Is “dark” fuel the same as “bad” fuel and can I still use it?

Written by Capt. Jeff Werner

This is a question that many of us have asked more than once. And what we really want to know is, “Will the stuff damage my engines?” Engines are expensive, ruining an engine is costly and operating an engine on bad fuel is not wise.

We all know that most engine failures start in the fuel tank. When all the mechanical parts are in good operating condition, the cooling and lube systems are working, the lube oil clean and there is a sufficient supply of clean air getting to the combustion chamber; a diesel engine could run almost forever. The only limiting factor is fuel quality. Dark fuel is symptomatic of poor quality and even though, in most cases it can be used, fuel in this condition will provide poor combustion and filtration problems.

Dark fuel is in general indicative of oxidation and the process of fuel degradation is in a far advanced stage. Hazy fuel is indicative of water emulsified in the fuel. In general, dark hazy fuel will not damage your engine. It indicates however, poor fuel quality, which will definitely not provide you with peak engine performance.

Using less than optimal fuel quality negatively impacts engine efficiency and accelerates the process that makes new engines old.

Diesel fuel ranges from colorless, to amber or light brown color, depending on the crude oil and the refinery process used to produce it. In addition, dyes may be added to change the fuel color for tax identification purposes.

In time, stored fuel will darken due to oxidation, repolymerization and agglomeration of certain components. The darkening is accompanied by a formation of sediment that clogs filters and causes poor combustion. Fuel and oil distributors suggest that if diesel fuel is stored for emergency use, it should be replaced with fresh fuel within a year, unless fuel conditioning is used.

The University of Idaho conducted tests on the life expectancy of fuels to determine the time line of degradation of stored #2 diesel fuel. The results indicated 26% degradation after 28 days.

Capt. Jeff Werner, a Licensed USCG Master, has sailed professionally throughout the world.  He is also the Diesel Doctor, helping to keep your boat’s fuel clean and bright.

If you have additional questions for Capt. Jeff Werner, please email him