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Top 10 8.3L Cummins Problems

Written By, Mike McGlothlin


The 8.3L Cummins is best known as a venerable power plant with day-in, day-out durability and above average power output. If you run one of these in your equipment, chances are it will be on the road, in the field, or out at sea for a long time. But as is often the case here at Big Bear Engine Company, we’re well apprised of the common failure points of even the most reliable medium and heavy-duty diesel engines—and the 8.3L platform is no exception. However, few of this time-tested inline-six design’s shortcomings are catastrophic, and most of them can easily be rectified or avoided. From the 8.3L’s propensity to blow head gaskets in certain applications to the ISC’s CAPS system failures to all of the issues associated with modern age emissions equipment, we’re covering everything below—along with providing a permanent remedy.


Problem #1: Head Gasket Failure

What Causes It?

Aside from old age, neglect, or abuse taking its toll in an on-highway application, 8.3L8.3L Cummins Block Engine Rebuild head gasket failure is most prevalent in agricultural equipment. In this particular engine configuration, sub-par coolant flow, filthy operating conditions, and low vehicle speed (i.e. little airflow across the radiator) often leads to overheating. If ignored long enough, excessive coolant temps will eventually warp the cylinder head and break the head gasket’s seal on top of the block. In-field head gasket repairs are rampant on 8.3L’s that live in an agricultural environment and, while not the most difficult task to perform, can be time consuming given how buried the engine is within the machine.


Where (And When) It Occurs

Typically, the 8.3L Cummins’ cylinder head will lift at the right front corner of the block.6CT Cummins Cylinder Head Rear However, rear leakage isn’t uncommon either. Regardless of where the head gasket fails, significant downtime is involved. Working to reduce its failure rate, Cummins issued an updated head gasket, as well as enhanced head bolt torque specs and a different torquing sequence. While the latter OEM level improvements helped, the conditions many ag-based 8.3L’s operate in remains the primary killer of head gaskets. In these applications, any point beyond the 7,000-hour mark becomes fair game to experience a head gasket failure.



While there is no way to engineer your way out of the 8.3L Cummins’ head gasketRefinished Cylinder Head Deck Surface 8.3L Cummins problem, you can keep it from becoming a common occurrence. Start with genuine Cummins replacement parts (namely the updated head gasket and head bolts), have the cylinder head resurfaced by a reputable machine shop, perform a proper inspection of the block’s deck surface, and follow Cummins’ torque specs and sequence to a T. Additionally, have the head magnafluxed to check for cracks and, if necessary, start with fresh intake and exhaust valves. Unstretched head bolts can be reused, but the safest bet is to start with fresh fasteners throughout.



Problem #2: Poor Coolant Flow

What Causes It

With agricultural renditions of the 8.3L Cummins being the most likely to experience headCombine Harvester 8.3L Cummins Water Pump Outlet gasket failure, it stands to reason that these versions also suffer from the poorest coolant flow to begin with. This is especially true in the 6CT found in combine harvesters. Overall, the cooling system is adequate, but the 90-degree radiator hose leaving the water pump is a huge bottleneck for coolant flow. In addition to its disappointing water outlet design, a debris-riddled working environment doesn’t help matters—and a plugged radiator with low airflow moving across it will inevitably lead to higher coolant temps.


What It Can Lead To

Obviously, poor coolant flow is only a contributor to the agricultural version of the 8.3LISC Cummins Cylinder Head Rebuild Cummins’ propensity to overheat, but it can create a situation which results in what was discussed earlier: a blown head gasket. Then of course comes the possibility of warping the cylinder head or causing hairline cracks to develop in its cast-iron construction. It’s also important to remember that any amount of significant coolant loss can lead to a myriad of engine problems, none of which will entail quick turnaround time or low repair costs.



You can’t change the design shortcomings of the agricultural-intended 8.3L’s coolantMechanical 6CT Cummins Truck Engine system, but you can keep the radiator clean… Just remember that, when cleaning it externally, the radiator should be flushed from both sides (looks can be deceiving). This also applies to RV configurations, where due to packaging constraints under the coach there wasn’t a lot of oversizing built into the radiator. In these vehicles you can’t get away with half of the radiator being plugged up and not allowing air to pass through it. Last but not least, always keep your water temperature gauge within sight.



