Diesel Engine Service Guide

The Do's & Don'ts of Diesel Engine Service & Repair

Disclaimer - automotive service and repair procedures present many dangerous and potentially life threatening situations. The following advice is nothing more than the opinions of the author. By continuing to view this content, you understand that Diesel Hub (dieselhub.com) expressly disclaims all liability for damages of any kind, including vehicle and/or personal injury.

DO wear PPE. Personal protective equipment (PPE) is necessary in any professional automotive service/repair environment. This includes, but is not limited to, chemical resistant gloves and protective eye wear (safety glasses). You are likely to be exposed to various chemicals and debris that may harm you. Protect yourself accordingly.

DO use synthetic oil when conventional is spec'd by the OEM. DON'T use conventional oil when the OEM spec calls for synthetic oil. Synthetic oil is far superior with regard to every chemical property than conventional petroleum based oils/fluids. Using an oil/fluid that exceeds manufacturer specifications may have significant benefits in the long run. However, using an oil/fluid that does not meet manufacturer specifications may have [expensive] repercussions.

DON'T use silicon or any liquid "gasket maker" in place of a required gasket. o-ring, or seal. This is a careless and sloppy method of sealing two surfaces. There are obvious circumstances in which silicon is commonly used in addition to a gasket (rear differential gasket, thermostat housing gasket, etc) and called for per OEM recommendations. Then there's the lazy man's answer to not purchasing the correct seal(s) to complete a task properly and thoroughly.

DON'T reuse gaskets, o-rings, or seals. There are few exceptions to this rule, one of which is certain transmission pan gaskets. This does not mean that all transmissions have reusable gaskets and you should refer to the OEM recommendations through a reputable service manual. If any gasket is reusable, it must be thoroughly inspected prior to reassembly and replaced as necessary if there is any damage, cracking, or tears. If in doubt, replace it. In the grand scheme of things, gaskets and o-rings are relatively inexpensive consumables in the service and repair industry.

DO follow the correct maintenance schedule for your vehicle. Each vehicle manufacturer provides two maintenance schedules in the owners manual or diesel supplement for their application. Countless third party service manuals incorrectly specify maintenance intervals. Furthermore, recall that there is a normal and an alternative "severe" or "heavy duty" maintenance schedule, which defines more stringent intervals for vehicles that are subjected to certain operating conditions. If you meet any of the criteria for the stricter maintenance plan, you must adhere to its standards; they are provided for good reason. Also note that most service intervals are given in miles OR months (and in rarer instances, hours of operation); do not postpone any procedure simply because you don't meet the mileage requirement, time is of equal importance. The following service schedules have been carefully compiled for each model year directly from the OEM specification:

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DO use oil, fuel, and transmission filters that meet or exceed OEM quality. In some instances, but not all, there are cheap alternatives to factory replacement parts. We all know that dealer markup on OEM branded products - Motorcraft, ACDelco, Fleetguard, etc - range from high to ridiculous. Seek a distributor or retailer that has OEM parts at a reasonable cost; a dealership is unlikely to match 3rd party prices. Inexpensive filters from an unknown manufacturer are likely to exhibit quality that matches their price; cheap. This is not to say there are not good alternatives on the market; Puralotor, Napa Gold filters, Donaldson, Baldwin all come highly recommended for most applications. Cheaper is not always better and certain brands should be avoided. One manufacturer, whom we probably shouldn't name, should be avoided specifically. Let's just say their oil filters are commonly referred to as the "orange can of death" - interpret accordingly.

DON'T forget to rotate your tires. While rotating tires can be an cumbersome task for a DIYer or shadetree mechanic without a vehicle lift, it is extremely important on 3/4 and 1 ton trucks. Recall that the weight of these vehicles is substantial and, especially for those that tow, tires can easily wear at different rates. Rotating the tires is imperative to maintain safe and confident handling characteristics on heavy duty trucks. Failure to rotate tires periodically ultimately results in uneven wear and possibly compromised handling, not to mention your tires are likely to last longer if you keep them rotated.

