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Fuel found in Engine Oil

20 years ago

I know this is a car-forum but I will ask about my boat engine which is very similar to a car engine. I have a Mercury O/B 4-stroke EFI 1.8L 4-cyl DOHC 16-valve 115 hp on my boat(2005 Trophy).

I've done several oil level checks before and it has always been at proper level. Then one day I checked and it was about an inch higher than the F mark on the dip stick. The engine had only about 20 hours that time. So I drained the oil and I noticed that oil/fuel mixture came out with strong gasoline smell. I am guessing that about 1 qt more than what it was supposed to be (5 qt) came out. The boat had not been used for about 2 months before this was found.

Can anyone guess how the gasline could enter into the crankcase on an EFI engine? The boat is stored outside on the driveway with a cover on and the fuel was about 70% full in the tank. The boat ran fine before and still runs fine now. Also, it hasn't happened again yet. Any long-term internal engine damage might have occurred that I don't notice now?

Thanks for your input.

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Comments (2)

jemdandy
20 years ago
I'm not familar enough with your 4-stroke EFI Mercury Boat engine to advise for sure, so I'll hazard some guesses.

How is your fuel system pressurized? Is it by electric fuel pump, or by a mechanical unit driven off a cam in the engine? If its cam driven, this is a possible source for a fuel leak directly into the crank case.

Typical fuel pressure for injectors is about 50 psi. The fuel log remains under pressure after engine shut-down. If an injector does not seal well, it can weep fuel into the intake manifold until the pressure is relieved. Also, since the fuel tank is a closed system, a temperaure rise can keep the system under pressure and dump more than the expected couple of cupfuls or so that was in the fuel log and lines.

If a cylinder was mis-firing, unburned fuel can work its way past the rings and slowly dilute the oil.

If the crank case breather system was not working as it should, fluids and condensed gasses (getting past the rings) will build up in the crank case oil.
mxyplx
20 years ago
DO NOT run that engine without extreme caution!

I had fuel in the crankcase twice with Detroit V-8 Diesels (what they called Silver 92's). First time the crankcase filled up and great clouds of vapor blew out the breather which I saw too late in the mirror on a foggy night and the engine gave up. Several thousand dollars worth.

Second time I caught it because the oil pressure dropped. I shut down and saved the engine but the crankcase was overfull ie well diluted.

First one was a hot engine under load. Second a cool engine; truck running empty.

Altho these were Diesels the cause was a broken fuel delivery tube. They vibrated till fatigue caused em to crack. They shoulda been replaced once a year whether they needed it or not.
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