Problem #3: Wet Sleeve Cavitation

What Fails

By and large, wet sleeve engines are far superior to their dry sleeve and parent bore8.3L Cummins Wet Sleeve Cylinder Liner brethren. However, that doesn’t mean they’re perfect. In the case of the 8.3L Cummins, cavitation is fairly common, and it can have devastating effects. Over time, general operation of the piston moving up and down within its respective sleeve (and combustion occurring, generating heat), pushes coolant away from the sleeve. As coolant is pushed away quickly, a low pressure is created behind it causing air bubbles to form. Then as the coolant is quickly pulled back in, the air bubbles collapse and the coolant itself impinges the sleeve, eventually eating away at the metal.


What It Leads To

Eventually, after prolonged intervals of cavitation, microscopic holes develop in the sleeveWet Sleeve Cavitation 8.3L Cummins Diesel Engine—and sooner or later one (or several) springs a leak. This immediately leads to coolant mixing with engine oil, a major no-no for any engine. When the engine begins “making oil,” the result can be bearing damage and/or a hydrolocked engine. Coolant loss and a rising oil level on the dipstick are telltale signs of sleeve failure. Of course, this is also the same way to detect that one of the sleeve’s O-rings, which are designed to separate coolant (top) from engine oil (bottom), has failed.



The biggest insurance measure you can take to prevent wet sleeve cavitation exists in thePeak Final Charge Engine Coolant antifreeze you use in the engine. Specifically, the proper dry chemical additive (DCA) should be used. Among other functions, DCA additives prevent cylinder sleeve pitting. But this isn’t a one-and-done proposition. DCA’s must be checked and maintained within the engine coolant to ensure they’re up to the task of protecting the engine, and especially your sleeves. Test strips (which we’ll note do expire) are highly affordable and should be added to every 8.3L owner’s (or mechanic’s) toolbox.



Problem #4: CAPS Injection Pump (’98-‘05)

What Is It

Cummins ISC CAPS Fuel PumpTightening on-highway emissions standards forced Cummins to kill off the 8.3L’s (6CT) mechanical inline injection pump in 1998. What it implemented instead was the Cummins Accumulator Pump System (or CAPS) on the ISC. The system, which features an electronically controlled, distributor-style injection pump, allowed Cummins to control injection events independently of engine speed. Similar to the Bosch VP44 that was added to the 5.9L 24-valve ISB Cummins, the electronic control unit is married to the CAPS pump itself. And also like the 5.9L ISB, downwind of the pump, mechanical fuel injectors remained in the mix.


Why It Fails

The primary cause of CAPS pump failure on an 8.3L ISC boils down to the engine’s self-8.3L ISC Cummins Electric Fuel Lift Pumppriming, electric lift pump. Although the lift pump only supplies low-pressure fuel to the CAPS pump for the first 30 seconds of engine run time (after that, a CAPS pump-married gear pump takes over fuel suction duties), with age and wear they tend to develop leaks. This allows air to infiltrate the injection system and, overtime, can take out the CAPS pump. Add to that the fact that once the lift pump kicks off, the aforementioned gear pump is on its own, pulling fuel from the tank (i.e. not an easy life).



A remanufactured CAPS pump alone costs north of $3,000. Factor in the labor required toAftermarket Electric Lift Pump 8.3L Cummins install it—especially in a motor home—and you’re looking at a hefty repair bill. Regular inspection and/or replacement of the OEM lift pump is recommended in order to ensure the supply pump isn’t leaking. A second option is to install an aftermarket electric lift pump. The key benefit here is that you get a pump that delivers constant low-pressure supply to the CAPS gear pump, instead of solely for engine startup. Furthermore, heat soak is a major killer of the CAPS pump—and low (or no) fuel pressure issues are notorious for causing excess heat.



Problem #5: Connecting Rod Failure

Why It Happens

First and foremost, connecting rod failure isn’t highly common with the 8.3L Cummins.Case IH 2388 Combine 8.3L Cummins However, it does occur more frequently in combine harvester applications. Here, engine operating conditions often entail higher rpm than what on-highway versions see, a filthy work environment and, due to poor coolant flow, excessive water temperature can (which in turn can lead to elevated engine oil temps). Couple all of that with big, heavy internal parts not exactly appreciating higher engine speed(s) and you start to see why rod failure is more common in combines, many of which turn 2,600 to 2,700 rpm out in the field.


When It Happens

Wrist Pin Bushing Failure Cummins ISL Engine DamageAfter living at those kinds of engine speeds for years, many 8.3L Cummins mills packed into combines become prime candidates for connecting rod failure. In these applications, somewhere between 7,000 and 10,000 operating hours seems to be the typical timeframe for failure. Whether the failure consists of a spun bearing, a failed rod bolt, or the rod itself cracking or bending, a windowed block is sometimes even the end result. Again, for several reasons the combine harvester applications run hotter (a huge radiator, sure, but no road-speed or airflow such as in truck applications) which, over time, is likely another contributing factor.



