Biodiesel; what happens when old fuel breaks down

Old bio-diesel, fuel system not flushed and cleaned enough

There have been a number of studies about biodiesel and how it degrades over time. Most biodiesel blends, B5 or B20, that we see in Oregon are a FAME (Fatty Acid Methyl Ester). We have found biodiesel will often degrade enough in 1 year to cause fuel system damage. This is faster than the laboratory tests we have read.

If you are burning several tanks of biodiesel a year, then we see few problems because the fuel is cycled through. The issues we see are related to very little use over a 1 year or longer period. For example, you have a tractor that you filled with fuel in the fall and the next summer you only used 1/2 of a tank. In the fall you top off the fuel tank. Now next spring comes along and at least 1/2 of the fuel is over 1 1/2 years old. That is when we often see the problems from old biofuel.

Many people assume since they are buying off-road fuel that there is no biodiesel in it. Oregon’s law mandates B5, which is a 5% bio blend and all fuel sold, except in the winter is biodiesel. In the winter biodiesel would gel and was up. The fuel you purchase at the pump may contain up to 20% biofuel. Unfortunately there is not a fuel additive on the market to extend the life of the biofuel component of the diesel.

When biodiesel is exposed to oxygen, metal, light or heat it accelerates the degradation process. As biodiesel degrades it becomes more acidic, sticky and corrosive. Biodiesel is more prone to absorb water versus petroleum diesel due to the esters in the fuel. The more water it absorbs, the more corrosive it becomes.

You are better off not topping off a fuel tank and dealing with condensation than dealing with degraded biodiesel. If your biodiesel is 1 year old or older, we suggest draining the fuel system and properly disposing of the old fuel, then filling the tank with fresh clean diesel.

If you have a no start issue after the engine has sat for an extended period of time it may be too late to flush the system. Broken down biodiesel will corrode injection pump parts and cause them to stick. It will often leave a sticky gooey residue on metal components, like the fuel tank and fuel filter housings.

If the old fuel has damaged the injection pump and injectors, the fuel system must be completely cleaned and flushed. Do it twice to be certain it is clean. We have seen injection pumps come back damaged after rebuilding due to incomplete cleaning of the fuel system.

The biodiesel fuel that damaged this pump was filtered in an attempt to “clean” it. Filtering will not remove acids or change PH levels. The fuel system must be cleaned thoroughly, flushed and fresh fuel put into the system.

This is another pump that was damaged after rebuild due to incomplete cleaning and flushing of the fuel system.

The engine was a no start due to sticky – stuck pumping plungers, caused by broken down biodiesel. Rotor is from a Stanadyne rotary injection pump.

Stuck fuel pressure regulator on a Bosch VP30 pump, caused by old biodiesel.

Rotor from Stanadyne injection pump. The rotor has been acid etched by degraded biodiesel. This amount of acid etched wear will cause low fuel delivery during cranking, and would cause a no start issue. The head and rotor, which is the heart of the pump, will have to be replaced.


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Hi Gordon,
We have not found a good solvent to cut the deposits, other than letting fresh clean diesel sit overnight or a couple of days, then re-flush the fuel system.

Bill Piper

Excellent. My E350 had old diesel in it when I last drove it 2 years ago. Now, it won’t start.
What’s the test for bad fuel? Is it a ph test or something else?


Hi Billy,
Did it have bio-diesel in it? If so, yes, it has broken down and could be causing the problem. If the diesel fuel in it is over 3 years old, then it can start to cause problems. Start with inspecting the fuel filter and fuel in it. There may be a local shop in your area that can test fuel, you may need to call around. Also take a look at our diagnostics information


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