New 4 stroke making oil

Is it possible for an engine to make oil because the fuel line is still connected between engine and tank, when engine is not in use? A mechanic advised me to disconnect the fuel line from the engine when not in use especially on warm days. He explained that the tank swells and forces gas into the engine through the hose. Then the gas somehow gets into the engine oil. Makes sense but just wondering if anyone has heard of this or experienced it with their engine.

It is possible, but not likely.. a few scenarios would have to be met.
1)The engine would have to be carburated.
2) the carburetor would have to be above the lower cylinder.
3) the intake valve would have to be open on the lower cylinder.
4) the needle and seat in the carburetor would have to be partially worn to allow some leakage.

So in theory, if the hot fuel tank could produce 5psi it would push fuel up to the engine. If the needle and seat were worn to allow leakage, the fuel could then gravity feed from the carburetor down to the lower cylinder. If the valve was open it could get into the cylinder.
Like I said, possible, but highly unlikely.

How many HP?
I’m thinking idling too long is a possibility?
During my break-in period I made that mistake.
No issues after normal operation.
Be sure to change your oil.


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Common issue with 4stroke outboards. Go to a forum that is more big motor oriented and you will see a lot of posts about this issue.

Either fuel or water condensate...with a 140f or equivalent temp thermo does not help..If ya troll for a while open her up and get some temperature into it.

Some get the oil analyzed to find out what the added liquid is..

Another cause is improper break-in and failing to set the rings properly! Push her up to plane, not like you stole it, but purposeful. Cruise a bit, back off plane & repeat the cycle. Repeat, repeat ... then a few up on plane like you mean it ... fast.

Do NOT ‘baby’ 4-strokes OBs or they can display the dreaded ‘making oil’ condition. And improperly set rings does nothing for a long engine life.

Also make sure it wears the right prop & at WOT that you can run revs to no more than 100 (ideally) to 200 (at worst) RPMs off (lower than) the max rated RPMs.

I have been battling this problem since the engine was new and now have over 80 hours on it. Just when I think the problem is gone it comes back. I was hoping that leaving the tank and hose connected to the engine when it’s not running was the culprit. I thought I did a good job of breaking it in but maybe I babied it too much. I will try putting it under a load as suggested and see it that will correct it. Thanks for all of the input.

on those the #1 cause is excessive idling. #2 is failed fuel pump. Low pressure pump mounts on the valve cover and runs off of a rocker arm. If the diaphragm ruptures, it'll still work but leaks fuel directly into the engine. This would be a large amount of fuel not just a little bit every once in a while. As said, open it up once in a while. You don't have to beat on it but it's good to open 'er up with a load on it.

Max RPM is 6000. Don't be afraid to run it 6100 or whatever, ain't gonna hurt it. You need to prop it correctly so that it will let you run 6000 or so at full throttle. If you are overpropped like a lot of them are, it is going to make oil.

Thanks for all of the excellent information.