In the Shop: Dealing With Diesel

January 26, 2013 05:47 AM
In the Shop: Dealing With Diesel

It’s not your imagination—modern diesel engines burning ultra-low-sulfur diesel (ULSD) fuel "gel" more easily on cold winter days than diesel engines built 20 or 30 years ago.

Gelling occurs when wax, a necessary component of No. 2 diesel fuel, crystallizes at low  temperatures. The "cloud point" of a particular diesel fuel is the temperature at which crystals of wax begin to form. The "gel point" is the temperature at which enough wax crystals develop to transform the fuel from liquid to semi-solid. The gel point for No. 2 diesel varies but ranges between 10°F and 20°F.

Most diesel fuel now sold in the U.S. is ULSD to meet the needs of Environmental Protection  Agency–mandated Tier III and newer diesel engines. The problem with these engines is that their precise, computer-controlled fuel injectors require fuel filtered to as little as 2 microns. Wax crystals in chilled diesel fuel measure 50 to 250 microns. As soon as temperatures drop to the cloud point in a 2-micron fuel filter, wax crystals collect and restrict or stop fuel flow.

The cloud point of diesel fuel varies among refineries. A refinery in Texas might produce No. 2 diesel fuel that clouds at 20°F, while No. 2 from a refinery in Louisiana might cloud at 10°F. Cloud and gel points also vary across the country because retailers use different formulations to create "winter-grade" No. 2 diesel fuel, which they generally begin selling in October. The cloud and gel points of winter-grade No. 2 diesel vary depending on the base fuel and anti-gel additives a retailer uses, but clouding is generally prevented down to 10°F.

The cloud and gel point can be lowered well below 0°F by blending No. 1 diesel fuel, kerosene or aftermarket anti-gel fuel additives with winter-grade No. 2 diesel. The cloud point for straight No. 1 diesel can be as low as –40°F, but using straight No. 1 diesel is not recommended because of the higher cost per gallon and lower lubricity.

Pesky droplets. A second cold-weather problem associated with modern diesel engines is that ULSD fuel is more hygroscopic than traditional diesel fuel. That means it attracts water. Most of the water is captured by filters, but microscopic droplets might remain suspended in the fuel. If those droplets freeze when fuel temperatures fall below 32°F, they can accumulate and plug fuel system fittings or filters long before the fuel is chilled to its cloud point. The best preventive measure is to switch to wintergrade diesel fuel before the first frost and begin using antigel fuel additives when temperatures fall below 20°F.

Do it yourself. Home-brewing winter-grade diesel fuel by adding No. 1 diesel fuel or kerosene to winter-grade No. 2 fuel is an option, but it carries expensive risks. ULSD No. 1 diesel fuel and ULSD kerosene have less lubricity than old-school No. 1 diesel fuel or kerosene, increasing the possibility of problems with Tier III diesel engines, which are extremely finicky about lubricity. Experts recommend using a special aftermarket diesel fuel additive to lower the cloud and gel points without decreasing the lubricative quality of the fuel.

It can be tricky to identify lubricative quality. According to the American Society for Testing and Materials, No. 1 or No. 2 diesel fuel should not exceed 460 microns of wear when tested in a high-frequency reciprocating rig (HFRR). The lower the score, the better the product’s lubricity. But because testing is voluntary, not every diesel fuel additive bears an HFRR rating. Finding an additive that combines adequate lubricity and good low-temperature antigelling characteristics might require patience and a good pair of reading glasses.

While I fully endorse biodiesel products and recommend them for their excellent lubricity, it’s  important to remember that biodiesel has higher (warmer) cloud and gel points, depending on the type of bioproducts used to create it and whether the blend is B100, B50 or B20. Combine this  with the fact the biodiesel is more hygroscopic than petroleum-based diesel fuel, and you can see why biodiesel should always be treated with appropriate coldweather additives.

Cool Tool

Strap-type wrenches are too clumsy for removing gelled, twist-on/off fuel filters in cold weather. The 22"-long Cobra slip-jaw pliers from Knipex are easy to handle while wearing gloves and make short work of stubborn filters up to 5" in diameter They’re handy for warm-weather tasks as well. $100 to $125.

Be sure to visit Dan’s "In The Shop" blog at, where he’ll share more tips and insights. Send comments and story suggestions to

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