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Old 19-01-2010, 07:52   #1
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Grinding Noise from Air Intake

My cruiser has 200 hours on the Mercruiser 5.7L Mpi motor. One day she was running quietly and the next time I took her out, I noticed a grinding noise that seemed to come from the air intake. The more throttle the more the gringing noise even though she runs well.. When in idle, she runs quietly. When cold, she stalls a couple of times but starts right up again. Dont know if the issues are related.
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Old 19-01-2010, 09:38   #2
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Can you describe the noise any more? A good way to find the source of the problem is to take a long screwdriver with one end against your ear and touch different places on the engine. There are a few things that can grind so try to cover a good bit of the engine including the accessories like the starter.
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Old 19-01-2010, 09:44   #3
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I used one of those mechanic type stethascopes and I checked the power steering pump, the water pump, generator and pullys as well as the tensioner. It all sounded good. When I take off the flame arrestor, there are 2 plates which look like the choke. When throttle is given, the plates open slightly and thats when you really hear the constant grinding noise. I thought that there may be lose/bad bearings but Im not sure. I really hear it through the intake at the top of the motor. Heck, Ive even dumped carb and choke cleaner down there.
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Old 19-01-2010, 10:27   #4
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I do not think the noise is in the motor If it is you will soon know Is this an io boat?
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Old 19-01-2010, 10:38   #5
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Yes. Luckily its on a trailer and I also have tow boat so when I am in the water, there are now worries in getting home. But that doesnt stop the grinding noise.......
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Old 19-01-2010, 10:44   #6
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You starter could be hanging up or my bet your outdrive has issues
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Old 19-01-2010, 11:03   #7
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Well, it wont hurt to look around the starter and the outdrive area. Actually, I hope that I dont find a problem there. Worse case scenario is that "something" breaks and then I can repair/replace it. I will continue to investigate. Let me know if you come across any other possibilities.
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Old 19-01-2010, 13:13   #8
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drain the oil from your outdrive
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Old 19-01-2010, 13:46   #9
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Quote:
Originally Posted by promax View Post
drain the oil from your outdrive
I believe it is the outdrive too but I don't know what this would tell him unless there is a ton of metal in it. If a race is spinning in the outdrive then that would cause a grinding noise. But wouldn't he have to disassemble it to find it? He might see some metal in the oil but that's also true for most functional outdrives.

Engines don't make grinding noises, they knock or rattle. I've never heard one grind. Has anyone?
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Old 19-01-2010, 14:05   #10
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The oil will be burnt There will be no mistake oil black and smell burnt
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Old 19-01-2010, 15:30   #11
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As suggested, check the oil in the engine and outdrive. The most common thing that I have run across when having a grinding noise on an engine is a starter that isn't fully disengaging.
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Old 20-01-2010, 07:53   #12
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Thanks for the help. Ill check it out this weekend. Until then, if anyone has other possibilities, let me know and Ill check them all out this weekend.
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Old 20-01-2010, 09:33   #13
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Just wondering if it could be broken ball bearings in the air intake fan (assuming you have one) - that would be the cheapest fix, too.
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