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Thread: Avgas in the crankcase

  1. #1

    Avgas in the crankcase

    Do to a malfunctioning float valve in the weber carb which sits on top of my Great Plains VW, 2180 cc engine, fuel has run into the crankcase. I detected it on preflight & did not start the engine, I imediately drained the oil and oil cooler. Does anyone have an opinion on any damage that may have occured to the internal parts of the engine such as the rod bearings, crank, etc.? I'm thinking (hopeing) it is probably O.K. since the engine did not run with the gas in the oil. Your thoughts please.
    Thank you.

  2. #2

    Join Date
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    Avgas is OK in the crankcase. In Continentals and Lycoming aircraft engines it is standard practice in really cold weather to add some avgas to the engine oil on shutdown to thin it for the next start. As the engine warms up it evaporates out the breather and the oil viscosity returns to the normal desired value.

    Avgas is just another petroleum product.

    Don't worry about it. Just fix your carb.

    Fly safe.

    Wes
    N78PS

  3. #3

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    Quote Originally Posted by planecaptain View Post
    Do to a malfunctioning float valve in the weber carb which sits on top of my Great Plains VW, 2180 cc engine, fuel has run into the crankcase. I detected it on preflight & did not start the engine, I imediately drained the oil and oil cooler. Does anyone have an opinion on any damage that may have occured to the internal parts of the engine such as the rod bearings, crank, etc.? I'm thinking (hopeing) it is probably O.K. since the engine did not run with the gas in the oil. Your thoughts please.
    Thank you.

    It's okay if fuel (cargas or avgas) gets into the crankcase and even okay if you had started/run the engine like that. You took the proactive step and all is well.


    A common practice in extreme cold temps is to dilute the engine oil with fuel before shutting the engine down. This keeps the oil viscosity low for the next startup, once running the the fuel burns off off you go into the wild blue yonder.

  4. #4
    rosiejerryrosie's Avatar
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    I had the same thing happen to an old lawn tractor that I had. I drained, count it, six quarts of oil/fuel out of the crankcase after having tried to run it (twice) and having oil/fuel shoot out of the exhaust. NO PROBLEMS. Drained it, filled it with fresh oil and has been running for two years now. I don't suspect you'll have any problems.
    Cheers,
    Jerry

    NC22375
    65LA out of 07N Pennsylvania

  5. #5

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    One thing that shouild be considered is how much leaked into the oil. If your crankcase nominally holds 4 quarts and you drain 5 or 6, it's one thing, but if it's 7, 8, 9 or so on, then it's another. My personal preference would be to drain and refil as normal, but then pull the plugs and spin the engine by hand enough to get fresh oil to the pump. Then spin it with the starter to see that you get full oil pressure. At that point, put the plugs back in, get it ready to fly and go do a test hop. Make the hop long enough to get the engine to full operating temp and keep it there for a while. If nothing weird shows up, you're done and go have fun.

  6. #6
    Neil's Avatar
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    I agree with others here. You did the right thing. This is a common occurrence on older British Motorcycles like the Triumph T-120 Bonneville where the carb is actually tipped toward the cylinder. They are of a dry sump design and sometimes the crankcase has been found to be full of fuel and the engine becomes a hydraulic pump and if you attempt to start it and blows fuel from any available opening. They live to run again after draining. Mine is going on over 40 years now.

    You might want to develop the habit of turning off the fuel with a down draft system. Many aircraft engines with horizontal induction have a sniffle valve that allows the induction system to drain in the event of fuel pooling in the intake.

  7. #7

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    I'd be a tad concerned about this. Probably once is okay, as long as you knew it was happening and stopped it immediately, but a danger I could see would be a leak you aren't aware of. I have a VW aircooled (bug) engine, in this case actually in a bug, which has a diaphragm style fuel pump. This leaked, causing the symptom of unexplained fluctuations, including apparent increases, in oil level, but I didn't figure out what caused this until it was too late. The fuel (mogas, not avgas, but I'd think avgas would do the same) displaced the oil in the bearings, rings, and valve guides, so they weren't lubricated, destroying them and causing the vehicle to blow an incredible amount of smoke out the exhausts, enough to create a smoke-screen over entire intersections. The engine never failed, seized, or threw a rod, I think I'm pretty lucky (I'd be concerned that other styles of engines might, those VW engines are pretty tough), but it required a complete overhaul before I could drive again. All because of a stupid fuel pump! If your oil ever smells like gas (avgas or mogas, whatever you're using), the level decreases at a slower-than-expected rate, or the level seems to increase, check it out!

  8. #8
    Larry Lyons's Avatar
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    For what it is worth this is a rather common occurrence in the construction industry. No big deal just drain refill go fly. If it had been run there could be problems but in your case you are home free. Larry
    No matter how far you push the envelope; its still stationary!

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