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Beltoy 04-03-2019 15:48

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
This is a painful post. Very frustrating. ejlindahl you have documented this situation very carefully and completely. I have changed the oil before on this lower unit. I can't imagine what caused it to plug up or else. I too have the problem with a 2004 Mercury short shaft. I tried putting carb cleaner in the top and then bottom holes and leaving it over night, neither worked. I also used compressed air all to no avail. I gave in and filled from the bottom hole.

By the way, I was using the pump that Mercury sells with its lube oil and it fell apart and made quite a mess. Amazon reviews mention this is a problem; and I was not impressed.

If someone does come up with a workable solution, I am all ears.

ejlindahl 04-03-2019 20:27

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Beltoy, Boy this thread keeps on giving. I'm glad you found it so that you can attack the problem as you see fit. You mention you can't imagine what caused the fill hole to be plugged. Somewhere in the thread I mentioned that I firmly believe that the bearing and its seal were not put in correctly or slipped down the drive shaft and now the stainless steel bearing carrier is blocking the fill hole. Look inside the hole and see if you see shiny metal, that is the bearing or carrier for the seal that is blocking the fill hole. You could probe in there with a dental pick to see if you "feel" any rubber material, that would be part of the seal. Don't mess with that too much, you do want that seal to do its job. You could disassemble the whole thing but that is a big job and may require special tools and or a bearing press. Not worth it in my opinion. Just fill/drain the oli by removing the prop shaft bearing cover and put in the correct amount of oil, but be careful not to pull the shaft out with the bevel gears and shifting dog and all, as I've pointed out above somewhere.

Beltoy 05-03-2019 07:46

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
ejlindahl , thanks for your reply. I did read all the comments and suggestions in this thread. I did feel the rubber gasket through the upper hole. What I don't get is why this would happen after 15 years of service. Why do you feel it is necessary to remove the prop shaft cover to put in the oil? I laid the unit on its side and filled the lower unit with 4.5 ounces of oil as per my manual. I put some in the top hole but it took virtually nothing.

ejlindahl 11-03-2019 17:27

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Beltoy, I can't imagine why it happened after 15 years either. Maybe a rapid a change in temperature and/or concurrent use. The way you filled it would be just fine. I don't recall why I did it thru the prop shaft cover but maybe it was faster than dribbling the oil thru the small fill hole.

eherlihy 29-08-2020 11:25

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I am glad that I stumbled across this thread in my search for a solution. I have the same problem with my 2002 Mercury 3.3 short shaft.

I have had the motor for the past ten years. While in the past I could get gear oil in the fill hole, the vent never worked well, and pressure would build up in the gearcase, causing a mess when I removed the pump. If I recall, I did get oil out of the vent hole when I first filed it after I bought the motor in 2011, but it has been difficult since then.

Yesterday I tried to solve the problem by cleaning out the vent hole with: first - a pick, then a punch, finally a drill. I did not get any chips of either metal or rubber out of the hole, but I was able to get a little air through when I applied 100 PSI of air pressure. 100 PSI also shifted the transmission into gear! I decided to quit before I did any real damage, and buttoned it up shortly after I started.

Like others, I finally gave up, and inverted the lower unit and filled it through the oil fill hole. I did not use a pump, but I snipped the very tip of a gear oil bottle and squeezed a little at a time into the fill hole.

I didn't measure the oil, I just stopped when the case was full. If the case were positioned upright, I do not believe that there are any spaces that would not be filled with oil, as I believe that the vent hole is at the highest point of the transmission.

ejlindahl 30-08-2020 10:01

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude

When you say you used a drill did you mean a drill bit chucked in a drill motor? Ie trying to drill a hole in the obstruction? Thats gutsy. as I'd be afraid of damaging something in there. It is a seal after all that keeps the oil in and saltwater out. I suspect that you got no chips of metal as I believe (as written upthread) that the vent hole is obstructed by the stainless steel outer carrier of the driveshaft bearing. If you've seen a bearing on a car or trailer wheel you know what I mean.

I believe that the bearing was not put in the right place on the drive shaft, ie; too high or too low so that it obstructs the vent hole. Alternatively I am beginning to think that the bearing may have been put in correctly upon manufacture but over time it has slipped down (or up) the shaft a fraction of an inch so that it now obstructs the vent hole. I am thinking this because there are some comments upthread by those who've owned the motor since new. They said they initially had no problem filling the lower leg with oil the normal way, (fill from the bottom hole til it leaks out the top "vent" hole). Then they say that they no longer can do this and have reported the same problem that caused me to start this thread. My work around described above (and yours) should work fine. It has so far!

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