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View Full Version : Loud Knocking coming from???


Like2bskiing
08-22-2013, 04:14 PM
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.

CantRepeat
08-22-2013, 04:19 PM
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?

Traxx822
08-22-2013, 04:27 PM
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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Traxx822
08-22-2013, 04:31 PM
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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Like2bskiing
08-22-2013, 04:54 PM
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.

TRBenj
08-22-2013, 04:58 PM
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".

Skir68
08-22-2013, 05:42 PM
perhaps you have a belt starting to separate and its hitting the inside of the motor box?

Mark rsa2au
08-23-2013, 06:23 AM
In Gear and under load, but not in neutral at any revs..? Id be checking gearbox!

JimN
08-23-2013, 06:34 AM
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.

Traxx822
08-23-2013, 10:15 AM
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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Like2bskiing
10-14-2013, 10:14 PM
Thanks everyone for all of the input. For the benefit of others who may have similar symptoms - a compression and leak down test indicated it was a blown head gasket. After removing the head - it was indeed a blown head gasket. You could see where the gasket had disappeared between two of the cylinders. Took the heads in and had them magnafluxed to make sure they were OK and not actually the cause of the gasket issue and they tested good. Hopefully will have it back together this weekend just in time to give it a test run and winterize it.

Cloaked
10-14-2013, 10:21 PM
Thanks everyone for all of the input. For the benefit of others who may have similar symptoms - a compression and leak down test indicated it was a blown head gasket. After removing the head - it was indeed a blown head gasket. You could see where the gasket had disappeared between two of the cylinders. Took the heads in and had them magnafluxed to make sure they were OK and not actually the cause of the gasket issue and they tested good. Hopefully will have it back together this weekend just in time to give it a test run and winterize it.Good deal. Glad to see you had an MT done on the heads.

mikeg205
10-14-2013, 11:49 PM
Not cheap but fixed -

Traxx822
10-17-2013, 12:37 AM
Thanks everyone for all of the input. For the benefit of others who may have similar symptoms - a compression and leak down test indicated it was a blown head gasket. After removing the head - it was indeed a blown head gasket. You could see where the gasket had disappeared between two of the cylinders. Took the heads in and had them magnafluxed to make sure they were OK and not actually the cause of the gasket issue and they tested good. Hopefully will have it back together this weekend just in time to give it a test run and winterize it.

Boy does this sound familiar. Make sure you do both sides. :D

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