The Mystery of the dropping Mercury Trim!

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Hi guys, 

Im in a bit of a situation trying to help out my father in law with a trim issue on his 2016 90HP  4 stroke Mercury outboard.

The problem is that the motor loses trim when you go higher than 10 knots or so (it trims itsself down, im guessing due to the thrust from the prop applying pressure to the hydraulic pump). You can trim up at 10 knots or so but as soon as you stop trimming up, the boat begins dropping trim fast. 

The trim level does not drop at low speeds.

There is no hydraulic leak anywhere that we can find, likewise, we have purged the trim mechanism a few times to ensure there is no air in the system.

We tried a soap spray over the ram and associated hydraulic trim components and this did not show any air leak (not that this would happen, but when you start running out of ideas you begin trying everything!).

We have tried leaving the motor trimmed up on dry land overnight to see if it would drop overnight ( like a small leak may) but the motor did not drop overnight at all.

We have even taken the motor to a dealer to have it looked at and he thinks that there is nothing wrong with the trim/tilt mechanism, thats how its supposed to work!?

I am having a hard time accepting that the trim on an outboard is meant to drop when travelling at more than 10 knots or so.

Does anyone have any suggestions we may have missed?


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I have a 2006 version of that motor.  I cant trim higher than about 3/4 up (on my gauge) when running over 2000rpm.  I found this when running in shallow water I just have to keep the revs below 2000 which means I cant go any faster than about 6-7 knots.  I just assumed it was a safety thing to prevent sucking air into the prop when its close to the surface of the water.


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only thing i can think of is dodgey down relay ,maybe swap with the up relay to see if any difference

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