Without a doubt, the 8.3L leads a miserable life in a combine. However, you can minimizeMotorhome 8.3L Cummins Radiator Cleaning the abuse it sees by avoiding excessive rpm if (or whenever) possible, maintaining a strict maintenance regimen, and by freshening up the bottom end ahead of schedule. Fresh rod bearings and new, correctly torqued rod bolts alone could go a long way in preventing connecting rod failure. And as part of routine maintenance, taking special care to ensure the radiator stays clean can work wonders in keeping coolant temperature in check.



Problem #6: Cracked Exhaust Manifold

What Happens

If our list was ordered according to failures most likely to occur with your 8.3L Cummins,Cracked 8.3L Exhaust Manifold this would be number 1. Discovering a hairline crack in its two-piece center section is extremely common. But that’s not all. It’s also typical for the exhaust manifold to warp over time, which leads to exhaust leaks at the gaskets—not to mention a drop in turbo drive pressure (i.e. lack of power). A third problem revolves around the fasteners that mount the exhaust manifold to the cylinder head. When attempting to remove the manifold, its mounting bolts are prone to breaking off.


Why It Happens

Age, countless heat cycles, and rust all serve to weaken the exhaust manifold. Those threeExhaust Manifold 8.3L 6CT Cummins Engine factors contribute most to the manifold’s tendency to develop a hairline crack, warp, and cause bolt corrosion, although improperly torqued manifold bolts can contribute as well. Both a cracked exhaust manifold and a warped manifold will cause a lack of performance due to the aforementioned loss of pressure required to drive the turbocharger. Beyond that, you get reduced fuel efficiency and a sooty mess covering the passenger side of the engine—not to mention a strong exhaust smell.



New Exhaust Manifold Big Bear Engine 8.3L CumminsThe easiest (not the cheapest) solution is to simply source a new exhaust manifold from Cummins. There are foreign-made alternatives, but most owners that attempt to save money by going this route usually end up buying two manifolds in the long run. On the other hand, a warped manifold can be rectified more affordably by having the mounting surfaces machined true at a reputable machine shop. Last but not least, always install new exhaust manifold bolts (and fresh gaskets) and torque them to the correct specification.



Problem #7: VGT Failure

What Happens

This failure doesn’t apply to the 6CT, which was equipped with the “old reliable” and fixedCummins ISC Holset VGT Turbo geometry Holset HX40 turbo. Rather, it’s for later renditions of the 8.3L ISC that enjoyed superior responsiveness across the rpm range thanks to variable geometry turbo technology. Unfortunately, the VGT on the ISC engine is prone to “sticking.” This either leaves the operator with a turbo that provides great low-end response but quickly runs out of steam up top, or a turbo that is very laggy down low but gradually comes to life at higher engine rpm. It’s a highly common failure with all VGT technology, not just the version of it employed on the 8.3L Cummins.


What Fails

A failed VGT actuator is usually the culprit behind most VGT issues. However, sector gearCummins ISC VGT Turbo Sector Gear problems—in the form of sticking or attempting to seize up before full travel is possible—can actually take out the actuator. This is why proper diagnosis is key. If the sector gear (shown) is bad, it’s a turbo issue (and possibly an actuator issue, too). If the turbine side of the VGT moves freely, it’s likely an actuator issue. As you may have already deduced, the latter issue is a much less expensive problem to have.



Depending on your turbo’s diagnosis and possibly even your mechanic’s recommendation,VGT Turbo Actuator 8.3L Cummins ISC actuator replacement or a complete turbo replacement (with actuator) will be in store. Unfortunately, VGT headaches are a part of life for all modern diesel engines. Their working environment, which calls for movement within the exhaust side of the turbocharger, eventually hinders the movable vanes (or nozzles) unable to operate at their full potential. Depending on the application, a VGT can be replaced by a more reliable fixed geometry alternative. However, the tremendous low-rpm drivability and responsiveness you get with a factory VGT will be sacrificed.



Problem #8: EGR Cooler Failure

What Happens

EGR Cooler Cummins ISC EmissionsThis shortcoming is reserved for ’07 and newer ISC engines with common-rail injection, as they were the first 8.3L Cummins’ to receive exhaust gas recirculation (EGR). Although EGR valve and sensor issues are common, EGR cooler failure is frequent and, arguably, more alarming. Over time, thermal expansion leads to fatiguing of the EGR cooler’s internal heat exchanger core and it can rupture. In most cases, when this happens coolant makes its way into the cylinders. From there, you get white smoke leaving the exhaust and a noticeable drop in the engine’s coolant level.


