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ejlindahl 29-12-2015 01:11

Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
1 Attachment(s)
I went to put new gear oil in my 3.3 Mercury lower unit and the oil would not go in the lower "fill hole". (I did remove the top "oil level" screw plug to allow air and/or oil to vent out the top.) When I removed my oil pump fill nozzle from the lower fill hole the oil sprayed back out under pressure. Closer inspection revealed the top "oil level" screw hole was clogged with a small amount of slushy, oil and water soaked, partially crystalized, salt. (The old oil I had previously drained was perfect/not milky. It drained very slowly over several days in my cold garage.) I removed the impeller pump housing (see #17 in the schematic below) that contains the bearing cup and bearing (#13) and rubber sealing grommet (#16) and cleaned out the slushy salt crystals in the screw hole. I found that the hole goes straight through to the bearing cup and that the rubber grommet blocks the passage of oil/venting air from the gear case. It seems that this grommet (and the #13 bearing race) should have holes in them to allow venting air or oil to come out from the gear case, thru the bearing and out the "oil level" hole. I am thinking that who ever installed the rubber grommet did not align the grommet's hole, (if there is one), and possibly the hole in the bearing race, (if there is one), with the "oil level" screw hole. (When I look in the hole I can see part of the bearing race as well as the rubber grommet - they completely block any possibility of air or oil to vent out).

I certainly could take the assembly to a shop to press the bearing out of the cup to allow me to inspect and properly align the grommet and bearing race. Or I could take a needle and try and poke through the rubber grommet and into a corresponding hole in the bearing race, (if there is one). But neither is a step I want to take before I seek advice from the forum.

Does anyone have experience with a problem like this or is familiar with the pathway of venting air and/or oil from the gear case to the upper "oil level" hole? My engine is a 1996 Merc 3.3 hp with a forward/neutral gear shift.

This link may be easier to read than the schematic below

Thank you,


mariner36bob 02-01-2016 10:29

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Should only have to vent the gear case once, at the time of filling with oil. Fill as normal, give a couple extra pumps to burp air and close with screw. No "in service" venting is needed or desired, it's a sealed reservoir....

Parts 2, 13, and 15-23 are all working together to keep water in pump housing and oil in the lower end.

Did you by chance determine how much oil came out? You may find some oil/sludge did not drain..

Can you reverse the fill procedure and fill from top to clear it out?

If you can get a little oil in it from the top hole then warm it up, put a little more in, warm it up again, little more etc. until your full enough to heat it up proper. Then drain all that crap oil out warm. I bet it will take the salt clog with it...

Draining cold likely left enough oil in it to not be scared trying to warm it up "empty". Does that make sense?

You've got to get it warm...

Worst one I've ever had was completely filled with peanut butter consistency emulsified oil. Only 1or2 ounces drained out, somewhat clean, the remainder was a white/grey sludge that required forcing it out the bottom hole via low pressure air into the top hole. Gross.

I bet you have a little salt in the vent screw hole is all.

ejlindahl 04-01-2016 11:23

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Thanks for the reply Mariner36 Bob. Great hint about measuring how much oil drained out and checking for sludge. I have removed part #17 and can see down into the gear case and it is pristine, no sludge, just bright clean metal with a remaining film of clear, light brown, gear oil, as if it just came out of a factory sealed oil can. Nice to see that on a 20 year old engine.

However, my problem is (when adding new gear oil, with the upper vent screw removed) there is no way for air or oil in the gear case to vent out through the water pump housing (#17) (and then through to the vent screw hole). The vent passage hole in the pump housing (#17) is mechanically blocked by the outer perimeter case of bearing #13 and the oil seal, (#16) both of which are pressed into the housing #17. When I Iook into this vent passage hole I can see the shiny metal outer bearing case and the soft black rubber seal totally blocking the passage, (there is definitely not a plug of crystalized salt for instance). Just to be sure I jammed the plastic tipped nozzle of my compressed air gun into this vent passage hole and at #150 psi no air passed by the bearing and seal to come out the other side, (this is an athwartship vent passage. I am not referring to the vertical axis where the drive shaft runs through the bearing and where there is and should be a tight seal of the rubber seal #16). The blockage of compressed air proves that venting air (or oil) can not get out of the gear case to the vent screw hole. There must be an alignment issue: I think there is (or should be) a small hole in the outer bearing case that is not lined up with the vent passage hole in the water pump housing, (#17) (sloppy manufacturing??). A small hole in the bearing case would allow oil or air to vent. I am hoping there is someone who is intimately familiar with this particular model of engine (and possibly specific to this inaugural year of the forward/neutral gear shift) that knows how the passage of venting oil/air is supposed to go through this pump housing/drive shaft bearing/seal assembly.

