The Ins and Outs of Fuel Contamination

Nov. 28, 2022

What happens when a vehicle’s fuel tank is filled with “bad” fuel? Well, it varies depending on how bad the fuel is and if the engine is gasoline- or diesel-powered. Fixes can range from cleaning the system with a fuel additive (isopropyl alcohol), flushing the system and refilling with “good” fuel, to needing to repair damage to the fuel system and/or engine.

What happens when a vehicle’s fuel tank is filled with “bad” fuel? Well, it varies depending on how bad the fuel is and if the engine is gasoline- or diesel-powered. Fixes can range from cleaning the system with a fuel additive (isopropyl alcohol), flushing the system and refilling with “good” fuel, to needing to repair damage to the fuel system and/or engine.

Issues resulting from bad fuel can range from difficulty in engine starting, sputtering/pinging sounds while idling or driving, stalling, notable reduced fuel mileage, acceleration trouble, or errant vehicle speed changes while driving. The cause may be water in the fuel, “old” gas or dirt particles in the fuel. If you’re lucky, the fix might simply involve replacing the fuel filter. One step worse, you may be replacing the fuel pump.

Water contamination likely leads the list with regard to fuel contamination. While issues have been previously reported on the East Coast in particular (from New Jersey to Florida, for example),  the issue can present anywhere. Water contamination is nasty. It can cause an array of problems aside from drivability issues, including fouling the fuel filter, damaging the fuel pump, or filter and fuel system issues caused by rust-laden, in-ground gas station tanks, fuel injector problems, etc. Old fuel (remember that gasoline has a shelf life) can cause sticky deposits that can easily create clogging issues in injectors. Water-laden diesel fuel can create herculean problems in high-pressure injection pumps and a diesel engine’s oh-so-expensive injectors. On GDI engines (gasoline direct injection), just as in diesel applications, fuel pressures are on the high side, in the thousands of ppm (parts per million). Any interruption of fuel pressure as a result of fuel contamination can result in the potential of ultra-expensive repairs. Water in fuel, which accumulates in the tank, results in rust buildup, and in warm/humid environments, microbial (algae) growth.

Gasoline engine injector O-rings depend on fuel for maintaining seal viability. Running E85 through non-flex-fuel designed injectors will degrade all soft materials. (Photo Credit: ASP)

In some cases, a contaminated fuel issue may easily be mis-diagnosed as a management system concern such as a bad fuel pump, a failed MAF sensor or a lean condition DTC, when the cause may be the result of water in the fuel, E85 in a non-E85 compatible system, diesel fuel in gasoline, gasoline in a diesel, dirt or algae contamination, etc.


Some (perhaps many) consumers are unaware of the difference between gasoline and E85 (85% ethanol and 15% gas). This fuel, designed for use only in “flex fuel” vehicles, is not compatible with the fuel systems in vehicles that are not designed for E85 use. Adding E85 into an incompatible fuel system can corrode aluminum surfaces and destroy fuel system O-rings, seals, gaskets, fuel pumps (due to reduced lubrication), paper filters, injectors and hoses.  E85 is also hygroscopic, which means that, similar to glycol brake fluid, it absorbs moisture from the atmosphere. Some ill-informed drivers may choose to add E85 simply because it’s a bit less expensive, or they assume that they’re being “environmentally friendly” since they’re using a “bio” fuel. Such decisions will eventually lead to expensive engine repairs.

GDI injectors run at extremely high pressures, including this Piezio injector from Bosch. Contamination such as dirt/grit/algae can not only affect injector performance but can damage the injectors. (courtesy Bosch)

Regardless of the type of gasoline fuel system (carbureted, port injection, TBI injection or direct injection), and especially if the vehicle is operated in warm and humid conditions, advise your customers to avoid parking their vehicles overnight with a low fuel level in the tank. The greater the surface area of the fuel tank walls that are exposed to air, the greater the chance of condensation forming and water sinking to the bottom of the tank. While in the real world this may be an impractical expectation, topping off the tank as frequently as possible reduces condensation issues. In other words, suggest that customers keep their tanks full whenever possible.


Speaking of diesel engines in particular, water contamination can wreak absolute havoc on expensive fuel system and engine components. Diesel fuel can contain two levels of water concentration — water in solution or “free” water. Diesel fuel can contain low ppm levels of water that may be dissolved in the fuel (water in solution). This can be the result of condensation in the tank and fuel temperature. Water that is not dissolved in the fuel (free water) accumulates in the bottom of the tank. If the fuel tank is made of steel, this can result in rust buildup and rust particles trying to pass through the system. Aside from naturally occurring in-tank condensation, the most common cause of water in diesel fuel is poorly maintained storage tanks from which the fuel is dispensed. Excessive water in diesel fuel can result in injector wear (in severe cases this can result in fuel injector tips exploding), sudden cooling of the engine that can cause intermittent thermal shocks which can damage the engine, and in cold temperatures, gelling of the fuel which makes it difficult for the fuel to flow through the system and ignite under cylinder pressure. While port injection on a gasoline engine may be in the 40-60 psi range and a GDI injection system may see in the range of 2,000 psi (high pressure needed to overcome combustion chamber pressure), a diesel injector may see anywhere from 3,000 upwards of a mind-boggling 30,000 psi. Given this level of pressure, it’s not difficult to understand how a clogged injector could literally explode under the wrong circumstances.

Diesel fuel injectors can be very expensive to replace. Rebuilding is the preferred cost-saving approach, but depending on damage, it’s not always an option.

While low part-per-million levels of water usually don’t pose a problem, free water can easily cause engine damage, filter pulling, loss of engine power and irreversible corrosion of fuel system components.

Tip: as noted earlier with regard to gasoline engines, it’s a good idea to top off the tank at the end of the day. This helps in getting rid of the humid/warm air in the tank, reducing/preventing condensation buildup. Excess water being pushed through a diesel engine’s fuel system reduces the lubrication needed for pumps and injectors that is normally provided by the diesel fuel.

Fuel additives are available specifically designed to clean up microbes and sludge. (courtesy Power Service)

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