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Oil In the Diesel Fuel

Dirty hand w wrench

Diesel engines are very robust. Quite often, they can literally pull all of the engine oil out of the system and burn through it with their regular fuel burn and barely even notice. This is what basically happened to user dlt630 in our forums when his 2001 Ford F-550 PowerStroke began sucking engine oil and burning it away.

The problem is relatively simple: fuel injector O-rings can fail, leading to engine oil being almost literally pumped into the fuel injector ports and into the chamber to be burned with the diesel fuel on combustion. When the cylinder is not on its burn cycle, the oil can be pulled through the system into the return fuel lines and sent back to the fuel tank, where it dilutes with the diesel and comes back again to be burned (or clogged in the water/fuel filters).

The first step to find out whether your fuel system is sucking the engine of its lubricating oil is to pull the fuel and water filters. Most likely, they will have a lot of oil residue in them or the fuel poured out of them will be black and murky rather than the usual amber color of diesel fuel.

The next step is to pressurize the fuel rails to test for leaks. Most well-outfitted shops working on diesel engines will have injector connectors for the shop's compressed air system. Using these, a technician or DIY mechanic can pull and check each pressure line across the cylinder heads. When fuel or air escapes from the fuel galley, you have a bad O-ring.

Many mechanics, finding one bad ring, will assume all on that head are bad and replace them as a matter of course. This is as much preventative maintenance as it is to keep the maintenance schedule simple.

Once the offending O-ring(s) have been replaced, recheck and then put the engine back together and try it again. Most of the time, this will fix oil getting into the fuel in a diesel engine.