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Why Mechanical Diesel Engines Are So Much More Reliable Than Electronic Versions

Written By, Mike McGlothlin


We’re now more than 20 years removed from the arrival of electronically controlled diesel engines, so why are mechanical diesel engines still part of the conversation? For one thing, mechanical diesels remain relevant because a solid majority of them are still ticking. For another, many OEM’s and remanufacturers continue to produce, repair or rebuild the mechanical mills of old. But exactly why do mechanical diesels continue to be so desirable? In a word, reliability. No matter how refined, clean-burning, and powerful new-age, electronically controlled diesel engines become, their added complexity, shorter injection and turbo system lifespans, and higher costs continue to keep their simpler, mechanical counterparts in high demand.

To be sure, there are undeniable performance advantages associated with electronically controlled diesel engines. But when it comes to million-mile durability, mechanical remains king. This time, we’re spotlighting why mechanical diesels are more reliable than their late-model electronic siblings. And because the internal hard parts are essentially the same between these two engine types, it means the pitfalls of an electronic engine often exist in the electronics themselves. Below, we’ll cover everything from common injector issues to VGT turbo failure, and even the crippling effect a faulty sensor can have on an electronically controlled engine.


Complexity Killed The Diesel


There aren’t fewer moving parts within an electronically controlled engine, but instead the way it’s controlled adds considerably more complexity to the overall equation. Intricate components such as a variable geometry turbocharger (VGT) and its accompanying actuator, solenoid-activated electronic injectors, and EGR, DPF, and SCR emissions-scrubbing devices are all along for the ride on modern day, electronically controlled diesels. And, associated with all of those systems there are countless sensors that can fail or produce false readings, an ECU that’s quick to pull fuel and de-rate the engine, and wiring harness issues that often surface as the engine ages.


Failed Sensors: The Silent Crippler

A great way to illustrate how mechanical diesels outlast electronic versions starts by highlighting the failure-prone parts that aren’t even present on mechanical power plants. In this case, we’re talking about sensors. Not crank or cam sensors (although those are important), but things like fuel rail pressure, pressure differential, EGT and NOx sensors. If either one of those vital sensors checks out on a fully emissions-compliant, late-model, high-pressure common rail diesel engine, you’ve got big trouble. Failed, failing, or faulty sensors trigger CEL’s that often result in the ECU pulling power and de-rating the engine until the problem is addressed. With more sensors you also increase the chances of facing corroded connections with age or high exposure to humidity—along with the drivability issues those issues pose.


Electronics Are Expensive

Initial purchase and later repairs are all higher with an electronically controlled diesel engine than with a mechanical one. To be fair, electronic engine issues can be easier to diagnose thanks to modern day computer software, but the bottom line is still the bottom line for most people—and that’s money. As electronically controlled engines age, chafed wiring, short-to-ground issues, bad connections, and even computer module failures are becoming more and more common. And once you’re troubleshooting a wiring issue or replacing an ECU you’re well into a four-digit repair bill. With a fully mechanical engine, there are very few (or no) microprocessors that can fry or fail with age.


Injector Longevity: 200,000 Miles (+/-) Vs. 400,000 Miles (+/-)

Once again, a major point to remember in the mechanical vs. electronic debate is that the hard parts inside these engines are basically all the same. The primary difference exists in the way the fuel is delivered. In mechanical engines (designated as such due to their use of mechanical injection), the fuel injectors operate according to pop-off pressure and almost always fire a single time per combustion event. In today’s electronically controlled engines, each injector can carry out multiple injection events per combustion cycle (as many as eight in some applications). Given how many injection events a single unit carries out in a modern electronically controlled diesel, it’s no wonder their injectors don’t last nearly as long. Their internal seals, seats, return springs and sealing surfaces see much more wear and tear.


