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I recently purchased a 2005 Montauk 170 with a 90HP Mercury 4 stroke on her. Love the boat - not that crazy about the motor since it is a carbureted engine and I spend most of my time trolling at slow speeds. Nonetheless, I had the engine checked out before the purchase. It ran good and had good compression.

My problem is gas in the oil. After noticing the oil way up on the dip stick, I drained 7 quarts from the engine that the manual says should have about 5 quarts in it. The presence of gasoline in the oil was obvious.

I have read this is not uncommon, especially at slow speeds. Possible causes are:
- A carburetor is bad causing the cyl(s) to fill with gas when the engine is shut down and slowly weep past the rings over time.
- A bad intake manifold gasket.
- Rings not properly seated, although if this were the case, seems like oil would be getting above the piston and the motor would be smoking more... the engine runs greats, especially at 3000 RPMs and above.
- Engine oil not getting hot enough.

Any comments or suggestions?

Thanks, Michael

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My best guess would be rings not yet seated.

If that's the case it is probably the easiest to correct. Try taking the boat out for a few hours and running it like a dog. Lots of WOT runs to make sure the engine is getting fully up to operating temperature. Read the break in procedure in your manual but ignore the part that limits high speed operation before running for xx hours.

Another possibility is a faulty thermostat not allowing the engine to get up to the proper temperature.

Are you able to reach the recommended WOT RPM?

I believe long hours of trolling is OK with your 4 stroke but I would give the engine a good high speed run after trolling while on the way back to the ramp.

Due to the very high pressures of compression unburned gasoline is far more likely to leak down into the crankcase than is oil to leak up into the combustion chamber.

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Thanks Butch. I can get to WOT - at or near 6,000 RPMs. I think you are right on target. That is the same advice of a marine mechanic friend of mine. I will give this a shot and let you know how it goes.

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Reports of the four stroke Mercury 90 making oil are very, very common. There has been much prior discussion of this phenomena. Among the explanations offered are:

- Rings not fully seated

- Faulty thermostat allowing the motor to run cool (this is corollary to the rings not seating)

- Low speed operation like trolling allowing the thermostat (defective or not) to open too soon and not allow engine temperature to rise to normal level (this is corollary to the two points above)

- Defective fuel pump

- Defective MAP sensor

We normally hear of these motors making oil that allows the level to rise above the FULL mark on the dip stick but draining 7 quarts out of a five quart capacity motor is rather shocking and makes me think the last two possibilities are more likely than the first three.

If you Google mercury "making oil" you will get a great number of hits but here is One Particularly Interesting Discussion.

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Thanks Tom. The 7 quarts surprised me too. At dead level, it was literally coming out of the dip stick port before I drained it into a graduated bucket. It should be noted this was after I had been trolling at idle speed for three or four hours. I don't think I even got her on plane that afternoon. I let the motor sit upright overnight and got a little more oil out yesterday before adding 5 quarts of new oil. After idling a few minutes, it measured right where it should on the dip stick.

I plan to get her in the water this morning and run her hard all day as much as possible. WOT 4 or 5 minutes at a time, back off for 10 or 15 minutes, and WOT again - back and forth. Then I will check the oil level and see what I have. This should help seat the rings if they need it and confirm the fuel is entering the crank case at low speed operation.

When I was a kid we used to check a thermostat by putting it in a pot on the stove and check the temperature of the water when it opened. Is it still that simple or should I just replace the thing for good measure?

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I did that exact thing last summer for our '07 ETEC as it was having overheat issues after long trolling. I had new t-stats anyway, but I think it's always good to diagnose each questionable component so you know for sure what role it's playing.

I should add that I boiled the new ones along with the old and watched each as the temp rose. The new ones opened about 7 degrees before the old ones. Not a lot, but enough to keep the latent heat buildup down so that no more alarms triggered. Feels good doing it that way vs just putting new parts in and hoping the issue gets resolved.