A cold filter plugging point test shows a fuel buyer or quality control lead how a diesel or heating fuel is likely to behave as temperatures drop. When a distillate cools, paraffin wax begins to come out of solution and form crystals. Eventually those crystals become large and numerous enough to restrict flow through a fuel filter. The cold filter plugging point (CFPP) is the laboratory measure of that point, reported in degrees Celsius.
CFPP sits alongside cloud point and pour point as one of the core cold flow properties checked during middle distillate testing. It appears in product specifications in many markets, is referenced in supply contracts, and helps show whether a cargo, tank, or blend suits the conditions where it will be stored and used. The sections below cover how the test is run, how it relates to other low-temperature measurements, and what to keep in mind when reading a result.
What Cold Filter Plugging Point Measures
ASTM D6371, Standard Test Method for Cold Filter Plugging Point of Diesel and Heating Fuels, describes CFPP as a way to estimate the lowest temperature at which a fuel will give trouble-free flow in certain fuel systems. In practical terms, it is the highest temperature at which a set volume of fuel, cooled under controlled conditions, fails to pass through a standardized wire mesh filter within 60 seconds.
Because the result is based on filterability, it describes a later stage of wax formation than the first visible haze in the fuel. For diesel, CFPP results are usually close to the temperature at which filter problems appear in service, with a few exceptions covered later in this article.
How the ASTM D6371 Procedure Works
The test can be run on manual or automated apparatus, and the equivalent methods IP 309 and EN 116 follow the same principle. The core steps are consistent across instruments:
- A specimen of the fuel is cooled at a specified rate in a cooling bath.
- Starting at a set test temperature, the specimen is drawn by vacuum into a pipet through a standardized 45 µm wire mesh filter.
- The fuel then drains back into the test jar, and the draw is repeated at every 1 °C drop in temperature.
- Testing ends when separated wax slows the flow enough that filling the pipet to 20 mL takes longer than 60 seconds, or when the fuel fails to return completely to the test jar before it cools by a further 1 °C.
- The temperature at which that last filtration began is recorded as the CFPP.
The method also describes comparing a sample tested as received with the same sample after heat treatment at 45 °C for 30 minutes. The difference between the two results can be used to investigate complaints of poor performance at low temperatures. ASTM D6371 is the method AmSpec uses to determine CFPP.
How CFPP Compares With Cloud Point and Pour Point
Cold flow performance is usually described with three related measurements, each tied to a different stage of wax formation:
- Cloud point: the temperature at which wax first becomes visible as a haze in a transparent fuel, measured by ASTM D2500 or an automated alternative such as ASTM D5771, D5772, or D5773. It marks the start of the temperature range where cold weather issues may begin.
- Cold filter plugging point: the temperature at which wax crystals restrict flow through a standardized filter, as described above.
- Pour point: the lowest temperature at which the fuel still pours, measured by ASTM D97. It is typically reached several degrees below CFPP.
In many untreated diesel fuels, cloud point and CFPP sit within a few degrees of each other. Cold flow improver additives can widen that gap by changing how wax crystals grow and cluster, which is why a treated fuel may show a CFPP well below its cloud point. Reporting all three values together gives a fuller picture of a fuel’s operating range than any single number.
Fuels and Applications Where CFPP Testing Comes Up
ASTM D6371 covers diesel and domestic heating fuels, including distillates treated with flow-improving additives. CFPP results are most often requested in these situations:
- Diesel supplied into markets with seasonal or climate-based specifications, such as the EN 590 grades and classes defined by CFPP limits.
- Heating oil and other fuel oil testing programs where product is held in outdoor tanks or lines through the cold months.
- Biodiesel and renewable blends, where feedstock and blend ratio influence cold flow behavior.
- Additive programs that need to confirm a cold flow improver is performing at the chosen treat rate.
Blends containing FAME deserve particular attention. Saturated fatty acid methyl esters, such as those from animal fats or palm oil, tend to crystallize at higher temperatures than the hydrocarbon portion of the fuel, so both the biodiesel content and its source can shift the CFPP of a finished blend. AmSpec’s Curiosity Corner explainer on FAME covers how these esters enter fuel streams and how their concentration is measured.
Reading a CFPP Result in Context
A CFPP value is an estimate tied to a standardized filter and cooling profile, so it is most useful when read alongside how the fuel will be stored, handled, and used. ASTM D6371 notes two situations where the result may differ from field performance:
- Fuel systems with a filter, such as a paper filter, installed in a location exposed to the weather.
- Fuels whose filter plugging temperature falls more than 12 °C below the cloud point.
Domestic heating installations are generally less demanding than vehicle fuel systems and often operate satisfactorily below the temperature a CFPP result indicates.
Sample handling also affects how much weight a result can carry. Wax can settle in tank bottoms and cold lines, so the laboratory value only describes the parcel if the sample represents it. Representative sampling during petroleum product inspection, with clear records of sample temperature and handling, supports a result that reflects the fuel being bought or sold.
Where cold flow improvers are in use, follow the additive supplier’s treat rate guidance, since overtreatment can work against the intended effect. A CFPP test on the treated fuel shows whether the additive is performing as expected.
Frequently Asked Questions About CFPP
What units is CFPP reported in?
CFPP is reported in degrees Celsius. Because the procedure tests the fuel at each 1 °C interval, results are given in whole degrees.
Does a lower CFPP mean a fuel will work in colder weather?
A lower CFPP means the fuel passed through the test filter at a lower temperature, which generally points to better low-temperature operability. Whether that is enough for a given application depends on the equipment, filter placement, storage conditions, and the specification or contract the fuel must meet.
Can CFPP be lowered after a fuel is produced?
Cold flow improver additives are commonly used to lower CFPP by modifying how wax crystals form and cluster. Blending with a lighter distillate, such as kerosene or No. 1 diesel, can also lower it. In either case, retesting the treated or blended fuel confirms the new value.
Building CFPP Into a Cold Weather Testing Plan
CFPP speaks directly to how a fuel will filter in cold conditions, which makes it a useful check before product moves into winter storage or service. Reading it together with cloud point, pour point, and the fuel’s blend composition gives buyers and QC teams a well-rounded view of how a product will handle the season ahead.
For biodiesel and blended fuels, AmSpec’s biodiesel testing covers ASTM D6751, BQ-9000, and EN 14214 work, with guidance on cold properties. To discuss CFPP or other cold flow testing for a specific product, contact the AmSpec team.