Mercury 20 HP Back pressure when filling lower unit.

[cwattsx5]

I think its a 1980 or 81, the S/N # is 2888354. It has not been started in at least 5 years and so I decited to change the gear oil in the lower unit and some other things to get it running. I pulled the upper plug (flush ) and the lower drain plug. Not much of anything came out. I started pumping in the gear oil and received some back pressure. The oil started seeping from a nut and bolt a little higher on the unit. It never came out the top hole. Im thinking it must have a clog or something. Can I use sea foam to loosen it? Can anyone please give me some advice. Thanks in advance.

 

[Alumarine]

Re: Mercury 20 HP Back pressure when filling lower unit.

I don't think the flush port is the oil vent.
It's for flushing fresh water through the motor I think.

Look around for the vent plug.

 

[rodbolt]

Re: Mercury 20 HP Back pressure when filling lower unit.

you pulled the wrong plug, yes the one labled flush is for cooling water when using a flush attachment,you failed to drain the unit as without the vent removed it takes hours to drain.

 

[cwattsx5]

Re: Mercury 20 HP Back pressure when filling lower unit.

What does the Vent plug look like, Is it a small screw by the flush plug?

 

[Alumarine]

Re: Mercury 20 HP Back pressure when filling lower unit.

It should be the same size as the drain plug.

 

[cwattsx5]

Re: Mercury 20 HP Back pressure when filling lower unit.

With your help I figured it out, Thank you

 

[charly1954]

Re: Mercury 20 HP Back pressure when filling lower unit.

It should be the same size as the drain plug.

I have a 1968 merc 200 20hp and my lower unit vent plug screw is at least half the size of the drain plug. The big screw right next to the gear lube vent screw is same size as my drain screw, its the flush screw plug not the vent. Found that out the hard way. I couldn't figure why the gear oil tube was hard to squeeze, nothing was going up into the unit, and when I removed it the oil sprayed out with pressure. I to thought the big screw plug was the vent, the small screw with about a 1/8" hole is the vent. WHen I removed the small screw I emptied the gear tube in about 15 seconds and oil ran out the small 1/8" hole.

 

[Dawson130]

Looking at mine today it’s a 78 and realized the vent screw is the smaller one to the left of the flush screw.

 

[GA_Boater]

Looking at mine today it’s a 78 and realized the vent screw is the smaller one to the left of the flush screw.

Same way a 6 years ago.

Closed.