Can't refill outboard gearbox!

I removed both plugs from my little Tohatsu 3.5 2 stroke gearbox and drained the oil.

Then I made an adapter to fit the oil canister spout tight into the lower oil hole. After trying to squeeze the oil into this lower hole - a small amount went in but no more!

Anyway, squeeze and squeeze but cannot force enough oil in to flow out from top hole, it just builds up pressure to force it out again from the bottom.

I just cannot see if there is a blockage or problem. Both plugs are removed - there is now oil everywhere!

 

Are you sure you have removed the correct top screw ? Don't know that engine, but some engines have three screws on the lower unit, one being to locate the gearbox innards properly, the others the upper and lower oil fill screws.

 

You've got me worried, so I went down immediately to look.

Its a very small outboard and there are only the two screws, the top is marked 'oil level' and the bottom marked 'oil'.

Both are removed for filling. I looked and poked into the top hole which has possibly a valve? at the bottom of its thread.

I am stumped. The next step must be to dismantle the top of the box where the impeller has its mounting in order to delve into the innards. I did try a sharp chisel to dislodge its gasket but even that doesn't want to budge!

I'm trying not to cause more damage than good - only wanted to replace the oil!!

 

I don't know what is thwarting your attempt to fill it. Try blowing some air into the bottom hole and see it it comes out the top hole, i can't see what could stop it. If the oil drained out easily, it should go in just as well.

 

It's possible some sludge has dropped down and is partly blocking the lower hole. Pour kerosene into the top hole and as it tries to drain out, put the lower plug back in. Let this sit for a few hours, hopefully dissolving the sludge. Remove the lower plug, then the upper one and let the kerosene drain out. Examine this liquid for gunk and stuff. If it's clean, try to force oil up through the lower hole again. If it has some junk in it again try to flush it with a thin petroleum product like kerosene. Don't poke any more tools into the holes. The only thing that can possibly be in there is sludge, which will break down with more refined petroleum products. Once you've unclogged the gear case, the "cleaner" will need to be flushed out, so start with transmission fluid, which is thin enough to do a good job. Remove the spark plug and slowly rotate the engine, while the trans fluid is draining. When you're sure everything has drained out, try to force 80 - 90 gear oil up the lower hole again.

Before doing any of this try tilting the engine up and see if it'll take any gear oil. The nugget of sludge may move enough to permit some flow in. If it does, keep the hose or bottle tip in the hole and lower to vertical again and continue squeezing the oil in.

 

Might be that when he shoves the nozzle of the filler bottle in the lower hole, it is bottoming out, and not allowing oil out the end.

 

The gearbox is removed, I can tip it any way up.

I have squeezed some oil through the bottom hole, but cannot continuously squeeze it to appear at the top oil level hole - it just builds pressure and blows back out.
I thought when the top bung is screwed in that vacuum prevented the oil draining back out from the bottom? Well the oil I have squeezed in the bottom hole certainly pours back out easily with the top bung screwed back in.

I don't know the internals of the gearbox - is there a valve inside the top hole?
The bottom hole appears quite clear and free of any blockage. The old oil poured out dark but but quite clear and free from particles.

Maybe now dismantle the the top of the box and delve into the internals to find how it works . . .

 

Try turning it upside down and filling it from the bottom hole which would then be on top. That is, fill it from the top hole which was formally the bottom hole. Crystal clear, no?

 

No, it is a really simple gearbox. I assume you have the "start in gear" model. There must be some sludge clogging it. Was there any oil flowing out of the bottom hole?

 

I can pour the oil in the bottom hole with the box upside down but how do I fill to the correct level? I would like to fill it as it should be filled!

It's a start in neutral model. As I mentioned above the old oil flowed out easily and contained no sludge or particles. I have already been able to squeeze some new oil in the bottom hole but only about say, 30 mls which flowed out again. Any more was under the pressure from my squeezing.

I presume there must be a one way valve which is blocked or not functioning. Something inside the box is preventing the air flow through the top hole so I'll dismantle it to see.

Thanks gents.

 

Well, it would be the same thing. When oil flows back out of the top hole, the thing is full.

What purpose would there be for a one way valve? I'm guessing none. Which way would the one way valve be regulating?

 

There's no one way or any other kind of valve, on the oil drain/fill holes.

 

http://tohatsu.com/tech_info/oil_change.html

According to this there is no level to fill it to just squeeze it in till it comes out of the top. Or not, unfortunately, in your case.

 

Fill from the top, slowly. Rotate the gear set, slowly, then check the oil level again. Do this several times, to insure no air bubbles are giving a false reading.

 

I've done several gear box oil changes on this engine, or derivative. If you have a problem, it may be worth taking the prop off and removing the back housing. Very simple to do a couple of bolts...

This should allow you to see the simple 45 deg cut gear and engagement. If the oil has not been able to go from the bottom to top fill/drain hole you may have a lot of sludge/mayonnaise inside the box. Clean this out, refit - empty but with light clean oil on teh meshing parts, and refill. You may or may not need to renew the washers on teh 'box depending on their material. The fibre ones tend to disintegrate but alloy ones can be reused a couple of times.

If what you drained was very 'milky' that is a good sign that water has entered the mix. Personally I like to change the g/box oil each year or max 2 years depending on use and hours. I assume you have changed the impeller? If the oil is in that bad a state it would be imperative to change it too.