Power trim won't hold against reverse or trim back down under full throttle.

Here's an odd (I think) problem I encountered today in the first time afloat in my new (to me) boat.

94 Johnson 50 hp. Had trim cylinder resealed prior to putting in the water for the first time.

The unit will trim against engine thrust at full throttle but will not trim back down until the engine is at perhaps half throttle as in a situation requiring trim down to reduce porpoising. Also, it will not hold against the reverse thrust when backing off the trailer with any significant engine RPM in reverse.

Otherwise it seems fine. Doesn't bleed down under thrust and seems to function fine otherwise.

Ideas?

Steve

Re: Power trim won't hold against reverse or trim back down under full throttle.

my first guess would be reverse check valve.
not sealing well enough for slider to overcome back pressure from prop. Which causes front check valve to stay closed. Which cause engine not to drop under load. In reverse is just directly bleeds by allowing engine to lift up in reverse.

However, I do not waste time trying to figure out which part is broken. I just rebuild the whole things from top to bottom and leave no room for error. Actually takes less time in the long run because you never chase the wrong problem or do the same job twice.

Re: Power trim won't hold against reverse or trim back down under full throttle.

In my factory manual it shows that there are externally accessible and internal check valves. Which are you referring to as a first step? If the externally accessible valve is the suspect can it be removed and cleaned?

Steve

Re: Power trim won't hold against reverse or trim back down under full throttle.

I'm posting this follow-up for the possible benefit of others.

Well I finally got my OMC shop manual for this engine and it states the following.

"When the unit is operated in the UP direction or when the engine is running in FORWARD gear, the following components must hold pressure:

o Up Check Valve
o Up Relief Valve
o Impact Relief Valves
o Piston O-Rings
o Expansion Relief Valve
o Manual Relief Valve."

When the unit is operated in the DOWN direction or when the engine is running in REVERSE gear, the following comnponents must hold pressure:

o Impact Relief Valves
o Up Relief Valve
o Piston O-Rings
o Down Check Valve
o Filter Valve and Seat
o Manual Release Valve."

If one does a process of elimination to consider those items that seem to function in the up direction (since it will trim up under full power) that leaves the Down Check Valve and the Filter Valve and Seat. I almost dug into the manifold to check those two valves but that the manual release valve actually has two sealing seats (one for up one for down hydraulic circuits) I would first attempt to ensure it was tight to spec. It was. So in the hope that I might flush by any debris that might be causing an o-ring or sealing surface to not seal I released it, cycled the engine manually up and down 3 times, ran the pump for 5 sec in up and in down directions, and reseated the valve.

IT WORKED!

Damn I'm either getting good or I'm very lucky. Actually the Army trained me mighty well in a whole lot of things. If we had a lottery in this state I'd buy a ticket!

Steve