Water in Diesel Fuel: Causes, Damage & Removal
Where tank water comes from, the three forms it takes, what it does to common-rail injectors, and how to test for it, drain it and keep it out.
Water is the most common contaminant in diesel, and some is in every tank. It gets in through condensation as the tank breathes, rain at loose caps and vents, and fuel that arrives wet. Dissolved water is invisible and mostly harmless. Free water is the problem: at 8.34 lb/gal it sinks below diesel (about 7.1) and collects at the tank bottom.
ASTM D975 caps water and sediment at 0.05% by volume, but fuel is also required to be visibly free of undissolved water. In a modern common-rail engine, water corrodes injectors, wears pumps, freezes in filters and feeds microbial growth. Drain it, filter it, and polish the fuel when it's already hazy.
Where water in diesel fuel comes from
The industry's own storage guide (CRC Report 667) puts it plainly: water is the most common contaminant in fuel systems, and it's common to find some in any storage system, however good the housekeeping. It arrives four ways.
Condensation from tank breathing
As fuel is drawn out and temperatures swing, the tank pulls in outside air through its vent. When that humid air cools to its dew point, the moisture condenses into water inside the tank. It's worse in humid regions and in tanks kept half full.
Vents, fill caps and fill wells
Rain gets in through open fill pipes, manways, uncapped vents and cracked vent caps. Fill wells that don't sit slightly above grade let runoff in.
Fuel that arrives wet
Fuel picks up water in the distribution system. Warm fuel holds more dissolved water, and when it lands in a cooler tank the excess drops out. A delivery also stirs up whatever water and sediment is already on the tank floor.
Overnight cooling
Run a machine in the heat of the day and it draws warm, humid air into a half-empty tank. Overnight, that moisture condenses and drops into the fuel. That's why John Deere's manuals say to fill the tank at the end of each day.
The three forms of water in diesel
Water moves between these forms as the fuel heats, cools and gets pumped. Caterpillar describes them as moisture in solution, free and dispersed moisture, and free water settled at the bottom of the tank.
| Form | What you see | How it behaves | How it comes out |
|---|---|---|---|
| Dissolved | Nothing. The fuel looks clear. | Held in solution up to a saturation limit that rises with temperature. Usually harmless below that limit. | Lab test only (Karl Fischer). Comes out on its own when the fuel cools. |
| Emulsified | Haze or cloudiness | Fine droplets suspended in the fuel. Pumping mixes settled water back in as droplets, along with dirt and rust from the tank bottom. | Settling time, coalescing or water-absorbing filters, polishing |
| Free | A separate layer at the bottom of a jar or tank | Heavier than diesel, so it sinks to the low point. Where corrosion and microbes start. | Draining the sump or separator, pumping off the tank bottom |
Cooling turns dissolved water into free water. Caterpillar's example: a fuel holding 100 ppm of dissolved water at 65°F can hold only about 30 ppm at 40°F, so 70 ppm comes out of solution as fine droplets that cloud the fuel and slowly settle. ASTM D975 notes the same thing: warm fuel saturated with water releases free water as it cools, or ice below 32°F.
Why biodiesel blends and ULSD make water harder to manage
- Biodiesel dissolves far more water. Its esters are polar, so B20 and higher blends hold much more dissolved water than straight petroleum diesel. Donaldson puts saturation anywhere from 50 to 1,800 ppm depending on temperature and biodiesel content. Wet biodiesel can also break down chemically. More in biodiesel vs diesel.
- ULSD holds water in tighter emulsions. Ultra-low sulfur diesel carries additives that act as surfactants. They keep water dispersed in smaller droplets and make traditional coalescing water separators much less effective, especially at high flow rates.
- Fuel chemistry matters. ASTM notes that a highly aromatic fuel can hold more dissolved water than a highly paraffinic one, and both can still meet D975.
- Renewable diesel dissolves very little. R99 is all hydrocarbon, and Neste reports it takes up water about like fossil diesel, with no extra handling needed. Free water still collects and still needs draining. See R99 renewable diesel.
How much water in diesel is too much?
