According to Marc Franke and other experts on alternative fuels, more recent diesel cars with high-pressure injection systems have a more difficult time burning straight vegetable oil (SVO) than older models. The fuel pumps struggle a bit more to propel SVO through the injectors than they do with biodiesel or conventional petroleum-based diesel. The diesel vehicles that work best are the older VWs and Mercedes that don't use direct injection. Instead, these older diesel vehicles have a pre-chamber to get the burn started in each cylinder and therefore don't need such high pressures.
Another problem, says Franke, is that the thicker SVO apparently doesn't burn completely and "cokes" (leaves deposits) on the valves and possibly the injectors. The unburned vegetable oil polymerizes into a hard goo that sticks to the engine.
I contacted a couple of individuals who use SVO in their diesel cars to determine whether these concerns were valid. One colleague who has burned SVO in a relatively new pickup truck reported no problems whatsoever in over 40,000 miles of driving. The other who has logged over 400,000 miles on a diesel truck also reported no problems. In fact, when the engine of his truck finally died, he hired a mechanic to disassemble it to see if he could detect problems. They could find none. Readers are advised to research this issue before switching to SVO.