The Key To Mechanical Injector Longevity: Simplicity

With just a few moving parts, mechanical fuel injectors are as simple as it gets in a diesel engine. They rely on spring-based actuation. Once pressure within the injector body reaches a predetermined level, a check valve lifts off of its seat, permitting fuel to travel through the nozzle, out the tip, and in-cylinder during the power stroke event. Mechanical fuel injectors are ancient technology at this point, but they still get the job done fairly efficiently and rarely fail in catastrophic fashion. In fact, the first thing to wear inside of a factory mechanical fuel injector is usually the spring.


Solenoid Valve Injectors: The Injector Of Choice In Modern Electronic Engines

Most modern, electronically controlled diesels feature solenoid-activated fuel injectors. Voltage sent to the solenoid valve initiates the flow of fuel into the inlet of the injector. Solenoid valve injectors are prevalent in common-rail engines and are capable of delivering highly precise quantities of fuel for optimum power production and emissions reduction. However, solenoid failure is a familiar problem, which often shows up in the form of hard-starts. When solenoid failure is unfolding, its respective injector is no longer sending the commanded quantity of fuel into the engine, nor is it operating within its emissions parameters. And if the solenoid fails completely, you’ve got an injector that will no longer fire at all. This is never the case with a mechanical injector.


Electronically Controlled Diesels are More Sensitive To Low-Quality Fuel

Due to their ultra-tight tolerances, modern electronically controlled, high-pressure common-rail fuel injectors (as well as their pumps) are very sensitive to poor fuel quality. They do not tolerate any sizeable contaminants or debris, whatsoever, and when they do see it high levels of internal damage can result—along with deterioration of the aforementioned solenoid valves. In contrast, mechanical injection systems tolerate sizable contaminants much better, thanks in large part to their looser internal tolerances—not to mention the fact that they were designed to run on North American LSD (low sulfur diesel) rather than ULSD (ultra low sulfur diesel).


VGT’s Are Problematic—And Most Electronically Controlled Engines Have One

As with common-rail injection, a variable geometry turbo (VGT) is a common sight on most modern, electronically controlled diesel engines. While this style turbocharger—with its movable vanes (or nozzles) able to redirect the flow of exhaust gases across the turbine wheel—provides instant spool up and exceptional transient response, high-mile reliability isn’t its strong suit. Whether the exhaust side succumbs to corrosion, rust, or soot and carbon buildup (which hampers its operation) or the actuator fails, it’s typically not a matter of if a VGT turbo will fail, but when. In direct contrast, most older, mechanical diesel engines are equipped with a tried and true fixed geometry turbocharger, with no movable vanes or electronics required to operate them.


Emissions Equipment: The Biggest Killer Of Modern Diesel Engines

Perhaps the biggest reason mechanical engines are more reliable than electronically controlled versions lies in the emissions equation. The emissions-curbing aftertreatment systems nearly all modern diesels are saddled with is something the mechanical engines of old never had to face. Exhaust Gas Recirculation (EGR), Diesel Particulate Filter (DPF), Diesel Oxidation Catalyst (DOC), and Selective Catalytic Reduction (SCR) components are part of a myriad of complex systems used to reduce nitrogen oxide and particulate matter tailpipe emissions—and every single one of them brings a host of reliability issues with it. Sensors failure regularly, DPF’s plug up, EGR valves stick and EGR coolers rupture, SCR system doser modules quit working, and on and on. And beyond the system failures, none of them do the engine itself any favors (imagine an engine ingesting its own soot and frequently experiencing high exhaust gas temperature).


An Unkillable Example: The Mechanical 6BT Cummins

To paint the perfect picture of a reliable diesel engine, look no further than the 6BT Cummins. The 5.9L (359 ci) inline-six benefits from a Bosch mechanical injection system and a Holset fixed geometry turbocharger. It’s also free of any emissions aftertreatment systems and requires no electronics to run. It built a million-mile reputation in the light and medium-duty truck world and was not only known as a road-going workhorse but an indestructible one in the construction, power generation, agriculture, and marine industries, too. Downtime is extremely rare with this engine, and when it is in need of repairs high parts availability makes for a quick and affordable fix. After more than a three-decade track-record, it’s safe to say that the 6BT Cummins has become the poster child for the ultimate, durable diesel engine.

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