The spec number is 0.05% by volume, water and sediment combined, measured by centrifuge (ASTM D2709) for #1 and #2 diesel under ASTM D975. The same standard has a workmanship clause: diesel "shall be visually free of undissolved water, sediment, and suspended matter," and fuel is expected to come in well under the 0.05% figure to pass it.
| Tank volume | 0.05% of it | What that means |
|---|---|---|
| 100 gal | ≈ 6.4 fl oz | Less than a cup of water and sediment combined |
| 1,000 gal | 0.5 gal | A typical generator base tank |
| 10,000 gal | 5 gal | A large bulk tank |
Calculated from the D975 limit. Equipment guidance draws its own lines. Caterpillar wants fuel dispensed into machines at 500 ppm water or less, close to the spec number but counting water alone. CRC lists more than 200 ppm of water reaching the first fuel filter as a level that may cause problems. The sources don't agree on one number; treat any visible free water as too much.
CRC's guide adds a useful point: fuel fails the visual clear-and-bright check long before it reaches the 0.05% centrifuge limit. So the practical answer is simple. If you can see water in a sample or measure it at the tank bottom, it's too much. Drain it and find out how it got in.
Engine damageWhat water does to common-rail injectors and pumps
Modern high-pressure common rail is far less forgiving than older pump-and-line injection. Bosch's commercial-vehicle and off-highway systems run at 1,800 to 2,500 bar (about 26,000 to 36,000 psi), and CRC reports metal-to-metal clearances as small as two microns. Caterpillar requires 4-micron absolute secondary filtration on its common-rail and unit-injector engines, and only strongly recommends it on older pump-and-line engines.
Rusted, sticking injectors
Bosch names corrosion caused by water in the fuel as one of the most common problems with diesel injectors. Water combines with acids in the fuel to attack steel and non-ferrous parts, and the corrosion debris scores or jams micron-level clearances.
Wear in pumps and injectors
Diesel lubricates the high-pressure pump and injector internals as it passes through. Water has much lower viscosity and lubricates poorly, so wet fuel accelerates abrasive wear on the parts that make rail pressure.
Pitting under pressure
Donaldson describes water flashing to vapor on hot surfaces and vapor bubbles imploding under sudden high pressure, which pits and fatigues metal surfaces. Water forced into microscopic cracks can also cause spalling.
Ice in filters and lines
Free water freezes at 32°F. When fuel cools fast, water coming out of solution forms fine ice crystals instead of droplets, and Caterpillar notes they plug filters the same way wax does. Wax is a separate problem; see what temp diesel gels.
Water also carries trouble with it. It dissolves salts and acids, and in tank bottoms it mixes with rust and dirt that get pumped into the fuel system every time the tank is drawn hard. The wider list of contaminants, from dirt to oxidation gums, is in our fuel contamination guide.
Diesel bugMicrobial growth starts where fuel meets water
Bacteria and fungi live in the water and feed on the fuel, so they grow at the fuel–water interface on the tank bottom. They don't need much: CRC notes the organisms are 0.5 to 3 microns across, so a few spots of condensation on a tank wall are enough. Their acids corrode steel tanks, and their slime plugs filters. They also make detergent-like compounds that emulsify more water into the fuel, which feeds the cycle.
Biocides work in the water phase and are recommended where growth has been found, but CRC advises against routine use. The lasting fix is removing the water the microbes need. If slime or sludge already coats the tank, see fuel polishing vs tank cleaning.
Symptoms of water in diesel fuel
What operators notice
- Water-in-fuel warning or a full separator bowl
- Filters plugging earlier than their interval
- Hard starting, rough idle, poor performance
- Frozen fuel lines or iced filters in a cold snap
- Corrosion in the separator or filter housing
What a tank check shows
- Hazy or cloudy fuel in a sample jar
- Water or sludge in a bottom sample
- Dark mats or slime on filter elements
- A set that starts, takes load, then chokes its filters
- Fuel that has sat a year or more untested
Generators get hit hardest because their fuel sits. A set that only runs for tests can hold the same fuel for a year or more while condensation builds up underneath it. Routine checks belong in your generator maintenance plan.
TestingHow to check diesel for water
- Clear-and-bright jar test. Draw about a quart into a clear glass container and look at it against light. Good diesel is clear and bright, with no haze, droplets or layer at the bottom. ASTM D4176 is the standard version of this check. CRC recommends it weekly on dispenser samples.
- Water-finding paste. Coat the end of the gauge stick, lower it to the tank floor, and the paste changes color where it touches water. CRC recommends sticking the tank before a delivery and again 30 minutes after, and weekly on underground tanks. Check the low end of the tank, or both ends if you don't know which way it tilts. Automatic tank gauges typically don't read water below about ¾ inch.
