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Oil leaking around lower unit, by IWombat

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  • Oil leaking around lower unit, by IWombat

    Oil leaking around the lower unit

    So, you have oil leaking out of, or around the lower unit on your two-stroke outboard and youíre wondering what to make of it all. Thereís only a few possibilities. Fortunately for you, most of the time itís pretty benign.

    Reason #1 - Unburned 2-stroke oil.
    This is by far the #1 reason for finding oil on your lower unit. 2-stroke outboards use a mixture of oil and gas. Not all of the fuel gets completely burned up. As a result, not all of the oil gets completely burned up either. Since gas is more volatile, it has a tendency to evaporate away, leaving just the oil or at the very least, a more concentrated gas-oil mixture. Combustion is even more incomplete at low RPM. What are you doing right before you put your boat on the trailer? Motoring around in the no wake zone and building up unburned oil.

    Unburned oil will normally be black and sooty. However, if thereís a lot of it (and this can happen when purging out all that fogging oil from winterizing) it can look amazingly like old lower unit lube.

    Normal places to see unburned oil are out the prop, out the top of the lower unit, and through some of the water drain holes in the lower unit. To add to the confusion, these are also normal places to see leaks due to Reason #2. If youíre not sure what the source is, inspect the lower unit oil as described later. If you find no water intrusion in the gearcase youíre most likely looking at unburned 2-stroke oil.

    Seeing unburned oil is fairly common and perfectly normal. It does not in and of itself point to anything wrong with your outboard.



    Reason #2 - Lower Unit Gearcase Lube

    The lower unit of your outboard, otherwise called the gearcase, contains its own lubrication. It is a self-contained unit with a series of seals and gaskets designed to keep lubrication in and water out. These seals can wear and fail. When they do it is possible for lubrication to make its way out. However, it is far more common to find water inside the lower unit than it is to find oil on the outside. Generally speaking, leaks in the gearcase are small in volume, and by the time youíre up on the trailer or at the dock all external traces of your leak have simply been washed away. However, it has most likely left its fingerprints inside the gearcase.

    Usually lower unit leaks are at least somewhat bi-directional, meaning water comes in and lube goes out. Water contaminated lower unit lubrication is milky looking and opaque. Uncontaminated lower unit lubrication is generally tan to grey and mostly translucent.

    Lube will float on top of any unmixed water in the gearcase leaving the bottom of the gearcase unlubricated. No water should EVER be found in the gearcase. If you pull the drain plug and you see water come out, youíve got a problem.

    If you suspect that lube is leaking out, and no water is leaking in, the definitive test is to pressurize the lower unit with air, submerge it in a tub of water, and look for bubbles. If it leaks out the prop shaft seal, you might inspect for, and remove any fishing line behind the prop and test again.


    This generally won't be the cause of an externally visible oil leak, but as long as we're on the subject . . .

    By far, the most common cause of water intrusion in the gearcase is failure to replace the seals around the fill/drain plugs. These should be replaced EVERY TIME you change the gearcase lube. If you find yourself with a water intrusion issue, put in fresh lube, replace the plug seals and inspect after the next use. If that doesnít remedy the situation youíre in for a lower unit rebuild.

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