Mercury 90 power trim doesn't hold the motor up.

[katch_me]

I have tried everything and looking for some fresh ideas. I have a 81' Mercury 90 6 cylinder, it has 2 trim/tilt cylinders and the pump w/ valve is inside the boat. 4 hoses. So the problem is that the motor will drop. When testing somethings it will fully drop in a couple days and somethings it will drop in a couple hours.

I have done many things and I can't seem to stop the problem and the problem always seems the same. What I have done.... tried 3 different control valves/pumps, replaced the o ring in the reverse lock, tried the 2 sets of cylinders, opened and replaced o rings in 4 cylinders, replaced the oil with 10w40, replaced and checked the hoses. As you can see I have tried almost everything. And yes, bleed the air out of the system many times.

I know the pump/control valves are used parts. The people I have bought them from says they work fine. I know there could be an internal problem, but 3 difference units I tried. You would think that one would be ok. Or at least the problem would be different.

A couple other tests I did; I removed the 4 hoses and connected the 2 small hoses together, and the 2 big hoses together. The motor still drops so I figured it was a cylinder problem. So I rebuilt the cylinders and still no luck.

I also applied 100psi of air to each of my cylinders. They all hold air at 100psi. I applied air to the control valve and plug one hole. The valve started with 100 psi and leaked down to 95 psi in 24 hours. Does that sound like a problem? I would think air would be a lot thinner than the oil.

When I move the cylinders without a load (detached from the motor) I find that one cylinder moves slightly slower then the other one. Is that a concern?

Thanks for any help! As you can see I have tried a lot of things!

 

[rat boat]

the different speeds of motion are fine. The air leak might or might not be, I don't know. Are there any pools of oil collecting around the system? If not try this, lift the motor, and then drain the oil reservoir. When it sinks, check the reservoir. If there is oil your valves might be shot. If no oil makes sure there is no air in the system also make sure you are using the right type of oil lines. On this forum, I heard of someone who used fuel lines instead of oil lines and they inflated like balones and then exploded.

 

[jimmbo]

So when you make the changes, One Item at a time, or the Shotgun approach and did several changes/replacements at once?

 

[katch_me]

So when you make the changes, One Item at a time, or the Shotgun approach and did several changes/replacements at once?

Yes, one item at a time.

I am still monitoring the air pressure in the control valve that I spoke about. The pressure seemed to raise slightly. So when I said it dropped 5 psi overnight I don't think that that is 100% accurate. I have to keep in mind the cheap gauge and air temp. To me it looks like it is holding well.

 

[katch_me]

the different speeds of motion are fine. The air leak might or might not be, I don't know. Are there any pools of oil collecting around the system? If not try this, lift the motor, and then drain the oil reservoir. When it sinks, check the reservoir. If there is oil your valves might be shot. If no oil makes sure there is no air in the system also make sure you are using the right type of oil lines. On this forum, I heard of someone who used fuel lines instead of oil lines and they inflated like balones and then exploded.

No external leaks

 

[katch_me]

That is a good suggestion. I could lift the motor up, drain the oil and check if any oil comes back. To separate the problem from the valves and cylinders.

When I open up the system and drain the cylinder it seems like there isn't a lot of oil in them. When I do this the cylinders are fully retracted. I will measure how much oil came out. And do a calculation. I still feel that there might be air in the system. Even though I have bleed them over and over again.

 

[katch_me]

That is a good suggestion. I could lift the motor up, drain the oil and check if any oil comes back. To separate the problem from the valves and cylinders.

When I open up the system and drain the cylinder it seems like there isn't a lot of oil in them. When I do this the cylinders are fully retracted. I will measure how much oil came out. And do a calculation. I still feel that there might be air in the system. Even though I have bleed them over and over again.

There seems to be enough oil in the cylinders, no air. The volume I get out seems to be the volume I calculate to be in there.

 

[Chris1956]

Gee, the only way the trim cylinders can "leak down" is if the control valve leaks backward or the teflon piston seals are leaking.

Are the teflon seals good? They take a spacial tool to insert the pistons into the cylinders, after they have been replaced.

Are the orings on the piston rods under the bolts (at the piston) in place?