Trim issues with 1978 mercury 800

tbonebrad

Seaman Apprentice
Joined
Jan 11, 2013
Messages
30
I have been having problems with my power trim on my 1978 mercury 800. The problem was that the cylinders would not hold the motor up for very long and seemed to have an internal leak. I took apart the trim cylinders a week ago and the orings were cracked so i replaced them. The system seemed to be better and would hold in the upright position overnight. This weekend i took the boat to the lake and when i took off the transom saver and trim locks, the motor dropped to the ground. I was able to trim it back up and i decided to go out on the lake anyway.

I had control of the motor at first but after a while i could not trim it up or down anymore. I went back and pull on it and the motor was easy to pull and push up and down by hand. At this point we decided to head back to the dock and when i got the boat on the trailer the trim was stuck in a down position. I could not move it at all and not knowing exactly how it works i just opened up the up bleed screws and pushed the motor up and let the fluid get pushed out through the bleed screws.

So at this point i really dont know what to do. The pump motor has a strange sound when it cannot push the trim up or down. It sounds as if its turning freely without pushing anything, then something starts screeching every once in a while as if a gear is finally catching then coming loose again. There is a control nob on the pump that i can turn but it seems to have no effect on the motor(not really sure what its for anyway).

Do i need to just go ahead and replace the whole pump or what should be my next course of action?
 

Laddies

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Sep 10, 2004
Messages
12,218
Re: Trim issues with 1978 mercury 800

If the cyls are OK it sounds like the pump needs oil or the manual release valve is not screwed all the way out. If thats not the trouble it could be debrie in the pump, try to flush it with fresh oil and check the pressure it should hold 2000# up 700# down average. If it don't hold pressure take it apart and clean it just keep track of the settings on the 2 pressure relief vavles other than that theres not much to the pump.
 

tbonebrad

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Jan 11, 2013
Messages
30
Re: Trim issues with 1978 mercury 800

Thanks for the response Laddies. I have a couple of follow up questions however.

1. Where exactly is the manual release valve and how do I operate it?
2. How do I flush the system?
3. How would I check the pressure of the system?
 

RRitt

Captain
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Mar 30, 2006
Messages
3,319
Re: Trim issues with 1978 mercury 800

if the system is not low on fluid then it sounds like debris from your old o-rings. pumps don't just go bad overnight. they bleed down slowly and over time the bleed down gets faster and faster until the engine bleeds down almost as fast as it lifts. since your pump is not a slow and cointinuous deterioration then your failure is due to some kind of "event". Debris floating through a system and catching on a valve would be an "event".

from your description, I would guess .... the fluid intakes both have a ball bearing. When you suck fluid in the ball bearing drops down and the intake is open. When you reverse direction the ball bearing is pushed up and blocks the hole so that pumped fluid is forced into lift cylinders. I would guess that a piece of debris is lodged into the ball bearing and you are just pumping fluid in a circle. if the debris was in a check valve, then the system should still show some effort to move. If it was low on fluid then you would have mentioned it in your post. so ... intake debris seems most likely suspect.
 

Laddies

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Sep 10, 2004
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12,218
Re: Trim issues with 1978 mercury 800

The manual release valve is the knurled valve on the bottom of the pump, to flush the system remove the lines at the cyls and run mthe pump tillm it's empty 3 or 4 times
 

tbonebrad

Seaman Apprentice
Joined
Jan 11, 2013
Messages
30
Re: Trim issues with 1978 mercury 800

Ok, a little bit more possibly useful information. I bought this boat a couple of months ago and at that time there was nothing wrong with the trim. The second time I had the boat out I hit a log with the trim mostly up and the trim would not go down after that. I had to drive the boat back to the dock with the motor trimmed most of the way up. As soon as I pull the boat out of the water I was able to trim the motor up and down as if there was never an issue.

This last time I took it out however the opposite happened, it got stuck in the down position and I had to open up the bleed screws in order to pull the motor up. This seemed to me like I had a bad check valve or something. Debris getting in the check valve sounds like a perfect fit because when i changed the o-rings the fluid that came out appeared to have debris in it.

The manual release knob does not seem to do anything when I turn it all the way clockwise or all the way counter clockwise. What is it supposed to do?
 

Laddies

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Sep 10, 2004
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12,218
Re: Trim issues with 1978 mercury 800

The system is released when the knob is screw in, with it screwed out the pump should function normally. If you don't have a gauge to test the system, take the pump out of the boat and disassemble it clean it and reassemble it, remember to keep track of the pressure releif valve settings, springs and balls.
 

tbonebrad

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Joined
Jan 11, 2013
Messages
30
Re: Trim issues with 1978 mercury 800

As I was flushing the system I noticed that the strange sound I was hearing was actually the pump sucking air. After flushing the system and filling the reservoir up the system seems to be working good. I left it trimmed up over night and it was on the ground in the morning but I trimmed it up and checked on it an hour later and it was still up all the way. I think I can live with a slow leak, what do you guys think?
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,035
Re: Trim issues with 1978 mercury 800

Brad, Sometimes the teflon piston seals wear out, and allow the motor to trim down. They are replaceable with a special tool.
 
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