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10 Common Problems With Diesel Engines

By Featured Writer Mike McGlothlin


For the most part, diesel long-blocks are pretty solid, with rigid components like forged-steel crankshafts and connecting rods, mammoth-sized wrist pins, rod bearings, and main bearings, gear-driven accessories, and densely-cast gray-iron blocks proving nearly unbreakable. Sure there are freak instances of dropped valves, bent rods, and cracked blocks, but by-and-large most light-duty, medium-duty, and heavy-duty diesel engines are hard to kill. However, outside of structural integrity, there are a host of issues—both mechanically and electronically as well as operator-induced and factory engineered—that can shut any diesel engine down. Cummins, CAT, International, Detroit, John Deere, or Mack, none of the big name manufacturers are immune to a bit of inevitable downtime.


For that reason, we’ve compiled a list of the 10 most common problems a diesel engine will face in its lifetime. From common-knowledge, hard-starting issues to the inherent vibration problems we’ve all heard about, to the modern-day diesel’s emissions-related headaches, our list runs the gamut. Of course, contaminated fuel, injection system failures, and cooling system troubles made the cut, too, with electrical gremlins, sensor failure, and oil dilution not far behind. Whether it be due to old age, system complexity, or lack of maintenance, the following problems represent the everyday ailments diesel owners are forced to contend with. It may be one of the uglier Top 10 lists you’ll find, but someone had to do it…


1. Vibration

Cummins 24-valve Turbodiesel Oil Fill CapVibration is inherent to all diesel engines and is primarily due to the violent, uni-directional combustion forces acting within the cylinders. This is true even in inline-six configurations, engines that are naturally balanced. But while slight vibration alone isn’t a major worry, it’s what happens over time that can be a problem. Or, worse yet, an increase in engine vibration, which is often a sign of a larger problem.



Is Time Ever On Our Side?

Diesel Engine Rear Main SealUnfortunately, as far as aged diesel engines are concerned, the answer to this question is unequivocally “no.” As service hours on an engine mount, the chances of vibration causing a problem to surface go up also. Many times, years of vibrant engine operation culminate in a fuel or oil leak. Without proper maintenance, different models of engines can be prone to certain failure points; for example, the DD15 can produce oil leaks when loose gasket seals appear around the fuel pump, oil coolant system, or air compressor. For fuel, the leak predominantly starts where a hard line and a fitting meet, but the occasional hairline crack in a steel fuel line is always possible. For oil, rear main seals, oil cooler O-rings, and even rusted out or damaged oil pans are behind most leaks.



Causes of Increased Vibration

Valve Adjustment ISB CumminsWhile a fuel or oil leak can be traced back to the source and addressed quickly in most cases, an increase in engine vibration could be the sign of a much larger issue. Improper valve clearance, piston slap, injector misfire, and lack of compression can all contribute to inordinate amounts of engine vibration, and a process of elimination—whereby the easiest tests are run first—needs to take place for proper diagnosis.




2. Hard Starting

Glow Plug Diesel Fuel InjectorHard-start issues can usually be traced back to two primary points of causation with diesels: 1) batteries and starting aids, and 2) a lack of fuel. Diesel engines’ high compression ratios often require multiple batteries to be employed in order to provide enough cranking power to fire them up. However, failed or ailing glow plugs (if applicable) and glow plug relays or controllers (again, if applicable) can generate plenty of starting issues, especially in colder climates. To a lesser degree, non-functioning intake heater grids can cause longer crank times as well.



It’s All About The Fuel

Fuel Supply Pressure GaugeOutside of battery, glow plug, and grid heater issues, most hard-starting problems for diesels are fuel-related. A lack of low-pressure fuel supply and an inability to build adequate injection pressure will make a diesel a bear to turn over, and each of those respective issues can typically be followed back to a lift pump issue or a dead or dying injection pump. Because fuel injectors don’t normally all fail at once, the engine will start with an ill injector or two—though a problem will likely be noticeable, especially at idle.



The Cold Weather, Hard-Start Solution

1000 Watt Diesel Block HeaterNo diesel engine enjoys firing up in the cold, but you can keep yours happy by ensuring a set of fresh or well-tendered batteries are always on board. Taking things a step further, it pays big dividends to have your existing batteries load-tested before cold weather arrives (i.e. fall). And finally, use your engine’s block heater (if applicable). After all, it’s there for a reason…and a diesel engine with ECT and EOT sitting at 50 to 80 degrees F over ambient temperature will always start easier than one sitting below zero.




