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Biodiesel Conversion Kits are generally not needed to run biodiesel. A conversion is required when running straight vegetable oil. People often confuse the two.
Straight vegetable oil is too thick to flow through a fuel injector nozzle. In order for straight vegetable oil to work correctly in a diesel engine it is heated to a high temperature before it enters the fuel injector. In other words, a SV0 conversion mostly comprises of a heating system.
You might need a biodiesel conversion kit when:
Biodiesel has worse cold weather properties than fossil diesel. In extreme conditions this will cause the vehicle to stop due to the biodiesel freezing in the fuel lines. This may require that the vehicle is fitted with a fuel heating system to keep the biodiesel flowing. Certain manufacturers (like VW) do supply certain vehicles with these biodiesel conversions in Europe.
Another way around cold weather issues is to blend the biodiesel with normal diesel or kerosene. This in effect averages out the cold weather properties of both fuels. This is one of the reasons that a B20 or less blend is available at the pump in certain countries and not a B100 blend.
Prior to the introduction of low sulphur diesel it was common for motor manufacturers to use nitrile rubber in the fuel system. Biodiesel is incompatible with Nitrile and natural rubber. Any vehicle which still has these types of seals and pipes would need to have them changed for it to run biodiesel with no issue. There are biodiesel conversion kits available. Typically a failure on your fuel system is not a quick failure, the degeneration of the rubber takes a while before failure. This does mean that it is quite safe to run a test tankful of biodiesel in your vehicle.
When a seal is subjected to a solvent it will swell and the amount of swelling will depend on the specific liquid. A result of this is that if you switch often between biodiesel and normal diesel your seals will swell and shrink repeatedly and eventually fail. It is better to use one type of fuel for an extended period of time.
In summary, a biodiesel conversion kit should not be necessary unless you live in a cold area or drive a pre 1994 vehicle. Even then generally you would not need a biodiesel conversion, common sense in cold conditions will avoid problems. There is some further discussion on these issues in biodiesel cars.