Essential Diesel Service

Jan. 1, 2020
Ambitious shop owners are always looking for ways to fill their shop bays and increase revenues. At the same time, they must limit their risk as they are bringing in new work.

You don't have to be a diesel expert to provide diesel maintenance to your customers.

diiesel repair shop training technician training A/C training automotive aftermarket Ambitious shop owners are always looking for ways to fill their shop bays and increase revenues. At the same time, they must limit their risk as they are bringing in new work. Pitfalls abound, mostly due to a lack of product familiarity. And for the technician, the first time you lay a wrench on a car you've never worked on before can be an uncomfortable experience. For these reasons and more, we all tend to limit our work to stuff we already know about.

Of course, being able to pick and choose your work is often a luxury. If business is slow, your closely-guarded "comfort zone" is more likely to change and you may feel compelled to broaden your list of available services.

Diesel engine repair may be on your personal list of uncomfortable experiences. However, an informal analysis may reveal that there is an abundance of diesel vehicles in your area that you could be servicing. The really good news is that the average repair ticket for diesel vehicles is much higher than that for gasoline-powered cars. Diesel engine owners have paid more for their vehicle and are generally prepared to pay more for maintenance and repair. The reward potential is definitely high for this kind of work.

For those who are considering making the plunge into diesel engine service, it may make sense to limit your risk by starting out with simple maintenance. Selling maintenance service to diesel owners can help you and your technicians "get your chops up" and prepare your minds for higher dollar repairs. Beyond that, it is a great way to build your customer base if you pay attention to details and help prevent your customers from suffering huge repair bills. Read on if you want to know more about essential diesel service!

Clean Air Acts

A really quick way to ruin a diesel engine is through air leaks in the air intake system. Diesel engines breathe tremendous amounts of air, much more than a gasoline engine because they do not use a throttle plate to control engine speed and output. Unfiltered air that makes its way into a diesel's air induction system can erode turbocharger blades and cause rapid cylinder wear. A "dusted" diesel is effectively destroyed and will require replacement.
The customer may bring the vehicle to you with an air filter restriction light illuminated on the dash, but a diesel's air intake system should be inspected regardless. If equipped, check the air restriction gauge that is located downstream from the air filter. High air filter restriction causes a low pressure after the filter and will cause the plunger to move in the gauge body. The gauge is typically calibrated in terms of percentages, and a 100% reading means that filter maintenance is required.

Resist the temptation to blow out the original filter element with compressed air. This practice can result in small holes being created in the element. Be sure to clean out the filter housing and ducts as a new element is being installed, and inspect them carefully for cracks and other damage. Carefully reassemble the filter assembly, being sure to tighten all connections securely. If there is any doubt about whether the air intake system is sealed properly, try doing a final test of your work with a smoke machine.

Be sure to do a careful inspection of the air passages leading into the filter and clear any debris that may have collected there. This becomes more critical in the winter with the potential for ice and snow to accumulate in the air inlet. If the system checks OK, you can finish up by resetting the air restriction gauge before moving on to other service items.

Diesel Motor Oil

Use of the correct motor oil in your diesel engine has transitioned from important to critical. Tough emission control standards have cleaned up the diesel's exhaust, but have also created serious challenges for its motor oil. The real crunch started with the use of exhaust gas recirculation (EGR) on diesels to meet EPA 2004 NOx standards. CI-4 motor oil was specified for use in the original EGR-equipped diesels because of increased temperatures and soot loading in the oil, which also caused higher acid accumulation. Use of lube oils with less stringent specifications than CI-4 would lead to much more rapid wear in these engines.
The EPA 2007 standards set the bar even higher, as all on-road diesels were going to be equipped with diesel particulate filters (DPFs). DPFs trap diesel soot, but also accumulate ash that results from a number of classic lube oil additives. Phosphorous has been used for decades as an anti-wear agent, and sulfated ash has traditionally been used to neutralize acids that accumulate in the oil. Both of these additives (as well as sulfur) were limited in CJ-4 motor oil, which is specified for diesel engines equipped with a DPF. CJ-4 was also engineered to handle further increases in soot loads as engine makers relied more heavily on EGR to meet tighter NOx standards.

It is imperative that CJ-4 motor oil be used in all 2007 and newer on-road diesel engines that call for it. Use of something other than CJ-4 in these engines will result in rapid ash accumulation in the DPF, which will then require premature cleaning or replacement. It is important to note that CJ-4 is backwards compatible with CI-4 and CI-4 Plus lube oils, so it can be used for the earlier EGR diesels as well.

