Join Date: Jul 2007
Location: South Yorkshire
It varies from engine to engine, depending on the manufacturer of the fuel system and most especially the fuel pump.
Ford TDCi diesels use brands of fuel pumps which rely on the lubricity and viscosity specification of standard diesel. Veg oil is much thicker, and puts a lot of strain on the pump, even if diluted 50/50 with standard diesel. So the pumps are much more likely to fail if used with biofuel. When fuel pumps fail they distribute metal shrapnel throughout the fuel system. This clogs up the fuel pipes and injectors, which means you have to completely replace the entire fuel system at a cost of several thousand pounds, or replace the engine.
Other manufacturer's diesel cars sometimes use other brands of pumps which tend to be more biofuel friendly, so it's a bit of a lottery.
There's also the problem that biofuels get thicker when it's cold, so a proper biofuel conversion will include a fuel pre-heater to ensure the fuel is the correct viscosity before being fed into the engine. Proper biofuel conversions often have a switch so you can start the car up on normal diesel, then switch to biodiesel when the engine is warm and can more easily process the biofuel.
You may be lucky putting biofuel into a modern diesel that hasn't got a full biodiesel conversion, but most likely you'll just cause problems later on.
And yes you have to be VERY careful when buying a second hand common rail diesel in case it has been misfueled in the past. Misfuelling with petrol is another way to ruin the engine, and probably more common than veg-oil. I've heard of diesel cars being misfuelled accidentally, being run for a bit, breaking down, flushed out with diesel, then traded in or sent to auction. The engines may appear to run well, but once misfueled and run for a bit the engine is much more likely to fail in a few hundred miles or so.
Edited by: Ravenger