It’s spring, and a maintenance manager’s fancy turns to …


Spring brings warm weather, birds chirping, longer days and complaints of black sludge in the tanks of diesel-powered vehicles everywhere. But why, and why now?

For answers, Fleet Maintenance talked with Steve Muth, chief chemist for The Penray Companies, which makes and markets a wide variety of fuel and cooling system additives, service chemicals and other functional fluids for fleet use under their PowerFleet brand ( He explains that warm weather is indeed a harbinger of sludge to come.

“Today operators of diesel-powered trucks and equipment are using ultra-low sulfur diesel (ULSD) fuel and, in many cases, biodiesel fuel. Increasingly, drivers and fleet maintenance personnel are finding a black tar-like substance in the fuel. This substance is particularly noticeable in vehicles with transparent fuel filters, where it is immediately apparent.”

This substance is asphaltenes – an  organic substance that forms in ULSD and biofuels, particularly in new engines where fuel re-circulates and is heated by the process, he explains. Asphaltenes develop as the result of changes in the refining process that remove much of the sulfur from today’s diesel fuels, and these can be harmful to diesel engines.

“Asphaltenes can clog fuel filters but, more importantly, can clog fuel injectors and other critical fuel system components. The result is degraded engine performance and fuel economy and higher maintenance costs, along with the ‘hidden’ cost of increased downtime.



“While we can’t change the way diesel fuel is refined or blended, there are steps that can be taken to offset the development and potential damage resulting from the formation of asphaltenes,” says Muth. “The occurrence of these harmful contaminants can be prevented with the use of an appropriate additive, like Penray’s Total Diesel Fuel System Cleaner, which is specifically compounded to dissolve and prevent the future formation of asphaltenes.”

With such a product, vehicle maintenance personnel can take proactive steps to prevent the development of this material and keep fuel systems clean and functioning as designed, he says. Once existing accumulations of asphaltenes have been dissolved, a maintenance level of the appropriate additive will prevent their recurrence.