Replacing gear oil in 1977 mercury outboard motor, no oil going out top vent hole

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steve91

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Oct 20, 2012
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We were replacing the gear oil in a 1977 mercury outboard motor we just bought. Firstly, barely anything came out after we removed both vent screws, a little bit of water and oil. We pumped the new oil through the bottom hole but after a while instead of oil going through the top vent, it was backing out through the pump. We couldn't get any more oil in even though nothing was coming out the top vent. Any ideas why?

Thank you
 
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kvjimbo

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Oct 11, 2010
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Re: Replacing gear oil in 1977 mercury outboard motor, no oil going out top vent hole

Maybe you could try probing the fill hole for an obstruction or filling through the vent hole.

If you don't have any success you will need to open up the lower unit.
 
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Maxz695

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Re: Replacing gear oil in 1977 mercury outboard motor, no oil going out top vent hole

O ring is on the round surface of the waterpump base This would be just above the driveshaft bearing where the vent is View attachment 172637 If you said your getting oil out of the water pump then your O ring on the pump base plate is leaking and possibly it or salt sand build up is obstructing the vent hole. If it didn,t come out the vent hole it definitley has debris blocking it. Try to get it out it could be anything in there. Question : did you see any metal chips or shavings mixed with the oil water removed from the LU?
 
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Moody Blue

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Re: Replacing gear oil in 1977 mercury outboard motor, no oil going out top vent hole

Take a good hard look at your lower unit. Possibly what you think is the vent screw is actually a flush port. The earlier Merc lower units had three ports on them. Drain/fill, vent and flush. The flush port had a larger plastic screw plug while the drain/fill and vent ports had smaller metal screw plugs.
 
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Chris1956

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Mar 25, 2004
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Re: Replacing gear oil in 1977 mercury outboard motor, no oil going out top vent hole

A 1977 Gearcase shouldn't have a flush port, I wouldn't think. You removed the upper and lower stainless steel plugs? The vent plug is above the anti-ventilation plate. The lower plug is under the "bullet-shaped" portion of the gearcase.
 
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steve91

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Oct 20, 2012
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Re: Replacing gear oil in 1977 mercury outboard motor, no oil going out top vent hole

Moody Blue said:
Take a good hard look at your lower unit. Possibly what you think is the vent screw is actually a flush port. The earlier Merc lower units had three ports on them. Drain/fill, vent and flush. The flush port had a larger plastic screw plug while the drain/fill and vent ports had smaller metal screw plugs.
Click to expand...

Thanks Moody Blue and Chris1956, that was our issue, we had the flush port open not realizing the small metal screw right below it was the vent. Thanks again for posting
 
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