3. Emissions Equipment

Diesel Emissions EquipmentThis problem area doesn’t really apply to diesels produced prior to 2003-2004, but since these issues have been a part of life for on-highway engines for nearly two decades now, it would be negligent not to include it on our list. Exhaust gas recirculation and the elaborate exhaust after-treatment technologies that include diesel oxidation catalysts, diesel particulate filters, and selective catalytic reduction have become major weak links for the modern diesel engine. Even as manufacturers continue to refine and improve their emissions-fighting technology, most of the systems’ components remain prone to premature failure.




Exhaust Gas Recirculation CoolerTo meet tougher federal NOx emission standards that went into effect in 2004, on-highway diesel engine manufacturers turned to exhaust gas recirculation (EGR) to get the job done. By rerouting a portion of exhaust flow back into the engine’s intake tract (exhaust being almost completely void of oxygen), combustion temperature within the cylinders is reduced, which lowers NOx. The problem is that diesel exhaust is laced with soot, carbon, and vapor from blow-by, which clings to the walls of EGR coolers and especially EGR valves, eventually rendering them inoperable. We’ll also note that EGR is cooled off using engine coolant, which taxes the antifreeze considerably.




Diesel Particulate FilterThe OEM solution to more stringent federal particulate matter emission standards was the diesel particulate filter (DPF), a downstream exhaust device used to capture and store soot produced during combustion. Over time, the soot accumulation has to be burned off, and it’s transformed into fine ash through a process called regeneration. Regeneration can be either active or passive, but the former type requires that excess fuel be consumed, not to mention that the excess fuel is either injected on a cylinder’s exhaust stroke or through an additional fuel injector mounted downstream in the exhaust system. We’ll deal with the sensor side of DPF systems later on in our list.




4. Water: The Worst Offender Of Contaminated Fuel

Rusted Diesel Fuel SystemAny form of debris infiltrating a fuel system is bad news, but the worst contaminant for a diesel to contend with is water—and this is precisely why water separators come equipped on virtually every diesel engine. But why is water so problematic? Because when it comes into contact with metal it leads to rust. Internal corrosion of fuel lines, pickup tubes, injection pumps, lift pumps, and injectors can precipitate their failure, cost you thousands in repairs, and, in a worst-case scenario, even destroy the engine.



A Perpetual Problem

Biodiesel Water In Fuel ContaminationLuckily for us, extensive studies have been conducted concerning water-in-fuel issues and we know that 99 percent of all fuel contamination issues begin in the fuel tank. Dirt, sand, grime, and other debris added via the filler neck is almost always captured by the engine’s fuel filter. However, water presents a unique challenge because, at the molecular level, diesel fuel is attracted to H2O. But on top of being attracted to water, diesel fuel will even absorb it. Biodiesel, which possesses an even higher affinity for water than regular number 2 diesel, compounds the problem further—as most fuel stations carry a biodiesel mix at the pump. Needless to say, water has always been and will always be an issue for diesel engines.



Keep Your Diesel Filtered And Clean

Fuel Water Separator DrainLike many issues on our list, proper maintenance goes a long way in preventing them. Because an unchanged fuel filter can only absorb so much water before it’s no longer effective, observing regularly scheduled fuel filter and water separator changes is of utmost importance. In addition, routine water separator drain intervals (if applicable) should be adhered to. It also pays to get your fuel from a trusted source, such as a high-volume filling station. High-traffic stations go through more fuel, which means the fuel they receive is always fresh. In closing, make sure to keep the fuel tank full whenever possible, as water vapor forms quickly in an empty or even a tank that’s half full.




5. Sensor Failure

Diesel Exhaust Aftertreatment SystemTo keep modern electronically controlled diesel engines equipped with exhaust after-treatment systems working flawlessly, a myriad of sensors have to be in good working order. But as many late-model diesel owners have found out the hard way, one failed sensor can lead to considerable headaches and lots of downtimes. Poor performance, limp mode, or complete inoperability, and check engine lights (CEL’s), and stored diagnostic trouble codes (DTC’s) are highly common when an emissions-related sensor stops working. For on-highway applications produced after 2007, the sensor failure problem is rampant.



The Life Or Death Sensors: EGT, NOx, And Pressure Differential

DPF EGT NOx SensorsTemperature, NOx, pressure differential, and even oxygen sensors are all required for proper diesel exhaust after-treatment system functionality. Failure of any one of these sensors can cripple engine performance. Temperature sensors are positioned before and after the diesel particulate filter to measure exhaust gas temperature (EGT). NOx sensors are used to determine the proper dosing of diesel exhaust fluid injected in applications that use selective catalytic reduction (SCR). Pressure differential sensors are also positioned upstream and downstream of the DPF, and they measure exhaust backpressure, with the pre-DPF sensor being the point of data collection for initiating the DPF regeneration process we mentioned earlier.



