Written By, Mike McGlothlin
Essentially a mid-range version of the 3406, Caterpillar’s 3306 engine is one of the most reliable and over-engineered diesel engines the company ever built. Properly maintained, this 10.5L inline-six can go 20,000 hours or more before requiring an overhaul. And when CAT 3306 engine repair is finally necessary, it’s an engine that’s extremely easy to work on. That said, the CAT 3306 isn’t without its faults. Most of them—including cracked cylinder heads, dropped valves and liner failures—are common diesel engine failure symptoms that tend to surface due to old age. Other issues, such as failed water pumps, leaky front mains, and leaking pre-chambers, have long been known as pattern problems on what is otherwise a very dependable platform. Below, we’ll spell out the Top 10 quirks of the CAT 3306 and follow each one with the best solution.
HIGHLIGHTS
Most Common Problems:
*Failed Water Pump
*Leaking Pre-Chambers
*Leaking Front Main Seal
*Failed Glow Plugs
*Aftercooler Failure
*Failed Oil Coole
Most Expensive Problem:
*Cracked Cylinder Head
*Dropped Valves
Less-Frequent Headaches:
*Failed Injection Pump
*Cracked Exhaust Manifold
*LinerFailure
*Worn Valve Guides
Problem#1: Water Pump
The water pump used on the CAT 3306 is gear-driven. Without question, this makes it considerably more reliable than one driven via a belt, but that doesn’t make it indestructible. A 3306 water pump is known to fail in both textbook fashion (with leakage coming from the weep hole) and catastrophic fashion. Key symptoms of water pump failure include coolant loss(be it from engine coolant puddling under the engine or by noticing a lower coolant level in reservoir)and coolant mixing with engine oil.
What Happens
In its most explosive form of failure, the water pump gears can actually break off the water pump shaft. With the shaft no longer connected to the impeller, engine coolant temperature begins to rise immediately, and if it goes unchecked (oris neglected) an overheat scenario can play out in short order. At high miles and elevated operating hours, the water pump often chec
ks out the old fashion way. Worn seals and internal corrosion are the culprits here, sometimes due to improper or poor quality coolant, as a result of cavitation, or simply as a result of old age
Solution(s)
While water pump failure can’t be permanently avoided, this vital component’s life can certainly be prolonged by running the proper coolant type specified by CAT. When the pump does check out, it pays to have an experienced mechanic perform the CAT 3306 engine repair. The most time-consuming part of the repair entails removing the water pump from the engine. In order to best access all the water pump’s mounting fasteners, it behooves the person performing the work to first remove the front cover. It also goes without saying that a water pump repair is an ideal time to replace the thermostat, too.
Problem #2: Water Jacket Aftercooler
Different types of charge air cooling were offered on the CAT 3306 engine depending on application, but we’re covering the water jacket after cooler variety here. This aftercooler, which features a plate-fin internal heat exchanger, relies on engine coolant to help drop compressed air temperatures leaving the turbo and that are bound for the cylinder head. Integrated onto the top of the 3306, it’s highly efficient and very effective, but it can fail or have its performance hindered in several ways. Fluid and airflow restrictions, internal corrosion, or a failed core can all occur.
What Happens
Screenshot
The most common issue associated with this water to air style charge air cooler has to do with mineral deposits from the engine coolant. These deposits, usually due to low-quality or improper coolant, can block internal passageways within the aftercooler core. This blockage obstructs flow, hurting the unit’s ability to effectively drop intake temperatures. A cracked internal core is another issue, which typically occurs due to corrosion and age, and that causes rapid coolant loss and excessive smoke out of the exhaust. A less severe failure issue, but one that is still fairly common, stems from a torn boot in the charge air system. The drop in boost pressure that results from this will reduce combustion efficiency and increase both fuel consumption and emissions.
Solution(s)
Finding the best remedy for an aftercooler-related problem boils down to the right mechanic and the correct tools. One key diagnostic tool for the water jacket aftercooler is the Caterpillar-built, Thermistor Thermometer Group (PN 9S9102), which can measure the output of the airflow traveling through the after cooler. If you’re hoping to avoid any of the a forementioned aftercooler issues altogether, ensure your 3306 is running the propercoolant (with the right additive package), keep regular tabs on the engine’s coolant level, and perform routine checks of the turbo system, specifically checking for torn boots or loose connection points.
