Ask SAIL: A Diesel Conundrum

A Diesel Conundrum

Q: I’m trying to determine why the diesel fuel for my 4.108 low-line Perkins is getting black. I use the word “black” instead of dirty, because the fuel looks as if it is getting crankcase oil (or soot) mixed in it. I’ve taken a sample of the fuel and let it sit for several months, but whatever is suspended in it doesn’t settle to the bottom. This is not the typical algae sludge that grows in fuel when water gets in the tank, and it gradually occurs even after I drain all the fuel out and refill the tank. The engine runs great. The fuel color doesn’t seem to have any negative impact, other than bugging me. Here is what I’ve done:

Replaced the aluminum fuel tank—despite being refilled with brand-new fuel, it started getting dirty right away, I believe from running the engine

Replaced the fuel lines (when the tank was replaced)

Bypassed the mechanical fuel pump and installed a marine grade electric pump. The fuel lines no longer run through the mechanical pump, hence there is no opening there to the crankcase

The engine has a primary Racor fuel/water separator and a secondary filter. I replace filters yearly and they are always clean

Replaced all injectors and the high-pressure pump

Is it possible for crankcase oil (or blow-by soot) to get into the fuel and be returned to the fuel tank? I’ve traced the lines and do not see where or how this could occur. If what I’m seeing isn’t oil, do you have any ideas what it might be?

Jim Brady, via



I confess this not only is a new one on me, but I’m at a bit of a loss. The only place I can see any possible interaction between the fuel and the engine oil or carbon in the engine is at the injectors, where excess diesel is recycled back to the fuel tank (through the injector bleed-off lines). Even so, I can’t think of an explanation for this. Perhaps one of our readers has a better idea?

Ed note: If anyone has the answer to this mystery, please let us know by e-mailing it to

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