Any one out there familiar with my specific situation please chime in. If not my solution is to button it all back up then fill the gear case with oil before I install the aft sealing collar of the prop shaft. And I will take M36Bobs advice to heat the oil and gear case up to around 100 degrees F to make the oil fills every nook and cranny.

PS: M36bob mentioned a little salt in the vent screw hole. I did have some of that which I previously mentioned as ďslushy salt crystalsĒ but I have cleaned that and everything else thoroughly including the hole in the pump base/housing #17 that lines up with the vent screw hole. Also, when I removed the water pump housing (#17) from the drive leg/gear case the drive shaft also came with it. So now I have to disassemble the prop shaft assembly to reinsert the pinion gear onto the driveshaft. A shop manual I later read says you can avoid this by holding down the driveshaft while removing #17 from the gear case. However in my situation the drive shaft seemed pressed onto the bearing and the bearing is pressed into part 17 and in any case it was very hard to remove the very tight fitting 17 from the drive leg/gear case. So I donít think it is actually possible to do this.


mariner36bob 04-01-2016 17:09

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I hope there's a Merc tech on here to help you but I am 100% confident you're looking in the wrong place.

Should be no reason to disassemble pump or remove shaft for oil change, but I get it....find the problem. :)

No oil passes through the water pump. Period. Parts 15-23 are all working together to keep water in pump housing and oil in the lower end. (My mistake above post, 2&13 are bearings not seals).

Oil in lower unit simply keeps drive shaft spline to prop shaft bearings lubed, it has no need to go up.

Just to be sure, you've removed both screws #6 right? Both are #3 flat head. No offense I've done it myself, removed the "wash down" bolt thinking it was oil vent...

This sentence makes me ask if you've accidentally removed bolt #8 or #3 which are 10 mil hex head bolts:
"I have cleaned that and everything else thoroughly including the hole in the pump base/housing #17 that lines up with the vent screw hole."

Hope any of this helps.

ggray 04-01-2016 20:28

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Well, this is real interesting.

I have a Merc 3.3 that hasn't been used in some years. But some years ago, I had the same problem. Drained the oil, then couldn't fill through the bottom hole.My motor may be a bit older than yours, also with the F-N shift.

Since it was not being used much, I just forced some oil in. Been a long time and can't remember what I did. Maybe repeated pumping of oil in the top hole, motor lying on its side. Just can't remember.

Again, since it wasn't being used much, I didn't analyze so much as you have.

Seems like now is a good time to study further and we can compare notes.

One other thing. For a short period of time concurrently with ownership of the Merc, I also had a Nissan 2.5. Virtually the same motor. The Merc used a WHOLE lot more fuel, and I finally discovered (or thought I discovered) that the Merc had a final drive that was geared down lower, which seemed strange to me, although it would explain the higher fuel use. Same prop, more horsepower, and turning slower?? Specs called for same rpm, so WTF?

Some time later, when I no longer owned the Nissan so couldn't confirm my earlier "findings", a mechanic on one of these forums with knowledge of these motors said that all these motors had the same final drive. No ifs ands or buts. I found it hard to argue, so I questioned my ratio determination. Still an unanswered question.

