigma that all CAT parts are expensive remains, some of it justified.
High Dollar Examples
Crankshaft Main Bearing Failure Cat 3406e Diesel Engine Repair
Going back to the problems that stem from improper liner protrusion, the 3406 gets expensive when new counter bores have to be cut. This is because specialty tooling is required and the cylinder pack and cylinder head have to be removed. This means new hard parts, gaskets, and considerable labor time. Reverting back to the 3406’s common leak points, addressing a front or rear structure leak can get expensive also, as (once again) a host of new parts are required. An added expense for CAT 3406 owners interested in reaching the million-mile mark is to roll fresh main (and even rod) bearings in every 250,000 miles.
Solution(s)
Cat 3406e IPD Aftermarket Engine Parts Piston Packs Wet Liners
The best solution for CAT 3406 problems begins with avoiding failures, and it all starts with good maintenance practices. That said, when new parts are required, the marketplace isn’t what it used to be. While some questionable aftermarket parts are being produced, there are various high-quality aftermarket alternatives to genuine CAT parts these days. Just do your homework and spend your hard-earned money with a reputable manufacturer. Of course, when the budget allows (and a great warranty is sought), Caterpillar OEM and remanufactured components often provide the most peace of mind.
