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Fuel contamination in oil

I have a volvo penta engine in my boat. It’s basically a chevy 350 fuel injected eng with a few marine modifications. I have been getting high fuel contamination (6% in 12 hours of use) in the oil and no-one can figure out how. It ruined one engine and threatens my fresh rebilb. I have had the injectors indivually check and also the fuel pressure and residual pressure. Any ideas out there? Thanks, Elliott

Is the engine warming up correctly? Thermostat? Hard to imagine that affecting two different engines, though.

The engine warms up properly, and runs beautifully too. There was a theory that one or more injector leaks into the cyls after the engine is turned off. It would have to leak past the rings and into the crankcase, little by each time the engine is turned off. Having the injectors tested pretty much ruled this out. There is not another place that fuel can enter, right? Elliott

Does the fuel pump shut off when the key is off? What type of injection?

The fuel pump turns off right away. I’m not sure what to call the injection. It is NOT throttle body type. It has a control unit mounted on it…EZ

Check for a leaking fuel pressure regulator. If the diaphram is leaking in the regulator, excess gas will be drawn into the engine while it’s running, and gas will be pushed into the engine after it’s turned off through the vacuum hose.


I’m guessing you have an electrical fuel pump.

But in case you don’t (because it’s an old chevy 350), then look at the rubber diaphragm that separates the fuel side and engine oil side of the mechanical fuel pump. That diaphragm can begin to crack, allowing fuel to enter the engine oil.

It is an electric fuel pump. The fuel pressure regulator was supposedly checked at the Volvo Penta dealership (two times). It was one of my suspicions from the beginning, so maybe I should re-check it myself. EZ

Any other ideas out there? The only way I have been able to use my boat lately is to change the oil every 10 hours… EZ

There is 6% contamination with what? Do you know for a fact that the contamination is fuel? Could it be coolant?

My engine oil has a 6% contamination with fuel. I took a sample and sent it out to a lab to be analized. There was no coolant present. It is thinning out the oil and causing severe wear. I did not know it was happening and it ruined the bearings in my previous engine in 18 hours! Elliott

This from Ford engines;
Back when the fuel pressure regulator was a vacuum operated diaphram, If the f/psr reg was bad , you would get “peanut butter your chocolate”. ( fuel in your oil )

I’m unfamiliar with your setup , but it sure sounds like those old ford fuel pressure regulators.

I agree with Tester about the pressure regulator. Disconnect the vacuum hose to it and see if there’s gas in it or if it smells strongly of raw gas.

I am no expert, but it sounds like excessive blow-by contamination to me. If that is the case, the there is too much space between the rings and the pistons. The engine might need a rebuild if that is the case.

Gasoline will not accumulate in motor oil unless the source is massive. Any small amount will boil out of the hot oil very quickly and be drawn off by the PVC system. Does the oil level on the dipstick increase?? if not, I think the “6% contamination” test report might be in error…

6% of 6 quarts of oil is about 12oz of fuel.
If you burn 2 gal/hr of fuel for 12 hours that’s about 0.4% of the fuel used ending up in in the crankcase.

If it were a crankcase ventilation problem I would also expect excessive moisture in the oil too.

It could be running very rich. How do the plugs look? Does it have an O2 sensor?

When this destroyed your engine the last time…was there any sign of cylinder wash??? That’s a sure sign of a leaky injector.

I’ll take a look at that, Thanks! EZ

That’s what I have been thinking, that any excess fuel would burn off. The level on the dipstick does increase, about one quart in 12- 15 hrs of use. I can feel and see that the oil is thin. The lab is a reputable lab, and I took the sample under the condtions that they reccomended. EZ

Any other ideas before I terminate this discussion? Thanks, EZ