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Old 08-22-2013, 03:14 PM
Like2bskiing Like2bskiing is offline
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Loud Knocking coming from???

I have a 1995 Maristar 200 with 500 hours. I have read quite a few threads on this but cannot quite narrow it down for sure. I am hoping the experts on this forum can help or have some ideas for me to test. I have a loud ďknockingĒ sound coming from the motor area only when in the water, under load, and greater than 2500 RPM. It sounds so bad you would expect pistons to come flying out at any minute. However, other than the noise the motor starts/runs good.

It is not there at all when idling; it is not there at all in neutral at any RPM even WOT; it is not there at all in gear but out of the water and at any RPM even WOT (the boat is stored on a lift so I am able to run it in gear out of the water for a few seconds just to test).
I donít think it is lifters/rods/motor because it is not there at any RPM when in neutral. I donít think it is transmission or shaft related because itís not there in gear at any RPM but not in the water.

I am thinking exhaust manifold leak since it is only there under load but I cannot find any visible traces of a leak. I have also tried listening for it with a hose to my ear listening for noticeable changes at different points but I canít find anything.

Is there something else it could be? Is there a good way to rule out or find exhaust leak? Any suggestions? Thanks in advance.
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Old 08-22-2013, 03:19 PM
CantRepeat's Avatar
CantRepeat CantRepeat is online now
 
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Quote:
Originally Posted by Like2bskiing View Post
I have a 1995 Maristar 200 with 500 hours. I have read quite a few threads on this but cannot quite narrow it down for sure. I am hoping the experts on this forum can help or have some ideas for me to test. I have a loud ďknockingĒ sound coming from the motor area only when in the water, under load, and greater than 2500 RPM. It sounds so bad you would expect pistons to come flying out at any minute. However, other than the noise the motor starts/runs good.

It is not there at all when idling; it is not there at all in neutral at any RPM even WOT; it is not there at all in gear but out of the water and at any RPM even WOT (the boat is stored on a lift so I am able to run it in gear out of the water for a few seconds just to test).
I donít think it is lifters/rods/motor because it is not there at any RPM when in neutral. I donít think it is transmission or shaft related because itís not there in gear at any RPM but not in the water.

I am thinking exhaust manifold leak since it is only there under load but I cannot find any visible traces of a leak. I have also tried listening for it with a hose to my ear listening for noticeable changes at different points but I canít find anything.

Is there something else it could be? Is there a good way to rule out or find exhaust leak? Any suggestions? Thanks in advance.
You really do not want to run the boat in gear while out of the water. You'll burn up the cutlass bearings in your strut doing that.

What is the oil pressure?
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Old 08-22-2013, 03:27 PM
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Traxx822 Traxx822 is offline
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Sounds like exhaust gasket. Open the hatch, drive until you hear the noise. then have a partner feel around the manifold gaskets for any leaks.

to be honest thought, do not do it for long.

Test compression in your cylinders too. Might help you find the problem.


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Old 08-22-2013, 03:31 PM
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Traxx822 Traxx822 is offline
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Originally Posted by Traxx822 View Post
Sounds like exhaust gasket. Open the hatch, drive until you hear the noise. then have a partner feel around the manifold gaskets for any leaks.

to be honest thought, do not do it for long.

Test compression in your cylinders too. Might help you find the problem.


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Wait, did you say it only does this when it's out of water.

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Old 08-22-2013, 03:54 PM
Like2bskiing Like2bskiing is offline
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Thanks CantRepeat - I know itís a bad idea to run it in gear out of water. It was for a second just to see if the noise was there or not so hopefully no issue.

It only does it when in the water under load.

The oil pressure seemed normal but I will look again tonight and try to get a better reading from the gauge.
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Old 08-22-2013, 03:58 PM
TRBenj TRBenj is offline
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All exhaust manifold leaks I have encountered sound more like a "ticking" than a "knock" and they start out being much more prevalent at idle than they do at higher speeds. So that would be the opposite of what the OP is describing.

Things certainly do change drastically when you put a load on the engine and powertrain, so I would not rule anything out just because it doesnt occur "out of the water" or "in neutral".
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Old 08-22-2013, 04:42 PM
Skir68 Skir68 is offline
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perhaps you have a belt starting to separate and its hitting the inside of the motor box?
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Old 08-23-2013, 05:23 AM
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In Gear and under load, but not in neutral at any revs..? Id be checking gearbox!
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Old 08-23-2013, 05:34 AM
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Quote:
Originally Posted by Like2bskiing View Post
I have a 1995 Maristar 200 with 500 hours. I have read quite a few threads on this but cannot quite narrow it down for sure. I am hoping the experts on this forum can help or have some ideas for me to test. I have a loud ďknockingĒ sound coming from the motor area only when in the water, under load, and greater than 2500 RPM. It sounds so bad you would expect pistons to come flying out at any minute. However, other than the noise the motor starts/runs good.

It is not there at all when idling; it is not there at all in neutral at any RPM even WOT; it is not there at all in gear but out of the water and at any RPM even WOT (the boat is stored on a lift so I am able to run it in gear out of the water for a few seconds just to test).
I donít think it is lifters/rods/motor because it is not there at any RPM when in neutral. I donít think it is transmission or shaft related because itís not there in gear at any RPM but not in the water.

I am thinking exhaust manifold leak since it is only there under load but I cannot find any visible traces of a leak. I have also tried listening for it with a hose to my ear listening for noticeable changes at different points but I canít find anything.

Is there something else it could be? Is there a good way to rule out or find exhaust leak? Any suggestions? Thanks in advance.
Most of the time, a knock will be worse under load, not in neutral.

Look for a mechanic's stethoscope at an auto parts store- it can pinpoint various noises a lot better than a piece of hose.
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Old 08-23-2013, 09:15 AM
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Traxx822 Traxx822 is offline
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Honestly, when I blew my head gasket it did the same thing. Sounded like the piston was going to come out of the engine and pop on a seat next to me. At idle or neutral under no load it was faint.

Test compression and see if you can locate the issue there.



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