Cold weather impacts diesel engines more severely than gasoline engines, and this isn’t solely because of the engines themselves. By design, diesel engines are more difficult to turn over and start, and low temperatures make this challenge even greater. In fact, studies suggest that at zero degrees Fahrenheit, a diesel engine can be up to five times harder to start compared to at 80 degrees. Additionally, other conditions related to diesel fuel also come into play.
Startup Compression Temperatures
Diesel engines ignite fuel using hot air generated by compression rather than a spark, as in gasoline engines. Because air heats up when compressed, diesels rely on much higher compression ratios—often about double those of gas engines—to achieve ignition temperatures. This higher compression makes it more difficult for the starter motor to turn the engine, demanding greater electrical output from the battery. For this reason, many diesel-powered vehicles, especially larger pickup trucks, are equipped with two batteries.
Cold weather worsens the issue since batteries lose a significant amount of their capacity in low temperatures—estimates suggest as much as a 60% drop at zero degrees Fahrenheit compared to 80 degrees. While this power loss impacts both gas and diesel engines, the extra demands of a diesel put even more strain on the battery in cold conditions.
Because compression alone might not heat cold air sufficiently to ignite fuel, many diesels use glow plugs. These small electric heaters, located at the top of the cylinder, warm the chamber before startup. However, glow plugs also consume battery power, meaning the battery must deliver energy before the engine even begins cranking.
Additionally, low temperatures thicken engine oil, increasing friction between moving parts and making it harder to circulate—an issue that affects both diesel and gasoline engines.
Fuel
Both gasoline and diesel engines can be affected by water in the fuel. In cold weather, moisture can condense inside the fuel tank, forming droplets that may mix with the fuel and freeze in the lines, causing blockages. To reduce this risk, it helps to keep the tank at least half full or to use a fuel additive designed to prevent fuel line freezing.
Diesel engines, however, face an additional challenge that gasoline engines do not. Standard No. 2 diesel fuel contains paraffin wax to improve combustion efficiency. Unfortunately, low temperatures can cause this wax to solidify into a gel-like form, which may clog filters and fuel lines. To counteract this, many fuel stations switch to No. 1 diesel—or “winter diesel”—during colder months. This type of diesel is more refined, less prone to gelling, and typically more expensive. In some cases, stations may offer a blend of No. 1 and No. 2 diesel depending on the weather.
Drivers can also use anti-gel additives to reduce the risk of fuel gelling, but these must be added before filling the tank so they mix properly, and ideally before temperatures drop below freezing. Without this precaution, a tank of No. 2 diesel purchased in the fall could end up gelling in the middle of winter.
DEF
Beginning with many 2010 models, diesel vehicles are required to use diesel exhaust fluid (DEF) to comply with emissions standards. DEF is sprayed into the exhaust system, so it doesn’t influence how the engine starts or runs directly. The challenge, however, is that DEF is primarily water and can freeze at around 12°F. To make matters more restrictive, a diesel vehicle will not start if the DEF tank is empty.
Although DEF inside the vehicle’s tank is usually safe from freezing—often thanks to built-in heaters—the same cannot be said for DEF containers stored in a cold garage. If you let the DEF tank run low and then attempt to refill it with fluid that has frozen in storage, you’ll be stuck until it thaws.
Diesel engines, especially in cars and larger pickups, are admired for their fuel economy and towing capability, but these advantages come with added expenses and occasional challenges.
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