Problem #3: Cracked Cylinder Head
Screenshot
This is the type of failure that can happen with any diesel engine that sees regular abuse. By abuse, we mean excessive heat, be it due to elevated exhaust gas temperature or sky-high coolant temps. While highly durable, the CAT 3306 is still susceptible to cracks forming in its cast-iron cylinder head. In many cases, an engine being operated with a blown head gasket enables the kind of overheating that can cause hairline cracks to develop on the head’s deck surface. Additionally, prolonged overheating can warp the cylinder head beyond repair.
What Happens
The problem originates when hairline cracks form between the intake and exhaust valves, where the highest levels of abuse (i.e. heat) are concentrated. When these cracks spread into the valve seat region, the risk of losing a valve seat and ultimately dropping a valve can become a reality. At that point, much more than a new cylinder head will be on the repair bill.In other areas of the 3306’s cylinder head, even the slightest of cracks can lead to considerable coolant loss and overheating during engine operation.
Solution(s)
Unfortunately, cylinder head cracks are often only discovered during an extensive CAT 3306 engine repair, such as a blown head gasket, a top-end reseal, or an engine overhaul. If and when a cracked cylinder head is discovered, sourcing a new (OEM or aftermarket) head, or locating a used one are the most widely recommended options. Repairing a cracked head isn’t cost-effective, feasible, or wise. One quick note on pre-chamber (PC) engines vs. direct injection (DI) versions: the heads between these two 3306 mills don’t interchange…
Problem #4: Liner Failure
Screenshot
One of the best traits of most heavy-duty diesel engines is their use of wet liners. The CAT3306 has them. Unfortunately, while they play a key role in million-mile durability, they don’t last forever—especially if cavitation or any level of abuse is present. Cavitation in particular leads to liner pitting. This erosion from the outside-in will eventually allow coolant to mix with engine oil and vice versa. Aside from cavitation, leaking liner O-ring scan also enable coolant to mix with engine oil. In any engine, exposing the rod and main bearings to antifreeze is never a good thing.
What Happens
The wet liners in the 3306 are made of ductile iron. As with all iron, it can beg into rust. In the case of cylinder liners, the side that’s exposed to engine coolant is more susceptible to corrosion. During normal engine operation, miniscule bubbles form within the coolant. When those bubbles pop (or explode), they are detrimental to the structural integrity of the iron liner. This process, known as cavitation, carried out millions of times and across thousands of operating hours, causes metal fatigue. Eventually, pitting begins, followed by pin holes in the liner. At that point, oil will mix with coolant when the engine is running, and coolant will infiltrate the oil system when it’s not running.
Solution(s)
If a cracked liner is discovered quickly and no collateral damage has been incurred (i.e., trashed bearings or hydrolock), the liner can be replaced, even in the field, to get the 3306 up and running again. Only CAT-sourced replacement components, or aftermarket parts that are comparable in OEM quality, should be used in a liner repair. Additionally, checking and establishing proper liner protrusion is critical before everything is pieced back together. If it's not, head gasket issues could be the result. It goes without saying that fresh coolant—and the correct type—should get the call as well.
Problem #5: LeakingPre-Chambers
Two very distinct versions of the 3306 were produced: Indirect Injection or Pre-Chamber (PC) engines and Direct Injection (DI) engines. This problem applies only to the CAT 3306 with pre-chambers. In PC engines, a pre-chamber exists in the cylinder head and, thanks to the fuel injection spraying into the pre-chamber (rather than on top of the piston), this is where combustion begins. After being initiated in the pre-chamber, combustion carries downward into the cylinder beneath it. And now that you know that, know this: there are two types of PC engines, old and new. Old PC engines featured a two-piece pre-chamber assembly and came without glow plugs. Later PC engines were equipped with one-piece pre-chambers and glow plugs.
What Happens
A pre-chamber is highly reliant on a good seal at the cylinder head. With age, they tend to spring leaks—and when they do, they create a mess of coolant around the affected unit on top of the engine. There are two sealing points on a pre-chamber, with an O-ring located at the base of it and an O-ring positioned on the body of it. These O-rings inevitably become brittle over time, often corroding or cracking and breaking the perfect seal they once had. Occasionally, pre-chambers fail structurally, oftentimes by a one-piece unit blowing apart at the point where it was furnace-brazed (to make it a single assembly) or after developing a pinhole and pressurizing the cooling system.