Most Likely To Fail: EGT Sensor

Diesel Exhaust Temperature SensorSo which sensors fail most frequently? EGT sensors. Although they’re designed to work in extreme heat, their intense work environment often takes its toll on the thermocouple side of the piece, forcing them into early retirement. In some cases, EGT sensors are exposed to more than 1,600 degrees F—and very few components can withstand that kind of heat long-term. Other points of failure with EGT sensors (and indeed many other automotive sensors) stem from vibration or being disturbed when other areas of the exhaust system are being addressed or repaired.




6. Overheating

Overheating 5.9L Cummins Diesel EngineEveryone knows that diesel engines are intended for hard work, but tackling the toughest of tasks on a consistent basis without proper cooling system maintenance or inspection can lead to overheating. Ignoring or allowing coolant leaks to go unaddressed, a failing engine fan, a stuck thermostat(s), or a bad water pump can all contribute to overheating. To a lesser degree, the incorrect type of coolant can also be a contributing factor. If improperly diagnosed or left unsolved, an overheating scenario could potentially lead to hard-part failure such as a warped cylinder head.



Find The Leak

Water Pump Failure Duramax Diesel EngineOlder engines, especially those with hundreds of thousands of miles on the clock, are often still making use of the factory radiator hoses. For external inspections of leaks, this is the best place to start, followed by analyzing the condition of the hose clamps and then the radiator itself. If the source of a coolant leak brings you to the engine’s water pump, examine the weep hole (if applicable, or possible). If your search lands you at the thermostat housing, check it over for cracks. Of course, internal coolant loss could mean you’re looking at a blown head gasket or a cracked EGR cooler. More on the former issue in a bit.



The Cooling Stack

Caterpillar 3406 Class 8 Diesel EngineIn the midst of a busy work schedule, many diesel engines don’t receive the attention they truly deserve, and a plugged cooling stack due to neglect or a dirty work environment is a great example of that fact. Positioned out in front of the engine you’ll find the charge air cooler (i.e. intercooler), a large radiator, an A/C condenser, and in some applications a transmission cooler or even an oil cooler. These air-cooled heat exchangers are all fighting for adequate airflow to begin with, but when you restrict them further it’s a recipe for overheating. For the right candidate, thorough cleaning of the engine’s cooling stack—which entails disassembly, compressed air and low-pressure water with the use of a mild detergent—can bring operating temps at full load back down to where they need to be.




7. Electrical Gremlins

Diesel Engine Electrical ProblemsNow that we’re decades into full-on electronic control of diesel engines, wiring issues are surfacing throughout the repair industry. Bad connections, wire chafing, short-to-ground issues, and hard-to-identify intermittent electrical flare-ups are all in a day’s work for many of today’s diesel technicians. Tracking down electrical gremlins can be highly time-consuming with so many onboard systems present in late-model diesel vehicles. What’s worse is that what ends up being a simple wiring fix can initially lead you to believe a major engine component is bad, such as an injector. Proper diagnosis is paramount here.



Start Slow

24-Valve Cummins Dodge Ram 2500 PDCMany times, a DTC or a blown fuse will guide you to the troubled area. In other instances, it might be obvious where you need to start. Regardless, before tearing into anything first look at your electrical connection points. Finding a loose connection or a corroded connector is more common than you think, and is always a welcomed “easy” fix. Damaged pins or wires at or near the electrical connection points can also quickly be discovered. In these cases you don’t even necessarily have to know what the wiring is for, you just have to address the obvious.



Chafed Wiring

Chafed Diesel Engine Wiring HarnessThis problem often stems from number 1 on our list: vibration. Years and years of pulsation combined with the friction of a wire rubbing against a valve cover or anchor point can eventually wear a hole through the tape, wire loom, and/or a wire’s outer coating. At that point, the exposed wire’s contact with its surroundings can cause a textbook example of a short-to-ground. This is extremely common with engine harnesses. And while the issue is sometimes time-consuming to track down, the fix is cheap, often calling for electrical tape or an additional wire loom (or both).




8. Diluted Engine Oil

Diluted Diesel Engine OilThis problem ties in with our “Emissions Equipment” entry due to many modern-era diesel engines making oil (as in, the level on the dipstick keeps rising) thanks to their regeneration events. If you recall, regeneration is the process that turns soot collected in the diesel particulate filter into a fine ash. To pull off this process, some engine manufacturers chose to inject excess fuel into the exhaust stroke of a given cylinder, which lights off the diesel oxidation catalyst downstream of the engine. The problem with this is that it washes out the cylinder wall, and some of the fuel inevitably squeezes past the piston rings and dilutes the engine oil.