But I'd be curious what final drive ratio others find in their Tohatsu clones, of which their were many.

ejlindahl 05-01-2016 17:08

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
5 Attachment(s)


My 3.3 is 1996 which I think is the first year they made it with a neutral gear. I guess previous to that it was just direct drive/forward only. Perhaps my drive leg vent blockage was a “teething” problem that was corrected in later models as I haven't seen any reference to it other than you and maybe one other person that described the same inability to fill the leg. (I think this person’s shop just took off the rear prop shaft cover and dumped in oil to fill it, which is what I will do if nothing else.) (I have had one other teething problem which is the unfinished/unmilled female bearing surface of the plastic shift lever pivot)

I don't know about the gear ratio issues but 2.5 vs 3.3 is 32% more HP so it would use that much more gas. Also, I've seen on this forum a post that only difference between the two is the the slot that the throttle lever travels thru. On the 2.5 the slot in the face plate is simply shorter than the 3.3 and if you file the slot longer so the throttle can be pushed lower down on its travel, the motor will rev up faster and produce more power. This is just internet hearsay so don’t hold me to it.

OK so I finally took some pictures of the issue they are below but here is a link with further description on Photo bucket photo bucket: I wish I knew how to insert the photo individually below each of my descriptions but with my lousy skill set I can't figure it out

If anyone can tell me how to correct this problem of the top gear case vent being plugged (perhaps with a bearing with a small [vent] hole in the outer case), please let me know. Ideally I’d like to find a Mercury mechanic of that era who came across this problem, circa 1996 or a little later. Surely many of these engines were serviced by shops and someone would have found the same problem as mine.

First photo below is of the assembly. The shift rod has been removed. You can see the water pump housing on the drive shaft. The drive shaft is stuck or more likely pressed onto the bearing and the bearing is pressed into the lower cylindrical part of the pump housing. This housing is the bottom part of the water pump.

Next photo below is the top of the drive leg where the water pump housing fits into. You can see the small vent hole in the circular chamber where the drive shaft runs through. One solution would be to gouge/router/grind a channel down from the vent hole to and past the lip in the chamber where the round part of the pump housing goes. This would allow air and oil to vent up to the hole. There is such a tight fit of the round part of the pump housing and the circular chamber above the lip that no oll can pass this way without some kind of channel.

Next below is the pump housing above the drive leg. You can see where the hole in the housing that holds the drive shaft bearing and seal will line up with the vent screw hole on the side of the drive leg. However the drive shaft bearing and rubber seal that is pressed into the housing completely blocks any passage of venting oil or air, even 150 psi of compressed air!

Next below is the pump housing partially inserted into the drive leg, again with the holes lining up.

Last photo below is a close up of the hole in the pump housing. If you click on the image you can see the glint of the outer case of the drive shaft bearing on the bottom, inside the hole, and part of the rubber seal above it. The bearing and seal are press fit into the housing and they completely block the passage of venting air and or oil. This is my problem that I want to fix. I put my compressed air gun in the hole and even at 150 psi no air got thru there! sigh. The other, small "weep hole" just allows incidental seawater to drain out of the interior of the drive leg, it is not part of the water pump/cooling water system.

The preview function on this forum does not display inserted pictures so I don't know if they will actually show up. So I will will add them here as attachments and you can figure it out what they are from my text above. If allowed I'll edit out any redundancy afterwards

I'll repeat: If anyone can tell me how to correct this problem (perhaps with a new bearing with a small [vent] hole in the outer case or maybe there is a hole but its just not aligned properly), please let me know. Ideally I’d like to find a Mercury mechanic of that era who came across this problem, circa 1996 or a little later. Surely many of these engines were serviced by shops and someone would have found the same problem as mine.

Thank you for any help on why this vent is not working.

mariner36bob 05-01-2016 18:38

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
You've been very polite in not telling me to go f* myself.
My replies were made with the utmost sincerity and good intent.
However your photographs are very helpful in showing I have no idea what I'm talking about in regards to this outboard.
That's a head scratcher for sure. And I wish you the best of luck with it.

ejlindahl 05-01-2016 19:02

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
Hi M36Bob,

You made me laugh. I hadn't take your comments that way at all. Like I said you had a couple good ideas. But I gotta figure this out. I'll take it to a local shop and see if they'll advise me. I'm happy to cough up 15 mins of shop time for a good lead or solution from them.