What It Can Lead To

Because a cracked EGR cooler leads to quick and excessive coolant consumption,Inline Six Diesel Engine Overheat Diagram overheating can occur if the problem is neglected long enough. And as everyone knows, overheating has the potential to warp a cylinder head and facilitate a blown head gasket. Worse yet, when a damaged EGR coolant allows coolant to enter the engine it inevitably makes its way through the turbo and EGR valve. It’s not uncommon for a cracked EGR cooler to plug up the EGR valve or cause a VGT-related issue thanks to the rapid accumulation of carbon and soot from coolant being present in the intake and exhaust tracts.



Other than periodically replacing the EGR cooler, there is no way to be sure theBig Bear Diesel Engine Coolant Leak component will last hundreds of thousands of miles. However, by paying careful attention to your engine’s coolant level, watching for coolant leaks, and immediately shutting the engine down should you notice white smoke leaving the exhaust system, you can preserve the life of the EGR valve and turbocharger in the event of an EGR cooler failure. Related issues that contribute to higher operating temperatures (namely coolant temp and exhaust gas temp) should also be addressed as soon as possible, as that creates extra wear and tear on the EGR cooler—not to mention the rest of the engine.



Problem #9: DPF Failure

What Is It

Beginning in 2007, virtually every engine manufacturer began adding a diesel particulatePlugged DPF Cummins ISC Diesel filter (DPF) to ensure EPA particulate matter (PM) standards could be met. The DPF, which is part of an elaborate exhaust aftertreatment system, essentially consists of a giant canister that traps the soot that would otherwise leave the tailpipe or exhaust stack. Even though what’s known as a regeneration process converts stored PM into a fine ash (to consume less space within the DPF), eventually the owner has no other choice but to remove and clean, or outright replace the DPF. Note: many on-highway, ’07-later model ISC and ISL Cummins engines used the same DPF.


How They Fail

Cracked Diesel Particulate Filter Cummins ISCEarly versions of EPA 2007-compliant ISC Cummins engines were prone to various DPF-related failures, but the biggest issue was the DPF itself cracking. The outer walls of these initial DPF’s were prone to cracks and would subsequently leak smoke and soot into the atmosphere. In the 2008-2009 timeframe, Cummins released an updated DPF for the ISC with a stronger overall structure and that featured more robust internal substrate material. Other forms of failure are textbook for any DPF: 1) being plugged solid with ash and, 2) faulty differential pressure sensors.



Regular maintenance goes a long way with any diesel engine, but in the modern era the listCummins ISC DPF Cleaning of items to check off has been expanded to include the emissions system. You can never guess when a pressure differential sensor will go south, but you can observe scheduled DPF cleanings (and/or replacement) to keep your ISC humming along at full steam. And although it may seem unrelated to the DPF, a faulty or failing fuel injector that contaminates the aftertreatment system can kill a DPF in short order. Long story short, take care of your engine—from air intake to tailpipe—and its reliability will be maximized.



Problem #10: Oil Leaks

Why They Happen

Though nowhere near as catastrophic as a connecting rod failure or a dead injection pump,Cummins 8.3L Engine Oil Leaks oil leaks are the pesky, often-inevitable gift that keeps on giving for the 8.3L Cummins platform. Factor in age, vibration, duty cycles, decades’ worth of expansion and contraction, abuse or all of the above and you get O-rings, gaskets and seals that can no longer seal as well as they once did. Though it’s a bit of an eye soar, it’s nothing like an oil consumption problem. No, a minor external oil leak won’t force your 6CT or ISC off the road.


Where They Happen

Based on age alone, a 6CT Cummins—even one that’s already been overhauled or resealedRear Main Seal 8.3L Cummins 6CT (maybe even more than once)—is more likely to be suffering from significant oil seepage than a newer, common-rail ISC would. The older 8.3L’s were notorious for springing a leak at the rear main seal (shown) or from the rear cam plug. We’ll note here that the latter source of an oil leak is regularly misdiagnosed as a rear main seal leak... Other trouble spots include the front cover, the gear housing case, and the injection pump.



Because they don’t necessarily affect the engine’s ability to perform, most oil leaks goCummins 6CT 8.3L Diesel Engine neglected. But once they can no longer be ignored, the only real fix is to dig into the engine. Whether you’re addressing one seal or all of them in a complete rebuild or reseal, just make sure to use genuine Cummins replacement parts. An 8.3L Cummins can last forever if properly maintained—and proper maintenance begins with the use of Cummins-tested and Cummins-proven components. The other piece of the puzzle is enlisting the help of a competent mechanic from a reputable dealership or independent shop.

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