Solution(s)
Fresh O-rings solve the most common sealing issues associated with pre-chamber failure. On older CAT 3306 engines (again, the versions without glow plugs and that utilized a two-piece pre-chamber assembly), the O-rings can be replaced without removing the entire pre-chamber. However, on later model PC engines (the 3306’s with glow plugs and one-piece pre-chamber assemblies), the entire pre-chamber must be pulled. In addition to replacing the top and bottom O-rings on a late-model pre-chamber, its copper sealing ring requires replacement. Note that special tooling is also recommended when servicing the pre-chambers.
Problem #6: GlowPlugs
As you now know, not all CAT 3306 engines came with glow plugs. However, it can be argued that every 3306 living in colder climates needs them. In cooler temps, they are notorious hard-starters, especially those with lower compression ratios. For a 3306 that is equipped with glow plugs, they appreciate a 20 to 30-second cycle of heat from the glow plugs prior to firing off for the first time each day. For many owners, cycling the glow plugs is even required when ambient temps dip below 70 degrees F.
What Happens
Like everything else, nothing lasts forever. And depending on how old they are or how often they’re used, glow plugs will fail at some point. The heating coil is the most common failure point on a glow plug—the heating element that’s present inside the component’s steel tube (i.e., body). If the glow plugs are regularly subjected to prolonged, repeat intervals of use (you can manually actuate the glow plugs on a 3306 engine), their service life will be reduced. In other instances, glow plug failure can be traced back to bad wiring.
Solution(s)
Screenshot
Prior to replacing a suspected faulty glow plug, proper diagnosis calls for the use of a multimeter and testing them for resistance to confirm if they’re bad or not (measuring ohms here). Inspect all wiring as well, replacing anything that appears suspect. Replacing the glow plugs on a CAT 3306 is very straightforward. They are easy to access on the engine, although removal of old versions should be performed with some restraint due to potential seizure within the cylinder head. Warm up the engine prior to loosening them and always torque the new units to spec.
Problem #7: Dropped Valves
Screenshot
Most of the problems on this list enter the picture very late in a 3306 engine’s lifespan. It goes without saying that dropped valves are one of these failures. In fact, a dropped valve usually only occurs after a 3306 has accumulated an obscene amount of operating hours or miles. With a reputation for lasting 20,000 hours or more before requiring an overhaul, many owners and operators expect the 3306 to last forever. Eventually, however, something is bound to fail in an engine with this much usage. Just an FYI: 20,000 operating hours equates to 1.1 miles traveled...
What Happens
Screenshot
Sometimes compounded by cracks within the cylinder head that have spread into the valve seat region, dropped valve scenarios often unfold after a valve seat breaks apart or fails completely. Catastrophic in nature, dropped valves can result in piston-to-valve contact. As with any engine, anytime these two components make (metal-on-metal) contact it can spell disaster. In addition to broken valves and bent or broken valve stems, it’s not uncommon for the engine to also endure deck surface damage on the cylinder head, piston damage, liner gouging and, in extreme cases, even connecting rod failure.
Solution(s)
While there is no saving a high-mile engine that’s been neglected or regularly exposed to abuse (excessive heat and/or rpm), keeping the overhead of a 3306 properly adjusted can help avoid catastrophes like dropped valves. Following this type of failure, expect to pull and resurface the cylinder head at the very least, but don’t be surprised if the head isn’t salvageable. Still, having to source a new or remanufactured Cummins diesel engine, particularly a cylinder head is getting off cheap with this kind of failure. Many times, a new head, liner, and a complete overhaul of the affected cylinder is in store.
Problem #8: Front Main Seal
Screenshot
Throughout the life of a CAT 3306, the crankshaft’s front main seal may need to be replaced more than once. It’s the number one source for an oil leak on the company’s revered 10.5L, whether it’s been in operation for decades or only a few years. Beyond it being one of the more frequent problem areas on the engine, a leaking front main seal is one that calls for specialized or high-quality tooling in order to access it. With the crankshaft damper being of a tapered design, its fitment on the crankshaft is ultra-tight. A heavy-duty puller is required to remove it.
