Thats odd.

Diesels (at least old Perkins ones, which is all I've had) have low pressure return fuel lines from the engine to the tank, for excess fuel to bleed back.

Dont know that engine but my best guess would be leakage of oil from the hydralic pressure side of the injector to the fuel side, or perhaps the engine has an oil-fuel heat exchanger to preheat the fuel, which is leaking.

A start might be to confirm oil in the fuel return line, which may only be flexible hose since its low pressure.

Beyond that it MIGHT be worth checking injector function. On old engines you could do this just by sucessively cracking the junctions to the injectors on a running engine, but I'd guess procedures are more elaborate on a common-rail with maybe special caps required.

High pressure diesel spray can kill you.

Edited by edlithgow on 21/01/2019 at 00:20

Thanks for the reply, some people have suggested injector seals leaking, but I cant see hoe this would lead to oil in fuel.

Thanks

Common sense says that engine oil could only get into the fuel on the return side, or as suggested above where the fuel passes through some kind of pre heater or pre pump mounted on the engine before it reaches the high pressure pump, where the oil pressure would exceed the fuel pressure.

Failing injector? dunno it would have to be in a right state to be sucking back ( serious misfiring) and unless the cylinder itself was injesting oil via broken rings i still can't see this amount of oil somehow finding itself in the return pipe.

First thing i'd do is siphon some fuel out of the fuel tank itself and see if that's dirty, what i'd be looking for though is bio growth as well as engine oil...that oily reside in the filter is definately oil and not bio growth?

Secondly run the engine and using a small bottle taped in place, feed each injector return pipe in turn into said bottle and see if one of them dirties its fuel, you could also try taking sample of the pre pumped fuel before it reaches the HP injector pump.

Compression testing and plug in diagnostics, if you find no obvious answers, probably best left to a Diesel workshop if you can find such a thing in your area.

As Edlithgow above, take care, HP Diesel pressure is very dangerous, at the very least goggles and mechanics gloves, and be especially careful if you remove any HP pipes.