mercury lower unit oil change

I got a 79 20hp mercury motor yesterday. I went to change oil on th3 lower unit and unscrewed the drain plug. Very little oil came out. Unscrewed the bent plug and nothing. I hooked up my tube to pump oil into it and I cannot get it to take oil or very little. I went for a ride in the boat it was on yesterday and it seemed to shift and run good. Shaft seems to be tight with no play. Any idea why the lower unit won’t take oil? I hope this is an easy I feel stupid fix! Thanks. Shawn

Are you sure you took out both plugs and not another random screw? Can’t think of any reason an empty lower unit would not fill.

first make sure nothing is blocking the lower fill hole,use a tooth pick or such.
as far as the vent hole,it sticks in my head there was a flush screw about the same size as the vent or fill plug down low on those older mercs,if you removed it mistakenly thinking it was the vent,putting oil in will pressurize the lower unit and make filling it a bugger.

its been a while since I have worked on one that old,so I could be mistaken on the flush screw.

OK. Iowaboy idea sounds good? There is a plug about the same size on this mercury that does say “flush” that is what I removed thinking it was the vent plug. So where the heck is the vent plug? Thanks. Shawn

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Here is the vent plug. Other one is drain plug.

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Or maybe the last picture is the flush screw? It does says flush right there. I blew a little air in that hole and the air comes out the vents. So where the heck is the vent screw?

the screw in your picture is the fill/drain hole,the vent should be just below where the lower unit separates from the exhaust housing,or about one inch below the water pump housing,you have to remove that screw to completely drain it as well as a vacuum sets up in there preventing the oil from draining out.

it could be on yours the vent screw is on the other side of the lower unit but it will still be about where I said.

the vent is that little screw in your picture to the left of the flush screw,didnt see that pic until after my last post.sorry

OK. I removed the bigger screw next to the small vent hole screw and I cannot get oil to push into the lower fill screw. Ya the oil will flow if I don’t have it hooked up. If I try to pressurize the bigger vent hole towards the top I get no air to come out the bottom drain plug. The guy I bought it from had a new water pump put in before I bought it. Could that be an factor?

okay,the vent screw which is the little one has to be out.
you dont need to remove the flush plug,that is for flushing the cooling system only when used in saltwater or has been run through mud/sand.

hook up your oil jug to the lower screw hole and pump it full til it runs out of that little hole,if you cant push oil into it,I am wondering if there isnt something wrong with your pump when hooked up,something internal may be blocking it.

I have replaced hundreds of water pumps and never once saw where that would have an affect of any sort on being able to fill it with oil as the only thing the two have in common is the seal in the lower part of the water pump housing.

I want you to try and fill it from a regular tube of oil,if it takes oil then,you know there is something wrong with your pumper.

Hey this is what I will try tomorrow, of course. All the motors I’ve ever owned the vent plug was the bigger plug. Evinrudes. I’ve never seen a vent plug that small. Thanks!

you are welcome!! in over thirty eight years of doing this as well as working on cars,tractors and airplanes,I have had to ask for help along the way and am just paying back all of those that helped me.

before there are any smart alec comments on all of the things I have worked on,growing up on a farm will teach you to be self sufficient thats for sure,like they say,necessity is the mother of invention!! lol

I spent half hour on the phone with Iowaboy1. Class act guy that knows his stuff about motors. Phone time wasn’t about helping me with my motor, he did that on IDO in public forum. I called him and thanked him and we Bs’ed. Never met him but he knows his motors. Thanks!

Easy fix and my stupid.

Thanks IDO. Shawn

This is excellent. I once had a nice Canadian guy talk me through popping the flywheel off a motor. Good to have guys with some knowledge helping out from time to time.

For the record, was it just the wrong screw?

dfresh wrote:

For the record, was it just the wrong screw?

Yep it was if your keeping records.