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Author Topic:   Mercury: Oil System: Carburetors Flood With Oil
wopf10 posted 12-27-2007 08:23 AM ET (US)   Profile for wopf10   Send Email to wopf10  
Hi--I have a 1997 Mercury 150-HP outboard which has been a pig to start for some time. I left it sit for some months and could not start it at all. I found the fuel line and carburetor bowls flooded with two-stroke oil. Obviously, with the main oil reservoir remaining pressurized, the oil has made its way through its various components and displaced the fuel.

Do you have any advice as to what component is meant to prevent oil flowing when the engine if off?



jimh posted 12-27-2007 09:47 AM ET (US)     Profile for jimh  Send Email to jimh     
Steve- See this article for a detailed description of your Mercury oil system:

There is a [regulator] valve which collects engine crankcase pressure and pressurizes the remote oil tank, which as you know, is the method used to move the oil from the remote tank to the under-cowling reservoir.

wopf10 posted 12-27-2007 07:43 PM ET (US)     Profile for wopf10  Send Email to wopf10     
Hi jimh,

Thanks for your comments, much appreciated. And the reference text is very helpful.

I have placed a manometer in the line between the crankcase and the oil reservoir and found the pressure is acceptable. From the diagrams you provided, the pump appears to be what should stop the flow of oil to the fuel system when the engine is not running. If it is worn, it will allow oil to seep past. I assume there is an overhaul kit for the pump?

Do you know what type of pump it is? Gear pump? Rotary disc? Do you have any articles on the pump?

Thanks again,


jimh posted 12-27-2007 08:14 PM ET (US)     Profile for jimh  Send Email to jimh     
Steve--The oil mixing pump is described as a gear driven pump, but I am not familiar with its details.

The flow of oil from the under-cowling reservoir to the mixing pump is due to gravity, so that force is present all the time, no matter what the state of the engine is, running or not. So, yes, your observation that the flow of oil must be impeded by the pump is a good one. Otherwise gravity would cause the oil to flow past the pump. However, I wonder if there is some accumulation of pressure which is not relieved when your engine has shut off, and this extra pressure is causing the problem. One way to test this hypothesis is to spoil the seal of the system at the remote tank. This will remove any pressure. If you still have the oil leaking past the pump, then it is being caused just by the flow of gravity, not extra pressure. If that result is obtained, then you would have shown that the problem is likely in the pump.

[Deleted some comments about the 2-PSI check valve shown as "5" in the diagram. This valve opens if there is negative pressure or suction in the oil line greater than 2-PSI. This condition could occur if there was a blockage in the line upstream, for example, if the oil pick up became clogged.--jimh]

wopf10 posted 12-27-2007 08:40 PM ET (US)     Profile for wopf10  Send Email to wopf10     
Both oil tanks remain at crankcase pressure after the engine is stopped. A check valve in the pressurization line holds the pressure in the main oil tank which is equalized to the engine reservoir via the oil line. Therefore the pump must hold back oil at crankcase pressure.

I know of many people who complain that older mercs are hard to start with symptoms that match. This is probably an inherent problem.

You don't happen to have a manual with a diagram of the pumps internals?

I am now chasing a overhaul kit for the pump, but the dealers over my way (australia) will only sell me a complete pump. Are there any websites who sell parts?


gss036 posted 12-27-2007 11:12 PM ET (US)     Profile for gss036  Send Email to gss036     
I don't think there is a kit to rebuild the oil pump. I don't think that is the problem. I think that the main oil tank is holding pressure and not de-pressurizing as it is supposed to do. The check valve on the lower starboard side of the engine, (pulse fitting) is supposed to allow the pressure in the oil tank to bleed off after the engine is shut down. If that doesn't happen, then the pressure keeps pushing the oil through the system and into the carburetors.
You might try opening the oil tank cap when you come in from a trip and then see if it has the problem. But, this is usually the cause of excess oil in the fuel system.
I think you need to change the pulse fitting (2-PSI valve).
wopf10 posted 01-04-2008 12:19 AM ET (US)     Profile for wopf10  Send Email to wopf10     

Thanks for your post. I am a little confused about which item your are refering to? I assume your reference to the pulse fitting refers to the check valve between the crankcase and the air supply line to the main oil tank?

This can be seen on the drawing on this link labeled as crankcase pressure with one way check valve.



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