Finding the Acid Number in Diesel Fuel
An acid number test measures how much acidic byproduct has built up in diesel fuel as it oxidizes over time, and it’s one of the few tests aimed specifically at fuel that’s been sitting rather than fuel that just arrived. Stored fuel doesn’t always look or smell different as it ages, which is exactly why this test exists. This page covers what a rising acid number does to your equipment, how to spot the risk yourself, and how Fleet Core tests and corrects it.
Premature Failure of Fuel Pumps, Fuel Lines, and Injectors Can Indicate High Acid Diesel Fuel
Diesel fuel starts out with very little acidity, but oxidation from heat, air exposure, and time gradually raises it. That rising acid number, formally called total acid number or TAN, signals fuel that’s aging in place rather than being contaminated from an outside source.
Ag equipment is a common place this shows up. A tank that sits largely full but unused between planting and harvest ages for months at a stretch, and warm summer storage speeds up the oxidation that drives acid number higher. Biodiesel blends raise the stakes further, since biodiesel oxidizes faster than pure petroleum diesel, and a B20 blend stored through a full summer before use can show real degradation by the time it’s finally burned.
The damage from elevated acid number shows up gradually rather than all at once. Acidic byproducts corrode fuel pumps, injectors, and tank internals from the inside, and gum and varnish deposits build up that clog filters and injector tips. Equipment that’s run fine on fresh fuel for years can start showing filter and injector problems purely because the fuel sat too long, not because anything contaminated it.
How to Check Your Own Fuel for Rising Acid Number
We recommend calling Fleet Core for your complete diesel fuel test, but you can get a rough read on acid number through test kits available for sale online.
These kits are worth keeping on hand for equipment coming out of long storage, but treat the result as a screening number, not a precise one. The color-change endpoint gets harder to read as fuel darkens with oxidation, and red or green dyed off-road diesel can make that color shift even tougher to see clearly. A field kit is a good way to decide whether a sample is worth sending to a lab. It’s not precise enough to make a treatment decision on by itself, since Fleet Core’s lab result relies on a potentiometric method under ASTM D664 rather than a color change a person has to judge by eye.
Fuel filters are worth a closer look too during long storage windows. A gummy or varnish-like residue on a filter, rather than the usual dirt or water signs, points toward oxidation rather than contamination. Equipment coming out of seasonal storage that runs rough or hesitates, despite starting fine before it was parked, is worth testing before you assume it’s a battery or a starter problem instead.
Biodiesel blends deserve extra attention. If you know a tank has been holding a B20 or higher blend through a hot summer, treat that as a reason to run a field check, or send a sample out, rather than waiting for a symptom to show up.
Acid Number Ranges and What They Mean
Unlike water content or sulfur, there isn’t one universal acid number limit for finished diesel fuel the way ASTM D975 sets a hard cap for other properties. What matters is the direction a fuel is heading, not just where it sits on a given day.
There's no single legal limit for finished diesel. What matters is the direction a fuel's acid number is heading across tests.
| Acid Number Trend | What It Generally Indicates | Reference Point |
|---|---|---|
| Very low and stable | Fresh, healthy fuel with minimal oxidation | No universal limit set for finished diesel under ASTM D975 |
| Rising gradually | Fuel oxidizing in storage; worth monitoring more closely, especially biodiesel blends | A climbing trend matters more than any single reading |
| Elevated and still climbing | Real risk of corrosion and gum or varnish deposits; treatment or replacement worth considering | Often paired with visible filter or injector symptoms |
| B100 blendstock limit | Maximum acid number allowed in pure biodiesel before blending | 0.50 mg KOH/g maximum under ASTM D6751 |
Source: ASTM D6751, which sets the acid number limit for B100 biodiesel blendstock, the one hard number in an otherwise trend-based conversation for finished diesel.
The B100 figure is a useful anchor even for blended fuel, since it’s the one hard number in the acid number conversation. A blend made with biodiesel that already tested near or above that limit starts its life closer to the danger zone than fuel blended with fresh, compliant biodiesel.
What Happens After an Elevated Acid Number Is Confirmed
Once a test shows acid number rising or already elevated, the right response depends on how far it’s gone. Fuel that’s moderately elevated but still usable can often be treated with a stabilizing additive formulated to slow further oxidation, buying time before the next scheduled delivery or use.
Fuel that’s degraded too far to reverse doesn’t get a second chance from an additive. At that point, replacing the aged fuel through a fresh delivery is the more reliable fix, especially for equipment that’s about to go back into service after a long idle period. Fleets on FleetCore360 get fuel-quality checks scheduled specifically around long storage windows, like the gap between ag seasons, so this decision gets made on a data-backed trend instead of a guess about how long fuel has been sitting.
How Fleet Core Keeps the Fuel It Delivers From Aging Prematurely
The best way to avoid an acid number problem is fuel that hasn’t had the chance to oxidize before it reaches you. Fleet Core sources bulk fuel from terminals it has vetted and moves it through delivery on a schedule designed to minimize how long any batch sits before it reaches a customer’s tank.
That matters most for fleets ordering biodiesel blends, since biodiesel’s faster oxidation rate means freshness at delivery counts for more than it does with straight petroleum diesel. Fleet Core checks blended fuel before it goes out to confirm it isn’t already starting from an elevated baseline.
If you’re storing fuel for an extended period, especially through a hot summer or a long equipment idle season, that’s exactly when an acid number test earns its cost. Fleet Core’s diesel fuel testing services check acid number along with the other core diagnostics fleets rely on, with lab-confirmed results within 48 to 72 hours.
Frequently Asked Questions - Diesel Fuel Acid Number
What equipment problems does a high acid number in diesel fuel cause?
Elevated acid number corrodes fuel pumps, injectors, and tank internals over time, and it leads to gum and varnish deposits that clog filters and injector tips. These problems tend to show up gradually, often mistaken for an unrelated mechanical issue rather than a fuel quality problem.
How do you treat diesel fuel with a rising acid number?
Moderately elevated fuel can often be treated with a stabilizing additive that slows further oxidation, which buys time before the fuel is used or replaced. Fuel that’s degraded too far generally needs to be replaced outright, since additives can’t reverse damage that’s already been done.
What causes diesel fuel's acid number to increase?
Acid number rises as fuel oxidizes, a process driven by heat, air exposure, and time. Fuel sitting in storage for extended periods, especially through warm weather, oxidizes faster, and biodiesel blends oxidize faster still than pure petroleum diesel.
Does biodiesel affect acid number more than regular diesel?
Yes. Biodiesel and biodiesel blends generally oxidize and age faster than pure petroleum diesel, particularly when stored for extended periods in warm conditions. This makes acid number testing more important for fleets using B20 or higher blends stored through summer months.
How often should stored diesel fuel be tested for acid number?
Fuel stored for extended periods, especially through a full season or a hot summer, is worth testing before it goes back into service. Equipment left idle between uses, like ag or backup generator equipment, is a good candidate for a check specifically timed around that storage window.
Is there a legal limit for acid number in diesel fuel?
No single legal limit applies to finished diesel fuel the way it does for sulfur or water content. The closest hard number is ASTM D6751’s cap of 0.50 mg KOH/g for pure biodiesel blendstock, but for finished diesel, a rising trend across tests matters more than comparing a single reading to a fixed limit.