Problems and Conversion Options

As mentioned before, the main problem is the high viscosity of the vegetable oil as compared with diesel (and the lower ignition performance). Problems can result in a vehicle that is not designed for this. The engine won't start because:

• The injection fuel pump cannot suck up enough of the more viscous oil (this is typically 20-60 litres/hour for distributor injection pumps) and insufficient fuel is supplied to the high-pressure end of the injection pump. The performance of the engine drops or the engine does not even start, or may suddenly start ßtuttering" while driving. This may be caused by the following:
  1. Special restrictions in the fuel feed and outlet. The only remedy is to check the fuel hoses and replace them if necessary. Check all connections! The inner diameter of the hose should remain largely unchanged.

  2. Fuel hoses that are long and too narrow. As above: replace them. The available figures show that you are normally safe with inner diameters of 8 mm or more if the return is bypassed. Inner diameters of 10 mm or even 12 mm will be fine where there is an open return.

  3. Fuel filter is blocked. Either the original filter has been in place for too long or it may be too dirty, so that enough diesel can just still flow through, while more viscous vegetable oil simply will not flow through the filter fast enough. In this case, the cheapest and relatively easy solution may be to fit a new fuel filter. If this does not help, either bypass the return to reduce the amount of fuel sucked through (more details of this later), or the vegetable oil needs to be heated before it reaches the filter. 80^C is the ideal temperature.

  4. Minor leaks: An option would be to fit clear hoses for various sections of the suction paths to pinpoint leaks. This is the best way to see where air is getting into the system. An opaque polyamide hose with an inner diameter of 8 mm has proved to be a good option (you can buy this from me - see below).

• Glow plugs are too old. If in doubt, replace the glow plugs, or at least replace them when reaching the kilometres recommended by the vehicle manufacturer. Measuring the current alone will not give reliable information about the condition of the glow plugs. They will lose heating performance as they get older! The ignition temperatures of vegetable oils are a great deal higher than those of diesel. To allow for evaporation and combustion, it is important to achieve sufficiently high temperatures in the pre-chamber, otherwise the fuel will not be completely burned and the engine will not start easily or not at all. This will result in deposits in the engine, which can lead to considerable damage (piston rings sticking and no longer sealing properly, injection nozzles coked and no longer spraying properly, etc.).
  
  If you need new plugs anyway, you should fit plugs with an afterglow / glow duration relay. These plugs continue to glow for a while after starting and ensure that the engine runs smoother during the warm-up phase. This prevents damage to the engine and lowers fuel consumption.

• Insufficient sealing. The sealing properties could be insufficient if the engine is either in poor condition or too worn to start with the reduced ignition properties of vegetable oil. The starting point should be to clean the engine (possibly even twice) to loosen piston rings that may be stuck. If this is the problem, the sealing properties should clearly improve after cleaning. If this does not help, a piston ring could be cracked. Normal wear is the only probable cause if all other possibilities have been exhausted, and this would mean reconditioning the engine.

• Injection nozzles coked / injection pressure too low. If you suspect coking, again start by cleaning the engine. With a bit of luck, this will solve the problem. If this doesn't help, a technician should test the nozzles. The first step would be to check whether the nozzle holes are unobstructed and whether the nozzle sprays properly or whether the injection pressure has been correctly adjusted (this is adjusted in the injector using plain washers).
• The injection pump itself is not sealing. It is possible for the injection pump to start leaking at various points. This is frequently caused by the radial packing ring on the main shaft. If air penetrates in a vacuum (when using vegetable oil, the negative pressure on the suction side of the injection pump is markedly higher than when using diesel), then you will not see air penetrating in the suction system, but bubbles will be apparent in the return or the fuel will become milky white from emulsifying air when the return is bypassed. In this case the conical nipple should be replaced, which should solve the problem.

• Suction filter in tank is blocked. The tank needs to be opened and cleaned. This may be possible from the top via a screwed flange.

• Metallic noises from the engine compartment. The injection pump may have reached the end of its life. This happens fairly quickly in Lucas injection pumps (=CAV = Roto), if the oil is not preheated. Inline pumps such as those fitted in old Mercedes or distributor injection pumps by Bosch are the most suitable pumps. Preheating the oil will always extend the life of the pump!

  If the engine starts stuttering while driving, this is generally due to an increasingly blocked fuel filter or a newly formed leak in the fuel supply line. When it is cold, the airstream can cool the oil to the extent that it can no longer be sucked through the filter. The fuel filter should last a reasonable time when using refined oil or properly filtered fresh oil. The lifetime of the filter will be greatly reduced when using frying oil or recycled oil. The only solution is to improve the filtering process or to use better quality oil.