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Mercruiser 8.1 EFI 2005 390 MY Both Engines only advance to 3800 RPM

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bob wells

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Recently both engines only develop 3800 when advanced to full power which is 4600 rpm. All normal engines indications and no misfiring on either engine. I can think of only 2 possible problems common to both engines, one is fuel which is good and the other is the throttle which I know nothing about. Does anyone have an idea what the problem might be?
 
Docksidemarineservices said:
Bad fuel cooler. Put a fuel pressure gauge on the rail and run it. Bet the pressure is over 60psi.
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The problem appears to be massive growth on the boats bottom due to a bad bottom job by my marina. When the boat was pulled it looked like a shaggy dog lived under the boat. Even Though I had taken it out and run it hard in rough waters the bottom would not clear. This had happened before with previous owner but he seldom used the boat. Never did I think a bad bottom job, less than 2 years old, would cause such degrade in performance.
 
look at the increase in surface area (over the clean hull) that the growth causes.....that increases the load the engine sees proportionally.....so you never see the rated RPM....just physics....
 
makomark said:
look at the increase in surface area (over the clean hull) that the growth causes.....that increases the load the engine sees proportionally.....so you never see the rated RPM....just physics....
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Thank for the reminder since all the mechanics at my marina never thought growth on the bottom would effect RPM. I wish you worked near me so I could use your insight, it would have saved me a lot of time and frustration.
 
makomark said:
Thanks....I have no clue where you are though....
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My boat is located on the Lake of the Ozarks near mile marker 1. The marina sent me pictures of the bottom and was I shocked by the excessive build up on the bottom, sides, prop, prop shaft and exhaust ports. I have seen algae build up before, but never this bad. Now I am trying to determine why such heavy buildup in less than 2 years of a new bottom job. Any thoughts?
 
I'm out east and that's about a day & a half drive.....

I'd bet its a grass of some sort vs algae.....my gut says you need to use the boat more often......
 
I don't know about issues in fresh water, but in salt water bottom paint is an annual job. Getting 2 years out of a bottom paint job happens only with special multi season paint. Your language about the "new bottom job" leads me to speculate that you think that this is like painting a house. Its not...given the growth that you observed, its an annual job. Per Makomark... using your boat frequently will reduce, but not totally eliminate the problem. There is also the possibility that the marina did not use an appropriate formulation paint for your area.
 
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