Further Causes Of Oil Dilution

EGR Soot ContaminationThe fallout from diluting the engine oil may not be immediate, but once the problems caused by it are noticed it’s likely way too late. First, engine oil that’s been contaminated with diesel fuel loses some of its viscosity, which obviously isn’t good for bearing life (mains, rods, cam, or even turbo bearings). But it isn’t just diesel that contaminates engine oil. In modern diesel engines, EGR introduces a considerable amount of soot particles into the oil. Increased oxidation is also common in diluted engine oil, which by breaking down the base molecules within the oil effectively reduces its service life.



Fuel Dilution Is Extremely Common

Old engines or new, discovering fuel in the oil is commonplace and can happen for a lot of reasons. For one, a damaged cylinder, piston ring, or both is going to allow fuel left over from the combustion process to sneak into the crankcase. Short-trip operation, where the engine never reaches optimal operating temp is another common cause of fuel dilution. Additional reasons one might detect a higher percent fuel content (PFC), usually from the information provided on an oil analysis, can be linked back to excessive idle time, incomplete combustion, and improperly balanced or leaky fuel injectors. Interestingly, according to Amsoil, just 3.4 percent is the acceptable percent fuel content limit.

 Oil Analysis Soot Contamination



9. Injection System Failure

Diesel Injector Pop TestYou might’ve noticed that we’ve referenced the difference between old and new diesel engines multiple times on this list…and now we’re doing it again. Pound-for-pound, modern-era injection systems simply do not last as long as their predecessors. However, in their defense, the injectors and injection pumps of today have more jobs to perform—and they also have to do it under higher pressure (hence the terms high-pressure common-rail injection and high-pressure fuel pump). To be sure, the mechanical fuel injection components of yesteryear have their wear points, too, but the service life of most of those systems was noticeably longer.



Injector Failure

Failed 5.9L Cummins ISB Fuel InjectorsWhether they’re new-age common-rail or old-school pop-off style, poor maintenance can facilitate the early demise of any injector. Neglected fuel filter change intervals and low-quality diesel (including water contamination) are two leading causes behind injector failure, with deposit buildup and particle contamination not far behind (especially on common-rail injectors). Other than those contributors, the biggest killer of injectors is age and wear. Over time, internal wear items such as the ball seat, nozzle needle seat, and injector spring (to name a few) simply become worn out. Hard starting, rough idle, increased smoke, and added fuel consumption are all indicators of a bad injector.



Injection Pump Failure

Inline Diesel Injection Pump FailureA host of factors can contribute to the death of an injection pump. For starters, any lack of low-pressure fuel supply being sent to the pump can cause damage, as the injection pump relies on that 10-to-65 psi worth of supply pressure to not only lubricate the injection pump but to keep it cool as well. Then comes the poor quality fuel causality, with rust being a very common culprit in killing an injection pump (again, thanks to water). Then, just as with injectors, there are internal parts that simply wear out in time. But luckily, just as with fuel injectors, injection pumps can be rebuilt, bench-tested, and made like new again.




10. Head Gasket Failure

8.3L Cummins Block Engine RebuildThanks to the employment of multi-layer gaskets, large diameter bolts, and many times six fasteners per cylinder, most on-highway diesel engines aren’t notorious for blowing head gaskets. Still, others are infamous for lifting the cylinder head(s), pressurizing the cooling system, and costing the owner thousands in repairs (the Navistar-built 6.0L Power Stroke and CAT’s 3126 come to mind). However, even the seemingly “bulletproof” power plants of the light-duty, medium-duty, and heavy-duty segments can all succumb to this type of failure with age.



Why Head Gaskets Fail

Blown Head Gasket 6.7L CumminsIt can take hundreds of thousands of miles, but over the course of decades’ worth of heat cycles, the integrity of a diesel engine’s head gasket is perpetually being challenged. Eventually, be it due to the cylinder head flexing, shifting, or expanding one last time, the gasket’s ability to keep combustion in and coolant and oil out can fail to do its job. In other scenarios, where the engine has been reflashed (or mechanically turned up, depending on the engine) to produce more power, the limits of what the head bolts can handle are exceeded and they can stretch.



It’s A Tough Job

6.7L Cummins Multi-Layer Head GasketsLike the upper sections of each cylinder bore, the cylinder head is exposed to both extreme heat and immense cylinder pressure. But it’s even deeper than that. The head gasket also has to resist the forces from the head flexing, vibration, and violent combustion that can scuff the block’s deck and the cylinder head’s surface. On top of that, the head gasket has to withstand the squish from the 200,000-psi worth of clamping force imposed across the length of its surface courtesy of the head bolts. Finally, the head gasket additionally serves to seal the pressurized antifreeze and engine oil running through it. Without a doubt, the head gasket is one of the hardest working gaskets in a diesel engine.


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