115 Yamaha trim tilt, won't stay down PROBLEM SOLVED

[Bonefish1]

I have a 1999 115 Yamaha outboard, with the Showa intergrated trim tilt unit. This unit was resealed about a two months ago, and operated normally for two weeks, then it developed an oil leak from a corrosion spot on the pump body. I installed a new pump body. Now the unit will trim up and down okay, and will hold up trim position when under power, but will lift up when powered in reverse. The manual release valve is secure.
The local shop I purchased the parts from tells me that the problem is air in the system. I've spent a lot of time purging the system, and do not believe this is an air problem. I could diagnose this myself if I could find a schematic of the hydraulic circuits in this unit. Can anyone help?

BTW, I've been in the boat repair line of work for many years, and am not an amateur.


SOLUTION

Removed and inspected the manual relief valve: I replaced a badly damaged oring, the inner one. The unit now stays down while under throttle in reverse, also there is a change in the sound of the pump when the unit is trimmed fully down, as it was before. Problem solved, thought I'd pass it on.

 

[rodbolt]

Re: 115 Yamaha trim tilt, won't stay down

Re: 115 Yamaha trim tilt, won't stay down

could be air, that unit is difficult to bleed.
however I would look for debris in the tilt piston or the upper chamber check valve.
could also be a bad thermal valve or a bad memory piston valve or oring.
could also be a bad tilt piston oring.

 

[Bonefish1]

Re: 115 Yamaha trim tilt, won't stay down

Re: 115 Yamaha trim tilt, won't stay down

Thank you for replying, Rodbolt.

Yes, it rather acts like air, as when it lifts up in reverse, and throttle is reduced, the drive falls (slams) down rapidly. I'm having a difficult time believing I've not gotten all the air out of it; I've used every method I know of, most of them several times.

This is our main workboat, can't haul it out for too long as we are 2.5 mile offshore and need it daily. I'll give it another go this weekend.

Again, thanks.

 

[Bonefish1]

Re: 115 Yamaha trim tilt, won't stay down

Re: 115 Yamaha trim tilt, won't stay down

Thank you for replying, Rodbolt.

Yes, it rather acts like air, as when it lifts up in reverse, and throttle is reduced, the drive falls (slams) down rapidly. I'm having a difficult time believing I've not gotten all the air out of it; I've used every method I know of, most of them several times.

This is our main workboat, can't haul it out for too long as we are 2.5 mile offshore and need it daily. I'll give it another go this weekend.

Again, thanks.

I forgot to mention, if the unit is tilted up all the way, the pump pressures up and strains under load (normal) but tilted all the way down, it will not pressure up, very little change in the tone of the motor. Acts like a pressure relief valve is weak, but in the Showa unit I'm not sure which one that is.

 

[rodbolt]

Re: 115 Yamaha trim tilt, won't stay down

Re: 115 Yamaha trim tilt, won't stay down

showa unit doesnt use pressure on the down circuits.
so it wont stall down like it will up.
also wont take much over about 1800 RPM in reverse and up it comes.

 

[Bonefish1]

Re: 115 Yamaha trim tilt, won't stay down PROBLEM SOLVED

SOLUTION

Removed and inspected the manual relief valve: I replaced a badly damaged oring, the inner one. The unit now stays down while under throttle in reverse, also there is a change in the sound of the pump when the unit is trimmed fully down, as it was before. Problem solved, thought I'd pass it on.[/QUOTE]

 

[BaileysBoat]

Re: 115 Yamaha trim tilt, won't stay down PROBLEM SOLVED

SOLUTION Problem solved, thought I'd pass it on.

[/QUOTE]

Thanks for letting us know. Glad all is well.