Mercedes: 83 Mercedes Diesel, oil water separator, mercedes 300 diesel

Question
I have a 83 Mercedes 300 diesel.  The previous owner used waste veggie oil and
I think some unfiltered got in the regular fuel tank.  It starts OK but after it runs
for a few minutes it starts to knock in the engine.  Then after about 10 minutes
it has no power and shuts off. I think I should drain the tank is there a plug and
what do I flush it with?  Help!  Thanks JACK

Answer
John,
Have you cleaned the oil/water separator and changed the fuel filter. It sounds like you are ingesting some water into the engine. The knocking sound may be the poor ignition when the water is vaporized. I also think you could do damage to the engine by continuing in this process.
If waste oil can promote mold growth. Bio Diesel (Waste Veg. Oil that has been treated with a methanol process) is an excellent fuel tank cleaner. and engine injector cleaner.  You may need to remove the tank fuel cap and blow air back through the lines with kerosene to back wash the tank. I would think the if a plug exist, it should be clearly visible under the fuel tank.
Good Luck
Rob