Blue Crab 06-01-2016 05:43

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
My 3.3 awaits the answer. Thanks.

HappyMdRSailor 06-01-2016 06:02

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

Originally Posted by Blue Crab (Post 2007780)
My 3.3 awaits the answer. Thanks.

I would continue with recent methodology.... install inverted and let the oil drain from the fill hole to the vent...

tinkrman69 06-01-2016 08:20

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I'm not sure I get what the problem is. Is the bearing over heating and seizing up? lack of lubrication? in "drive leg" are you referring to the lower unit? there are two plugs in the lower unit. the one lowest on the unit is to drain the gear oil and the point to fill it from the upper plug is to vent during the filling process. it is also to indicate when it is full. filling past this point will cause excess pressure and cause the seals to shaft and output shaft seals... If you aren't having problems with the bearing and you can't fill the lower unit with oil look for the second plug. there will be a slotted screw head just below the gear housing section of the lower unit and one just above the cavitation plate on the lower unit. remove both, drain, press nozzle of gear lube tube in the lower hole...having the plug handy to reinsert when the nozzle is removed,,,and fill till the oil comes out the top hole.

The drive shaft is not "pressed" into the upper bearing. if it is seized into there bearing I would look to the seal. if it goes water will leak down to the bearing and corrode. I've seen it happen in such a small amount that it wasn't evident in the oil. in whack case pull it and replace the bearing and seal. Be carful in pulling the drive shaft. some shafts are just splined into the helix gear in the lower unit and can can just be pulled out if you move the lower unit or rotate the output shaft while its out the gear will become misaligned. some shafts have a 'C' clip on the end of the shaft to retain the gear. in that case you have to pull the put put shaft and bearing carrier to access the bottom end of the drive shaft. Lower units are pretty simple and haven't changed much since Evinrude made the first one back in Garry Coopers time. I don't know your age, but he was a famous actor back then and was the face for the Evinrude adds.

Keep it simple man and don't look for problems that aren't there.

ejlindahl 06-01-2016 17:09

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

Thank you (and others) for the replies.

I understand that there is an upper vent hole and lower fill hole in the lower unit (which I sometimes refer to as the drive leg).

The problem is that venting air (or excess oil) can not get thru to the upper vent hole. This vent hole is blocked/plugged by the upper drive shaft bearing and its rubber seal.

If you look at the last picture in my post #6 you will see 2 holes in the water pump housing. This housing also houses the upper driveshaft bearing and its seal. The larger of the 2 holes lines up with the upper vent hole in the lower unit. (Look at the pictures in post #6 and you can see it lining up). If you click on the last picture to show more detail you will see the glint of shiny metal that is the outer case of the bearing that is pressed into the housing. You will also see a bit of the black rubber seal in the hole. That seal and bearing case is what is blocking the passage of venting air (or excess oil) when I attempt to fill the lower unit from the bottom hole. I put 150 psi compressed air into that hole and no air went through it! I am now of the mind that there is some sort of perforated spacer missing between the seal and bearing. Or the seal could be installed improperly and the spacer is above the sealÖ. A correctly installed spacer would allow venting air (or oil )to rise up thru the ball bearing and out the hole in the housing and then on through to the upper vent screw hole.

The schematic in my original post shows this housing as #17. Click the word ďlinkĒ in the sentence just above the schematic to get a better look. There is an expanded view of the seal (#16) just to the left of the seal on the schematic. This expanded view has poor resolution but it has an odd shaped squiggly thing on the left side of the view. Its possible that this squiggle is a spacer that will allow air and oil to flow thru the vent hole. I have seen no mention of a spacer in the Clyner or Seloc books nor in this factory exploded view nor anything else Iíve read about this motor but there must be one to allow the lower unit to vent.

If there was a spacer then you would not see part of the black rubber seal through the hole in the housing in post #6. Instead you would see the spacer which would allow vent air and oil to pass through.

So, does anyone know if this seal has an integral, squiggly looking, perforated spacer? Iíll ask to see the seal at my local Merc shop but Iím sure they donít stock this part.