DO service your battery. Unsealed batteries are serviceable items are will last longer/perform better if the electrolyte level is maintained. Batteries are to be filled with distilled water only. Allowing the electrolyte level to drop too low may result in premature failure. There is a rumor (or lie, depending on how you look at it) in the automotive repair industry that a battery is a non-serviceable item. Unsealed lead acid batteries are serviceable and the electrolyte level should be checked and adjusted with every oil change. On a diesel engine, which draws a significant amount of current during cranking and glow plug pre-heat cycles, battery life expectancy is around 3 - 5 years. Don't expect to experience any significantly longer service life out of an AGM or gel battery; 3 - 5 years is fairly standard.

DON'T ignore brake noise. Squeaking brakes are annoying, but they could also indicate that it's time for a brake job. If your brakes are making noise, pull the tires off and give them a quick glance. If any brake pad has less than 1/8" of material left, they should be replaced. The more the pads wear, the more likely you will warp a brake rotor.

DON'T ignore a check engine light (CEL, also called a malfunction indicator lamp or MIL) just because the engine seems to be running normally. The CEL may indicate a minor malfunction of a non-critical vehicle system, but you won't know for sure until the DTC is pulled. An intermittent or non-essential concern can become a major problem further down the road; address problems promptly.

DON'T attempt to dilute DEF. Diesel exhaust fluid is a requirement of modern emissions equipment and thus there are fail safes in place to prevent engines from operating properly if the DEF quality is compromised. If you own a truck that requires DEF, the burden is on you to maintain the fluid level using a proper exhaust fluid. This is one battle you will never win.

DON'T run your truck out of diesel. Stick to the 1/4 tank rule; never let your fuel gauge fall below a quarter tank. In theory, you'll never run out this way and you'll always have a minimum of 1/4 tank of fuel in the event of an emergency. Starving a fuel pump (especially a high pressure injection pump) or injectors of fuel can quickly become an expensive mistake. Diesel fuel lubricates these items are they can wear extremely quickly when they run dry.

DO use anti-seize when called for in service procedures. Anti-seize or "never seize" is commonly used in two situations; 1) steel bolts threaded into aluminum castings. Galvanic corrosion occurs when two dissimilar metals are in contact. In other words, the threads will corrode more rapidly because the bolt and threads are different metallic materials. Anti-seize helps prevent this corrosion and, since aluminum threads strip easily, should be used accordingly. 2) exhaust components including exhaust manifold bolts, up-pipe bolts, up-pipe collector bolts, etc should be lightly coated in anti-seize. Exhaust system hardware is subjected to hundreds, if not thousands of heating-cooling cycles before they are removed. The result is an extremely tight, often seized bolt. Lightly coating these bolt and nut threads in a high temp anti-seize lubricant helps keep them from seizing over time, and just maybe you won't snap a head off when you go to remove one.

DO start bolts, nuts, and threaded components by hand. Never attempt to start a piece of hardware with a socket, wrench, or tool in general. The risk for cross-threading increases exponentially when an item is not threaded by hand first. When using a wrench/socket, it is often impossible to identify whether or not the bolt/nut is being cross-threaded. Threading the bolt by hand first ensures that it is being assembled properly, after which point a tool can be used with confidence.

DON'T ignore torque specs. OK, we admit; there are circumstances when there is absolutely no possible way to fit a torque wrench without pulling the entire engine. And there's those instances where pulling the engine wouldn't even help reach a particular piece of hardware. However, if it can be torqued to spec, it needs to be torqued to specs. Nut/bolt torque specs are provided for specific reasons; follow them whenever physically possible.

DO use jack stands. DON'T work under a hydraulic jack. In any industry, safety has one common attribute; redundancy. A jack stand is a redundancy, as should a jack fail the vehicle does not come crashing down on you. Jack stands are inexpensive insurance against the possibility of an otherwise life threatening event. If we were to work under a vehicle, for example, we would raise the axle by the rear differential and place jack stands beneath the left and right axle tube, then lower the axle onto the jack stands. Next, we would put a small amount of jack pressure on the differential, but not so much that the axle tubes lift off of the jack stands. Finally, we'd stack wooden blocks beneath the frame rails of the rear of the vehicle. Redundancy - a hydraulic jack, two jack stands, and a series of wooden blocks would have to fail/slide before the vehicle came crashing down on us. You only have one life, be smart before you crawl beneath a suspended vehicle.

