Bottom Line
Maintenance Q&A
I’ve had quite a few questions about additives for fuels and oils. While related, the two are really quite different. Let’s start with diesel fuel additives. Next issue, we’ll look at oil additives.

Paul Abelson
technical editor

Is all diesel fuel the same?
Regardless of the brand you buy, most diesel is pipeline fuel. The pump may say Mobil, BP, Shell or any other popular brand, but what you buy may have been refined by that or any other supplier. The companies refine the fuel from crude oil, treat it minimally with additives, then put it into the pipeline. When a marketer is scheduled to take diesel from the pipeline, it doesn’t matter who refined it (except for accounting purposes). At that point, No. 2 diesel (2-D) is a commodity product. Some additives are mixed with diesel at the refinery, but only to protect the pipeline, not to improve the fuel. Like refined sugar or white rice, all pipeline 2-D is the same.

It must meet minimal specifications, most of which are technical. Aromatics cannot exceed 35 percent by volume. The 90 percent distillation point cannot be greater than 640 degrees F. There are limits on flash point, viscosity, sediment and water, ash weight, sulfur content and corrosiveness.

Performance requirements are for a minimum cetane number of 40 and a cloud point relative to the minimum ambient temperatures for the area of use. All diesel, regardless of who refines or markets it, must meet these minimums when it leaves the refinery.

Are additives needed? If so, why?
Diesel changes over time. It starts to oxidize (form sludge) and promote growth of organisms. Also, diesel can be further improved, either to differentiate one brand from another or to achieve better performance in your truck. The Technology and Maintenance Council Recommended Practice RP-309A, “Preferred Fuel Specification 2-D,” expands on the minimums and adds a few more tests. Cetane, for example, should be 50, not 40. Less ash is allowed. Pour point and flash point requirements are added. There are prohibitions on bacteria and fungus. Fuel lubricity tests and results are listed, and BTU content (British Thermal Unit, the energy needed to heat one pound of water one degree F) must be at least 138,000 per gallon. Premium diesel, sold as meeting TMC preferred-grade 2-D specifications, is most often pipeline fuel treated with fuel additives. Each marketer has its own proprietary formula.

Knowing this, you can buy pipeline fuel and bring it up to Preferred Grade specifications with additives. Just make sure you use the right ones.

What do additives do?
Fuel additives can perform several important tasks, even if they don’t bring fuel to premium level. Depending on what chemicals they contain, additives can have detergent properties to help keep injectors, rings and piston crowns free of soot buildup. They can improve lubricity, minimizing wear throughout the fuel system. They help manage water, which enters fuel through condensation from the air inside fuel tanks.

How do additives control water?
Some additives, called de-emulsifiers, condition diesel so it cannot suspend water. Any water in the system will precipitate from the fuel to the low point, either in the tank or in the fuel line. When a de-emulsifier is used with a fuel-water separator (Raycor, Webb and Davco make them, just to name a few), water can never make it into the engine.

Other additives, called emulsifiers, break the water droplets into tiny particles so fine they can be carried through to the engine, where they turn to steam in the combustion process. An additive can have either de-emulsifiers or emulsifiers, but not both.

Do additives improve an engine’s performance?
By keeping injectors clean, additives maintain the designed performance of the engine. If injectors have become clogged enough to affect spray pattern, additives will clean the injectors to restore the original pattern, thus restoring performance to like-new levels. They do not raise performance above what the engine maker designed.

How do additives help in winter?
Additives can raise the cetane index of a fuel, helping it to combust more easily when cold. The most common use of additives is during winter, when cold weather promotes wax precipitation in the fuel. The paraffin molecules try to cling together to form a wax matrix. When the matrix is big enough and strong enough, it traps the balance of liquid fuel inside the matrix, gelling the fuel.

Anti-gel additives attach to the paraffin molecules, preventing them from clinging to each other. It would take far too much additive to keep all the wax in diesel from gelling, so there are limits on how low a temperature these additives will still work.

Read the labels on each container, to find out what properties each has and what it’s designed to do. Some additives are for winter use and have anti-gel chemicals. Others, made for year-round treatment, may not have any anti-gel.

How do additives stop organic growth?
A few have biocides to go after fungus and bacteria. These organisms enter fuel through the air and find a home where the water (source of oxygen) is in contact with the fuel (source of nourishment). Regular additives will minimize organic growth, but will not destroy what is there. Biocides are toxic chemicals that get rid of biological growth. Penray, one of the suppliers, advises that you read biocide labels thoroughly and follow all precautions when using them.

Are there any precautions when using fuel additives?
Always follow directions when using any additives. Some must go in first, so fuel entering the tank will mix them. Others will automatically distribute themselves when poured into a full tank. Many otherwise safe additives cause problems if fuel-to-additive ratios are doubled or tripled.

TMC RP-312, “Qualifying Questions to Minimize the Potential for Negative Side Effects from an Aftermarket Diesel Fuel Additive Package,” recognizes that not all additives are alike in strength or effect, and some may have chemicals that could attack rubber or elastomer parts. With the major brands (Howes, FPPF, Power Service and others) that have proven themselves over time, that is not a concern.

Grade 8 bolts
In our March edition, “Maintenance Q&A” reviewed torque, torque wrenches and fasteners. In it, I wrote, “use the highest grade fastener available. Even if the manual calls for grade 5, a grade 8 will provide a greater tolerance against over-tightening.” I went on to say “All I keep for automotive work are grade 8 bolts.”

A number of you took me to task for those comments, so let me explain my thinking. I have a relatively small toolbox, so there is not the room for a complete assortment of sizes and fasteners in all grades. In all but a few cases, the higher grade will do everything asked of the lower grade fastener, but not the other way around.

Grade 8s have greater tensile strength than grade 5. Most grade 8 bolts are through-hardened, making them stronger, but slightly more brittle than case-hardened grade 5s. The only time this may present a problem is in high-temperature applications, such as exhaust manifold or turbocharger mounting bolts. That is because the high temperature cycles slightly heat-treat the fasteners, making them more susceptible to shock loads.

Since grade 8 fasteners can take greater torque loads than grade 5, make sure you use a torque wrench, and use only the torque listed for the originally recommended fastener. If you over-torque, you won’t damage the fastener, but you could damage the parts you are attaching or fastening.

A few of you also admonished me, and rightly so, for not mentioning matching grades of nuts to the bolts. A grade 5 nut will not have the strength to apply a full clamping load on a grade 8 bolt. And a grade 8 nut could be too hard for a grade 5 bolt. It could distort the bolt’s threads or apply too great a stretch to the bolt. But if a grade 8 is used in place of a 5, you can get away with mixing only if you follow torque specification.

Do you have a maintenance question?
You can write to Paul Abelson, technical editor, in care of Land Line Magazine, PO Box 1000, 1 NW OOIDA Drive, Grain Valley, MO 64029; or you can fax information to (630) 983-7678; or e-mail your question to Please mark your message Attention Maintenance Q&A. Although we won’t be able to publish an answer to all questions inLand Line, we will answer as many as possible.