Read back through my posts. I think I have explained the problem pretty well.

Thanks for any advice

PS for Tinkerman69: From the reading Iíve done I think you are right; the drive shaft is probably not pressed into the bearing. However when I pried the housing (#17) out of the lower unit the drive shaft came with it so it is at least a very tight fit. At a minimum I will have to do some light hammering with a wood block to get the drive shaft out of the bearing. There is absolutely no sign of corrosion anywhere. The ball bearings, races and gears in all areas of the interior of the lower unit are bright shiny metal.

When the shaft came out of the lower unit the helix/pinion gear dropped down into the hub cavity. It does not have a ďCĒ clip. So I will have to open that up. The books describe holding the pinion gear in place while I lower the shaft spline on to it. Iím not sure if I have to do this with the lower unit upside down or right side as I will have to let go the pinion gear to reinstall the prop shaft. (I donít know if the pinion gear is below the prop shaft or above). Iíll figure that out when the time comes.

Iím 63 but didnít know Cooper stumped for Evinrude. He must have been a yachtsman like other Hollywood types.

I appreciate yours and everyones efforts to help me solve this riddle.


ggray 06-01-2016 20:49

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
I played a short while with this issue tonight, and not really sure what is going on.

I wondered if in the past, the end of the spout on the quart bottle was cut square and just the right length that it was mashed up against something in the final drive that prevented oil from entering, but that's not the case. The spout I used tonight had an angle cut on the end. It accepted oil at first...I could hear snapping and popping of air bubbles in the oil. Until the air passed, and then it was like hitting a brick wall, wouldn't accept any more oil.

I have seen the lower unit removed from only 3 different motors (so no expert) and it's been a while. But I don't recall seeing anything like what your pictures and the diagram show. As has been posted above, I thought the final drive was a separate, sealed unit and oil in it didn't travel further up.

(As an aside, about 30 years ago someone stole a Mercury off my boat on a mooring. Then about 15 years ago, the guy rebuilding our dock said he had found a Mercury in the mud. The guy who stole my motor had dropped it and didn't even try to recover it! The mud had protected a lot of the motor, but it did have some barnacles. I asked a mechanic if anything would be salvable, and he said likely only the lower unit as it was TOTALLY SEALED. I was able to sell it.)

Anyway, I turned the 3.3 upside down, and baby fed oil in the now top hole, a bit at the time until I think it's full. I've turned it right side up, and tomorrow, after the oil settles, hopefully, I'll open the top hole and let it drain for a day.

We still haven't answered the question.

ejlindahl 06-01-2016 23:20

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

The gear oil continues 5 or 6 inches up a ~1.5" round drive shaft chamber in the lower unit to the level of the upper drive shaft bearing. It lubricates this upper bearing so its important to completely fill the chamber.

There is a seal just above the bearing and keeps water from the exhaust housing above the lower unit out of the contiguous, oil filled, prop shaft and drive shaft chambers. The top vent hole that should be open when you fill the chambers from the lower fill hole is at the level of this top driveshaft bearing and that is why its important to keep filling oil in from below til it dribbles out of the upper vent hole. Unfortunately the bearing and seal block my top vent and thus, even if I could get oil in the unit, it is impossible to know if I've filled the it to the correct level. I'm doing further investigation tomorrow.

ggray 07-01-2016 08:29

Re: Unable to fill the lower unit with gear lube on my 3.3 Mercury Tohatsu, Evinrude
This seems a strange set up. Again, I've little experience working on lower units, But I haven't seen one like this. How does that top bearing get lubricated if it is that much higher than the oil level? Splash?

I'll soon be removing the lower unit on a 30hp Tohatsu to see how it may be different (need to install a new impellor). I want to see if there are any shaft bearings installed like what your diagram shows.

I appreciate your informing us of what you are going through. I've got too many things (boats) calling for my attention to get myself heavily involved in a motor that I don't need at the moment.

But I may take a moment to determine the gear ratio of the lower unit, and hope I can find the ratio of other Tohatsu clones. I still have a nagging suspicion that there is a difference.

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