Ultra-low sulfur diesel (ULSD), limited to 15 parts per million of sulfur, became the standard for highway diesel in the United States in 2006 and later for off-road, locomotive and marine fuel. The refining process that removes sulfur also removes some of the natural compounds that gave diesel its lubricating ability. To compensate, ASTM D975 added a lubricity requirement in 2005: a maximum wear scar of 520 microns in the high-frequency reciprocating rig (HFRR) test, and fuel suppliers add lubricity additives to meet it. (Fuel Oil News: Taking the mystery out of lubricity)
For most engines, on-spec ULSD works as intended. Owners of older engines with fuel-lubricated injection pumps pay closer attention, because those pumps were designed for fuel with more natural lubricity.
Why lubricity matters to an injection pump
Many injection pumps are lubricated and cooled by the fuel passing through them. Rotary pumps such as the Stanadyne DB2, Bosch VE and Lucas CAV DPA are classic examples, and modern common-rail pumps depend on fuel lubrication too. A thin film of fuel separates moving parts. If the fuel’s lubricity is poor, that film breaks down, and parts wear faster.
What the lubricity specification means
The HFRR test rubs a steel ball against a steel disc in the fuel and measures the resulting wear scar. A smaller scar means better lubricity. ASTM D975 caps the scar at 520 microns. European diesel specifications use a stricter 460-micron limit, which is often cited in discussions of why some pumps have struggled more in North America.
The limit is a maximum, not a typical value, and actual fuel lubricity varies by supplier and batch.
Older pumps and the fuel transition
When lower-sulfur fuels were introduced, some owners of older pumps saw seal leaks and wear issues. Lower-sulfur, lower-aromatic fuels can affect some seal materials and offer less natural lubricity. Many older pumps have since been resealed with materials suited to current fuel, but a pump that’s been sitting on original seals may still develop leaks. See diesel injection pump leaking fuel.
How owners protect older pumps
Buy fuel from reputable, high-volume suppliers. On-spec fuel meets the lubricity limit.
Consider a lubricity additive if your engine maker permits it. Choose a product designed for diesel lubricity and follow the dose on the label.
Be cautious with home remedies. Adding engine oil, two-stroke oil or other fluids to diesel is widely discussed among owners of older engines. These aren’t engine-maker-approved fuel treatments, and on engines with aftertreatment they can harm emissions systems.
Consider small biodiesel blends. Biodiesel improves lubricity even at low blend levels, and B5 or less is allowed in conventional diesel under D975. See biodiesel and diesel fuel injection.
Keep fuel clean and dry. Water and dirt cause far more pump damage than lubricity on on-spec fuel.
Signs of lubricity-related wear
Lubricity-related wear develops gradually: harder starting when hot, power fade, leaks and eventually poor governing or delivery. Those symptoms also come from age, water and dirt, so a pump that shows them should be tested rather than blamed on fuel.
Special cases
- Jet fuels and kerosene, used in some military and multifuel applications, generally have lower lubricity than diesel. See military diesel fuel injection.
- Antique tractors with original pumps that have sat for years may need resealing and rebuilding before they’re ready for modern fuel. See antique tractor injection pump rebuild.
Lubricity questions
Does every older diesel need a lubricity additive?
Not necessarily. On-spec ULSD meets the lubricity limit. Some owners of older fuel-lubricated pumps add a lubricity product as extra protection, where the engine maker allows it.
Did ULSD ruin older pumps?
For most pumps, no. Problems tended to show up as seal leaks in pumps with older seal materials, or as accelerated wear in pumps that were already worn. Water and dirt remain much bigger threats.
Is lubricity a concern for modern common-rail engines too?
Yes. Modern high-pressure pumps also rely on fuel lubrication, which is one reason fuel quality and water control matter so much on newer engines.
Further technical reference: Cummins: diesel fuel storage and transfer. Use the exact engine/injector/pump service information for applicable procedures and limits.
Have an older pump checked
Diesel Control Technicians tests, reseals, rebuilds and calibrates older mechanical injection pumps for trucks, tractors, military vehicles and industrial engines. Send the pump tag details and describe the symptoms, and we’ll advise how to ship it; testing will show what the pump needs.
Ask about pump or fuel-system service: Request a mail-in component review with the pump tag, engine application and diagnostic findings. Confirm supported testing or repair, intake terms and current timing.