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Author Topic:   Dual hoses in a 2-stroke oil system
Frank O posted 03-15-2015 11:29 PM ET (US)   Profile for Frank O   Send Email to Frank O  
My old 1994 Mercury Offshore 225HP carbureted outboard and my current 2011 OptiMax 250HP direct fuel injection outboard both use(d) a 2-stroke oil system with a reservoir located in the boat's console. Two hoses connect the engine to the reservoir.

How do these hoses work together to move the oil? Does the engine exert positive air pressure through one hose to push oil into the second hose and back to the engine? I know it's important to make sure the reservoir cap is tightly sealed.

Frank O posted 03-16-2015 12:24 AM ET (US)     Profile for Frank O  Send Email to Frank O     
Actually I just found this:

which appears to confirm my suspicion.

So, if there is a leak in the air pressure hose (the hose from the engine that pressurizes the oil reservoir tank), will inadequate oil be delivered to the engine through the oil line?

And, if there is a leak in the oil line, presumably excess oil will be forced out of the leak when the engine is turned on and pressurizes the oil reservoir?

floater88 posted 03-16-2015 05:40 AM ET (US)     Profile for floater88  Send Email to floater88     
Sounds right too me. I have the same motor in a 200 hp. My oil alarm went off one day so I cracked the screw where the lines enter the motor and bled off some oil and problem fixed. Must have been an air bubble cus I had just refilled my main jug.
RevengeFamily posted 03-16-2015 05:54 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     

I own and maintain twin 1999 Mercury 225 Optimax engines...

To answer your question, "Yes"... which you appear to have done on your own. The 2.5 gallon oil reservoir tank is pressurized through a 3/8" hose from the engine crankcase.

It is important to keep the cap on the oil reservoir tight as well as the on block oil tank cap tight. If the on block cap is loose, oil from the reservoir will be forced into the on block tank and leak making a royal mess while the engine is running... And if the cap on the reservoir tank is loose or cross threaded,resupply of oil will be slow or non existent to the on block tank.

The on block tank is the only tank with any type of alarm built into the Optimax system. If the oil level in the on block tank drops down 1.5-2.0 inches a float switch will activate a low oil alarm...

Hope this helps,

jimh posted 03-16-2015 08:46 AM ET (US)     Profile for jimh  Send Email to jimh     
To learn more about the Mercury oil-fuel mixing system, in addition to the reference article (mentioned above and given a hyperlink), I recommend reading the follow-up discussion related to that reference article, which is found at

Frank O posted 03-16-2015 07:39 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks. Apparently I have a leak in the oil supply hose running under my boat's deck. Mercury only sells the set of dual hoses in a single length, 9 1/2 feet, which is a stretch given the distance from my engine to the console and various bends along the way, etc. If I conclude I need to extend the hoses a couple of feet, does anyone know if there's an equivalent hose I could splice on the end that's rated for oil etc?

RevengeFamily posted 03-19-2015 05:50 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
Any hose that is rated for fuel is more than capable of handling two-stroke oil. Instead of splicing hose to hose, I would buy enough fuel hose to make the run from tank to outboard.--Norm
Frank O posted 03-23-2015 07:47 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Actually it sounds like Mercury makes longer hose sets, so I should be okay.

After I install the new hose pair, is it necessary to bleed the system? Or do you just make sure there is 2-stroke oil in the smaller reservoir within the outboard, and then run the engine until the oil gets pushed through the hose?

RevengeFamily posted 03-24-2015 07:38 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
To purge the new oil line of air is relatively straight forward...

After installing the new hose set, make sure the reserve oil tank is full of two stroke oil. You should remove approximately an two inches of two stroke oil in the on block reservoir... This will allow you to push oil through the new supply hose with out having it bubble out of the top of the reservoir...

With the engine cowling removed, start your Opti... You may get a low oil alarm at the helm station. This would be normal and a good test of the float switch that is inside the on block oil reservoir...

Allow the engine to run for a minute, this allows the pressure in the oil supply system to build... Now loosen the black cap on top of the on block reservoir... This allows the air to purge from the reservoir... After about 15-30 seconds you should see the oil level in the reservoir starting to rise as the oil is moving through your new hose... When the oil rises to within a 1/4 to 1/2 inch from the top, tighten the oil cap...

Turn off the engine, then restart it... The low oil alarm should not sound and as long as you keep your main reservoir from going empty, the oil level in the on block reservoir should not change... If you do run your main reservoir empty, you will need to repeat this purging procedure after refilling the main oil tank...

Hope this helps,

Frank O posted 03-24-2015 09:32 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Thanks much, Norm. Those instructions are very clear, and should be a big help.

RevengeFamily posted 03-25-2015 05:21 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
No worries Frank, Let us know how you make out...


Frank O posted 04-03-2015 06:07 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Norm, we got the hoses hooked up and air purged today. Worked like a charm. Thanks again for the help.
RevengeFamily posted 04-07-2015 07:56 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
Sounds good Frank, glad I could help...

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