DON'T jump start your diesel. One simple exception; mechanically controlled diesels can safely be jump started. Any modern and/or computer controlled engine should not be jump started unless it is an emergency situation. The complex series of electronic components in your engine is highly sensitive to surges and/or drops in voltage. Attempting to start a truck with low/dead batteries is bad enough for these components, and the strain of jump starting can quickly fry the circuitry in expensive PCMs, drivers, and control modules. Recharge or replace dead batteries, unless it's an absolute emergency.

DON'T use starting fluid (ether) to start a diesel. If a diesel only starts on starting fluid (you're extremely lucky it's still running if you needed to use ether to get it going) there is a serious problem. Diesel engines are fairly simply if you break it down to basics - fuel, air, compression. However, you also need to consider timing and starting aids (glow plugs or grid heater). Diesel engines utilize a relatively high compression ratio because they are compression ignition engines - there is no spark to assist in ignition. There's two important reasons why NO starting fluid, REGARDLESS or the product manufacturer's recommendations should ever be used in a diesel engine. 1) high compression = detonation. There's no spark in a diesel and combustion is not to be initiated until fuel is injected near TDC of the compression stroke. A diesel does not inhale an air-fuel mixture into the combustion chamber; it compresses air and injects fuel at a specific time. If there is an air-fuel mixture in the cylinder prior to the time that combustion should occur, the mixture may ignite long before an injection event is set to occur. The resulting "ping" or denotation event can cause catastrophic failure. 2) do you know why they're called glow plugs? Because they get so hot they glow red. Literally. When a volatile fuel like ether comes in contact with a heated glow plug, combustion is highly likely to occur. Once again, we have a timing issue - where is the piston when the mixture ignites?

DO use thread locker as specified in service procedures. Certain nuts and bolts require the use of thread locker, which keeps them from backing off do to vibration. Systems that generally require a thread locker include drivetrain components and brake calipers/brackets. If a bolt is removed and it has remnants of a thread locker on it, apply a light coat before reinstallation.

DO clean electrical connectors. Anytime a sensor is replaced or an electrical connector is removed, it should be cleaned with a light solvent. There are several aerosol cleaners on the market that will remove dirt, grease, and grime from electrical connectors and are completely safe to use on plastics. Do not use brake cleaner or heavier solvents as it will compromise plastic electrical connectors.

DO use dielectric grease. Electrical connectors should be packed lightly with dielectric grease, which will keep dirt, oil, and water from working into the connector.

DON'T use multi-vehicle ATF (automatic transmission fluid). ATF is a complex oil that contains, amongst other compounds, friction modifiers, anti-wear additives, anti-foam additives, and oxidation inhibitors to produce specific chemical properties. Multi-vehicle ATFs meet various manufacturer oil specifications, but may fall short in comparison to OEM grade fluids as they aren't specific to any one application. There's a reason that Ford's MERCON V transmission fluid differs from GM's DEXRON VI and that they are not interchangeable - so why use a fluid that claims it covers both manufacturer's specifications? The logic is missing.

DO check and maintain SCA/DCA engine coolant concentrations. Diesel engines typically require an engine coolant that protects against cavitation, which can slowly eat away the wall of a cylinder. Some engines are more susceptible than others. Certain coolants come pre-charged with an SCA/DCA and are maintenance free, while others will require periodic testing and maintenance.

DO replace glow plugs in sets, not individually. Would you replace a single spark plug in a gasoline engine, or would you replace the spark plug for every cylinder? Glow plug tips can swell and require tedious extraction procedures. If a single glow plug or series of glow plugs is detected to be faulty, replace them all. It's better to have a all glow plugs working "great" than 1 glow plug working great while the others perform "adequately". On a separate note, consider replacing them as a general service item before they fail.