Firstly, you need to have a diesel engine in your car. This should have a
fuel injection pump by Bosch, Stanadyne, Nippondenso or Diesel Kiki.
Fuel injection pumps by Lucas (= CAV = Roto) are unsuitable, especially
for single tank operation! However, if you can ensure that the vegetable
oil always reaches the fuel injection pump at 80^C, then a roto pump can
also last a long time. The manufacturer's name is usually stamped on the
It is recommended to test the individual tolerance threshold of the car. As a general rule of thumb, pre-chamber and swirl-chamber diesel engines have fewer problems than modern engines. Good results have been recorded for Mercedes equipped with inline-pumps. "Normal" direct fuel injection engines should be treated with care. However, in these, there are several examples of vehicles that run very well with only minimal conversion (such as the older Passat TDI). Very little is known about Commmon Rail and Pump Jet engines, so caution is advised.
The engine must be technically sound. This means that the timing belt must be intact, because it will have to keep up with higher stresses when used with vegetable oil. The glow plugs must be in good condition, preferably even new, or the engine will not start when cold. If you have to replace the spark plugs, it is best to choose ones that continue to glow after starting; this softens the cold-start and is gentler on the engine. The seal must be in good condition, i.e. the piston rings should not be sticky or cracked. The fuel injection nozzles must be adjusted to the correct pressure and the quality of the fuel should be good, i.e. there should be no deposits. If all these conditions have been met, there should be little risk in testing the performance.
To start with, drive until the fuel tank relatively empty, and add a small amount of vegetable oil, so that if the tank is half full of diesel, you achieve a mixture of approx. 10%. Now observe the engine reaction: how good is the cold-start? Is the idling constant even immediately after a cold-start? Is the engine knocking louder than usual? Is there smoke? Can the maximum speed still be reached? Are there unusual mechanical noises? If there are any of these signs, then stop! Immediately fill up with diesel. This should solve the problem. As a rule, an admixture of 10% should hardly be noticeable. Now try with 20% and so on. Initially, however, never fill up completely if a particular mixture has not yet been tested, because that way you can fill up with diesel and this will halve the mixture immediately. Please note that when it is cold, the vegetable oil will only mix with the diesel very slowly, if at all. So the vegetable oil should always be filled into the tank first, and then filled up with diesel. It would be even better to premix the mixture in a canister first (it's illegal, because this would be manufacturing fuel and the produced fuel would be subject to tax, but for trial purposes.... Mixing in the fuel tank is tax-free this is right for Germany, check the taxes in your own country!).
Once having tested the reaction in this way, you can decide on the mixture you want to drive with. The colder it gets, the worse the reaction will be. Vegetable oil thickens much faster and won't spray properly, so the ignition performance drops. In summer, you will have far fewer problems.
In the long-term, however, one generally won't get around pre-heating the fuel for 100% operation. Once again long-distance drivers who keep their cars in garages have the advantage. The worst cold starting is avoided and the engine remains nicely warm. The next chapter shows what can be converted.