Some diesel engine manufacturers (e.g.; Volkswagen) use a motor oil specification exclusive enough that only select products will meet it. You will need to educate yourself on what motor oil will work in each particular case, and be prepared to purchase oil from the dealership if necessary. Your customer should be apprised of the higher costs associated with an oil change in these situations.

Diesel Fuel

While a service technician often has little to do with the customer's fueling practices, it is a good idea to discuss diesel fuel requirements with them to prevent big problems down the line. Diesel injection system components are expensive to the point that every measure must be taken to ensure that only clean, water-free fuel makes its way into the customer's tank.

Diesel fuel has a natural affinity for water. The fuel will absorb it from moisture in the air and (since water is denser than diesel fuel) cause it to accumulate at the bottom of the fuel tank. This water can then be picked up by the fuel pump and sent with the fuel stream into the filters. One of the system fuel filter assemblies will have a water separator built into it, where entrained water is allowed to settle and collect. Water can then be drained from the water separator, preventing it from making its way into the high-pressure side of the system and causing serious damage.

Most diesel-powered vehicles have a water-in-fuel (WIF) light on the instrument panel. On these systems, there is a sensor located in the water separator that detects when the water level has risen above a certain threshold. The sensor then prompts the WIF light to illuminate, warning the driver that the water separator needs to be drained ASAP. Some systems can be drained by simply opening a valve at the water separator, where others require that the water be removed using a vacuum extractor. Regardless of the procedure, you should continue removing water until only diesel fuel is flowing from the water separator.

It is good policy to drain your customer's water separator whenever the vehicle makes its way into your service bays. Make a note of it on the repair order, letting the customer know that you have taken care of it for the time being. However, they should also be reminded that they need to be draining the water separator at least once per month on their own to prevent serious injection system damage.

Sidebar 1 Don't wait for the WIF light!

A major mistake that many diesel vehicle owners make is waiting for the water-in-fuel (WIF) light to illuminate before taking the time to drain the water separator. The problem with this approach is that some WIF systems (even on new vehicles) have proven to be unreliable and won't turn on the light no matter how much water is in the system.

If you don't think this is a big deal, consider having $15K in fuel system repairs on a newer common-rail injection system. If the high-pressure pump and all the injectors have to be replaced, the final bill could exceed this number. What about warranty, you ask? Let's just say that even the most benevolent engine manufacturer is not about to warranty bad fuel.

You might also ask how the dealership would know for sure that water has damaged the injection system. A good example is the Cummins 6.7 liter turbodiesel, where all the technician has to do is remove one of the high-pressure fuel pipes that thread into the cylinder head. If corrosion is found on the pipe, the system has digested water and your warranty claim is null and void.

The point is this: don't wait for the WIF light to turn on. Buy your fuel from a reputable source and be diligent about draining the vehicle's water separator on a regular basis. Following these rules of thumb could save you or your customer from a major repair bill.

Fuel filter service is another critical item. There is no substitute for quality when it comes to fuel filters, so OEM products are the safest bet. Be certain to change all the system filters, as some diesels will have both a primary and a secondary fuel filter.

Don't fill the filters with fuel during installation. Instead, install them dry and use the hand primer pump or the electric fuel pump to move fuel into the filters. In many cases, this can be accomplished by cycling the key a number of times to operate the electric pump and purge air from the system. If you have trouble getting the fuel filters primed, use scan tool bi-directional controls to turn on the electric pump and run it for a minute or two. Once the air has been purged from the system, start the engine and check for leaks.

Keeping Your Cool

Using the correct coolant and changing it at required intervals is another high priority maintenance item. Prediluted coolant works well, but it may not be available for your specific application. When mixing straight coolant, use only distilled water and keep it at a 50/50 concentration. Keep in mind that mixing different types of coolants is a no-no, and use of the wrong coolant can result in cavitation damage on the water jacket side of the engine cylinders.
The Ford 6.7 liter Powerstroke diesel has specific requirements for testing and servicing its coolant. The vehicle will keep track of coolant service intervals and alert the driver when the coolant needs to be tested. A test strip kit is used to determine the coolant condition, which may then call for a supplement to be added to the coolant. This cycle may be repeated, but eventually the coolant will have to be replaced when it reaches the end of its useful life. The lesson to be learned is that Ford is making it clear that coolant condition is absolutely critical to the life and health of the engine. Taking that attitude with any diesel is definitely worth the time and effort.


Essential diesel service may seem like unglamorous labor for an experienced technician. However, this work requires attention to detail and it should not be left to a novice. Simple mistakes can lead to extremely expensive failures.

Despite the hazards, remember that the reward potential is great as you expand your customer list and prepare yourself for more complex diesel repairs. Good luck with your adventures in diesel service!

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