- Bottom sample. Water sits at the bottom, so a sample from the top can look perfect while the pickup draws water. CRC recommends a monthly bottom sample for clarity and at least an annual microbial test.
- Lab test. A lab measures what you can't see: total water in ppm by Karl Fischer (ASTM D6304), water and sediment by centrifuge (ASTM D2709, which can't detect dissolved water), and microbial growth (ASTM D7687). Caterpillar recommends a full analysis of bulk storage fuel every three months.
FuelGo pulls a sample from your tank and sends it to a lab that checks it for water, sludge, bacteria and more. The results decide what the tank needs. See what our lab testing covers.
RemovalHow to get water out of diesel
- Drain the equipment. Drain the fuel/water separator. Caterpillar says to drain water separators daily, per the machine's manual. Change filters that have taken on water.
- Let the tank settle, then draw water off the low point. When CRC's guide finds high water or sediment, it calls for isolating the tank and letting it settle 1 hour per foot of fuel above the suction, up to 24 hours, before drawing water off. Dispose of the water and fuel mix according to applicable regulations.
- Filter what's left. Coalescing filters grow fine droplets into drops that fall out. Water-absorbing filters use a polymer that swells on contact and slows or stops flow, so ASTM notes they double as an alarm for free water. Expect coalescers to struggle with ULSD's tighter emulsions.
- Polish hazy fuel. When water is emulsified or microbes are present, fuel polishing circulates the fuel through filtration and water separation and back into the tank. See diesel fuel polishing.
- Clean or replace if needed. Sludge and microbial mats on the tank walls call for tank cleaning. Fuel too far gone gets pumped out and replaced. Which one you need: polishing vs tank cleaning.
How to keep water out of diesel tanks
- Keep tanks full. Less air space means less condensation. CRC, Caterpillar and John Deere all say it: top off storage tanks and fill equipment tanks at the end of the day.
- Fit desiccant breathers. A desiccant vent-breather dries the air the tank inhales. Caterpillar specifies 4-micron absolute breathers with water removal on bulk tanks.
- Seal every opening. Tight fill caps and gauge covers, vent caps that aren't cracked or missing, and fill wells slightly above grade. Diesel tanks should never share a vent line with gasoline tanks.
- Give water a place to go. Aboveground tanks need a low-point sump with a drain valve. Underground tanks are pitched so water collects at one end, but they settle over time, so check both ends.
- Drain on a schedule. CRC calls a no-water requirement impractical, but says to monitor water frequently and pump it out as soon as possible. It recommends sticking underground tanks weekly.
- Filter at the dispenser. CRC recommends a water-absorbing dispenser filter of 10 microns nominal or finer. Change it when flow drops to about 50–60% of normal.
- Check every delivery. Buy from a supplier you trust, stick the tank before and after the drop, and pause dispensing from a tank while it's being filled so stirred-up water doesn't go straight into equipment.
Water is only one reason stored fuel degrades. Oxidation, heat and time are covered in how long diesel lasts in storage.
FuelGoWater in your tank? We polish, clean and refill on site
FuelGo brings the equipment to your tank. We polish the fuel on site, clean sludge and water out of the tank itself, and pump the fuel out when it can't be saved, then refill with fresh dyed diesel, clear diesel or R99 renewable diesel on the same account. A regular maintenance plan paired with keep-full generator fuel delivery or bulk deliveries keeps the water from coming back.
Tell us about the tank and what you're seeing: get a free quote or call 800-720-4546. Dispatch runs 24/7/365, and a dispatcher replies within the hour.
Common questionsWater in diesel fuel FAQ
How much water in diesel is too much?
What are the symptoms of water in diesel fuel?
How do you get water out of a diesel tank?
Can water in diesel damage injectors?
Does diesel absorb water?
Can you run a diesel with water in the fuel?
Does water sink to the bottom of a diesel tank?
Is water in diesel worse in winter?
Sources: ASTM D975-24, Table 1 and Appendix X8 · CRC Report 667, Diesel Fuel Storage and Handling Guide · Caterpillar Machine Fluids Recommendations, SEBU6250-21 · Bosch, Common Rail Injectors: Potential Failures and Causes · Bosch, Modular common-rail system CRSN · Donaldson, How Water Damages Diesel Engines · Donaldson, Water in Diesel Fuel · NREL Biodiesel Handling and Use Guide, 6th ed. · Neste Renewable Diesel Handbook (2020) · John Deere operator's manual, fuel storage · USGS, water weight