Lost 10 MPH of top end speed

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Spinout

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Aug 13, 2005
Messages
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I have a 2000 Mercruiser 5.7 litre MPI 300 HP sitting in a Malibu 20.5 Sport Cuddy. The leg is the Bravo 3 with 26 pitch stainless steel props.<br />The engine has 240 hours into it.<br /><br />I've owner the boat since new & have maintained it very well.<br />Top speed was bragged to be 65 MPH although the most I've ever got was 64. 60 MPH was always a piece of cake even with 4 people in it. RPM was usually around 4800 if memory serves me right.<br /><br />The problem lies in the top speed. I don't know exactly when I noticed it but definetly in the past month the boat will not exceed 55 MPH & 4400 RPM. The engine does not miss or sputter at this point, just no go.<br /><br />All other areas of performance seem very normal. Good out of the hole, same RPM at 30MPH as it always was. No engine misses or hesitations. <br />Engine is fresh water cooled & runs usually around 160 degrees.<br /><br />Maintainence I've performed.<br /><br />LOF every end of season.<br />Just changed the fuel/water separator. This is the 3 time since new.<br />Leg serviced this spring, 3rd time since new.<br />Checked two spark plugs today & both appear to be fine, but at 240 hours they should be.<br /><br />First the hull. Excellent shape & not stored in the water. <br />Waxed twice per year including the hull.<br />Props have a few tiny knicks in them. No different since 2 years ago.<br /><br />My thoughts along with my techs thoughts was a fuel problem. ie: plugged fuel/water separator.<br />Possibly a fuel pump.<br />He did a pressure test & it's sitting at 34psi. <br />The specs in a car are about 65psi. I phoned a marine shop here & he thought 34 was about right for the boat though.<br /><br />Any help of course is what I'm looking for & all help is greatly appreciated..<br /><br />Thanks
 
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Laddies

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Sep 10, 2004
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12,218
Re: Lost 10 MPH of top end speed

Have you ck'd the engine for total spark advance, in some cases the module goes bad and won't give maximium spark advance at WOT
 
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wolf396

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Aug 8, 2005
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Re: Lost 10 MPH of top end speed

if you get it figured out can you give us an update? this sounds alot like what my boat is doing.
 
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Spinout

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Re: Lost 10 MPH of top end speed

if you get it figured out can you give us an update? this sounds alot like what my boat is doing.
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Absolutely! It's going in this Friday to have it scanned for codes etc.<br />The tech I talked to today said that the Bravo 3 has whats called a guardian system in that, say the water pressure drops it causes the system to drop 10% of power which makes some sense because RPM wise that's what I've lost.<br /><br />Anyhow I'm sure hoping these guys can figure it out without cleaning out my bank account :rolleyes:
 
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Spinout

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Re: Lost 10 MPH of top end speed

Update. The engine has been scanned & no codes have been stored in the memory. :( <br />The fuel pressure is ok & the timing is also fine.<br /><br />He replaced the distributor cap as it was a bit worn but it made no difference in my current problem. (Didn't think it would anyhow). <br />Today he phoned the Mercruiser techline & see what suggestions they may have.<br /><br />If they have no idea then I may replace the spark plugs even though the two I pulled appeared to be in excellent condition & if the plugs were in poor shape I'm pretty sure other problems would exist as well...<br /><br />The Guardian system I referred to in my last post didn't come out till 2001 so it's not that either<br /><br />I'll keep you informed. Hopefully somebody will read this & have some idea..
 
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rbezdon

Senior Chief Petty Officer
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Jun 20, 2004
Messages
689
Re: Lost 10 MPH of top end speed

Any chance you have old gas?? How long since the tank was empty and filled with fresh??
 
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lowshovel1

Petty Officer 2nd Class
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Dec 11, 2004
Messages
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Re: Lost 10 MPH of top end speed

did I missread or did you post you havent changed your plugs in 5 years?
 
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Spinout

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Re: Lost 10 MPH of top end speed

Any chance you have old gas?? How long since the tank was empty and filled with fresh??
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No chance. I've used the boat regularly since mid June & have used plenty of gas<br /><br />
did I missread or did you post you havent changed your plugs in 5 years?
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The life of a plug is more dependent on how many hours/miles it's been used rather than it's actual time life. While the plugs are still original since 1999 they only have 240 hours on them. If you were to convert that to approximate distance driving at say an average of 60 kilometers per hour or 40 miles per hour that would be roughly 15,000k or 10,000 miles. Spark plugs in todays engines have a recommended replacement of 50,000k or 30,000 miles. <br />I can't see why being in a boat should change that.<br /><br />However as I said previously if Mercruiser techline or you folks here don't have an answer the plugs will be changed. :)
 
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Purebreed

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Apr 21, 2005
Messages
34
Re: Lost 10 MPH of top end speed

I agree on the spark plug theory. In 500 hoours of use on my old 1987 Mercruiser I changes my plugs once. Not because they needed it just part of general maintenance.
 
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lowshovel1

Petty Officer 2nd Class
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Dec 11, 2004
Messages
135
Re: Lost 10 MPH of top end speed

I rplace mine every spring with complete tune up----marine operateing conditions a bit different than automotive.
 
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Spinout

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Re: Lost 10 MPH of top end speed

OK, compression test done. 175-180 right across the board. Awesome! Spark plugs changed & will test it out tomorrow evening. The marine I'm dealing with on this is still waiting for Mercruiser tech from Toronto to call. This is getting very frustrating :(
 
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rbezdon

Senior Chief Petty Officer
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Jun 20, 2004
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Re: Lost 10 MPH of top end speed

I hope that fixes it for you but I am skeptical on the plugs issue. I think both points are valid, time and operation hours. Time, plain and simple can cause the need to replace plugs. Oxidation, carbon deposits from short cold runs (not significant hours) can all lead to damaged plugs. This is just time related. Also, though operation hours are improtant as plugs do basically wear, especially under a big load and a boat engine is alomost always loaded heavily so the wear factor is higher than a car at 40MPH on a highway. Just good sense to replace them fairly frequently. Maybe once a year is "short" by some standards, but surely every couple of years. Heck, these thing sit and rust for 6 months of the year or more!!! (around here anyway, some of you Southern guys get a bit more time on the water than we do).
 
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Spinout

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Aug 13, 2005
Messages
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Re: Lost 10 MPH of top end speed

Ahem, well the problem seems resolved & excuse me while I wipe some serious egg from my face. :eek: After testing it tonight I got 4700 RPM & a speed of 58 MPH so it would appear the spark plugs were the culprit all along. <br />The mercruiser dealer figures the spark was carbon tracking down the side of the insulater rather than just on the tip.<br /><br />With the boat having a full tank of gas (200 litres), too much other stuff & the condition of the waters it would seem that the boat is up to snuff once again. <br />I'll know for certain when I unload most of the fuel & some extra weight <br />Dang good lesson for me IMO :rolleyes: <br />Thanks for all the help & input from you guys. I really appreciate it :D
 
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rbezdon

Senior Chief Petty Officer
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Jun 20, 2004
Messages
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Re: Lost 10 MPH of top end speed

Glad to hear it Spin, sounds good when somebody finds and repairs a problem and it is FIXED!!
 
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