What Happens
Excessive operating hours, high miles, constant NVH exposure, corrosion, and time can all take their toll on a3306 engine’s front main seal, and springing an oil leak at this location is all but inevitable. Most failing front main seals begin to seep subtly, which usually goes unnoticed by the owner or operator. However, in time the leak worsens, with the bottom front portion of the engine, as well as the oil pan, becoming covered in a layer of oil. For engines powering heavy equipment or trucks that get very little attention, it’s not uncommon for a leaking front main to go unnoticed until oil loss is discovered, either on the dipstick or in the form of a puddle under the machine.
Solution(s)
Obviously, a new front main seal is the only solution for a leaking one. However, a new wear ring must also be installed at the same time. The wear ring is made of steel and effectively provides a new surface for the crank seal to sit in. Starting over with a fresh front structure gasket set isn’t a bad idea at the time of service either, as is having an experienced CAT mechanic perform the work. It’s also worth noting that you’ll need ample space at the front of the engine in order to use the appropriate puller on the crankshaft damper.
Problem #9: Bad Injection Pump
Screenshot
There are a myriad of fuel system issues that can take an aged 3306 out of commission temporarily (out of spec injectors or air-in-fuel issues being common), but an injection pump failure will end any chances of making it back home or continuing work for the day. Internal component failures aren’t commonplace, but every part inside one of these injection pumps does have a lifespan. Other headaches associated with the injection pump include fuel leaks which, given the injection pump’s location near the exhaust manifold, amount to a fire hazard and require immediate attention.
What Happens
CAT’s 3306 uses a mechanical, sleeve metering style injection pump (not to be confused with a “scroll” type pump). This means the fuel rack relies on radial motion vs. linear movement (scroll pump) to send fuel to the sleeve. Inside the governor housing at the rear of the pump, the shaft that requires full travel/rotation (with throttle movement) in order to open the fuel rack is known to have its travel limited due to corrosion (rust) or debris (dirt, contamination). When the shaft cannot rotate properly, the operator will experience hard-starting, one of the clearest diesel engine failure symptoms on an aging 3306, and eventually no starting at all.
Solution(s)
Screenshot
With the right skillset, experience, and tools, the sleeve metering pump can be repaired in the field, especially if it’s only dirt or other debris hindering the pump’s performance. When rust is the culprit, any field fix will be more of a temporary Band-Aid. The best solution for restoring performance is to pull the injection pump and send it to a creditable shop for a complete rebuild. Of course, another option is to purchase a whole new pump—just make sure it’s sourced from a reputable name in the industry.
Problem #10: Oil Cooler, Valve Guides, And Cracked Exhaust Manifold
These failures aren’t everyday occurrences on the CAT 3306, but in their old age they tend to surface more and more. As for the oil cooler, when the internal core fails it typically sends engine oil through the radiator as well as into the coolant. Additionally, it’s not atypical to find coolant in the oil pan of a 3306-equipped piece of equipment that sits for prolonged periods. The valve guide issue is primarily reserved for engines with loads of hours and miles on the clock, in which the engine begins burning oil. Cracked exhaust manifolds typically surface due to age and excessive corrosion.
What Happens
1. Within the oil cooler, the relatively thin internal tubes that make up its core are susceptible to corrosion eating away at them over time, causing leaks and cross-contamination. 2. As for the valve guides, once worn due to high operating hours they tend to allow oil between the valve guides and valve stems, typically under low load conditions. This leads to moderate to high oil consumption. 3. When a through-crack develops in the exhaust manifold, a host of performance-debilitating things unfold. Among them is a drop in boost pressure, which results from the reduction in drive pressure that stems from the crack allowing exhaust to escape before reaching the turbocharger’s turbine (exhaust)wheel. Low power, reduced fuel economy, and a mess of soot on the passenger side of the engine are par for the course here.
Solution(s)
Addressing an internal failure of the oil cooler warrants a brand-new unit, but it’s a repair that is nowhere near as time-consuming or laborious as the fix for worn valve guides. For that, the head will need to be pulled, torn down, and completely overhauled. At that point, the discovery of cracks may even force the purchase of a new cylinder head. Add to that all the costs you would incur in a typical head gasket job. Without question, the best remedy for a cracked exhaust manifold is sourcing a new or reman unit, with a junkyard take-off being a cost-effective solution.
