Mercruiser 350 Mag MPI, 5.7L sucking air

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dmm192

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Jul 15, 2009
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I have a 2003 Mercruiser 350 Mag MPI engine in a 2004 Crownline. Out of the blue, I had trouble starting the engine and once the engine was running the engine starting making a loud sucking noise. The day before this noise started, I filled up with medium grade gas at the marina where I normally buy gas. I thought maybe the gas clogged the fuel filters, so I changed both the inline and the water separating filters - no help. The boat will still run and got up onto plane with 7 people aboard, but it was very sluggish from 2900 rpm to 3200 rpm. Once on plane, RPMs topped out as expected. But, this 'sucking noise' is loud and must be robbing me of performance. I removed the flame arrestor and the noise is definitely coming from inside the air intake.

I read other posts about checking carb sealing surfaces, etc.. but none seem applicable.

Any ideas?
 
P 0 P E Y E

P 0 P E Y E

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Joined
Jun 3, 2009
Messages
441
Re: Mercruiser 350 Mag MPI, 5.7L sucking air

Take a long look at your engine to make sure the PVC valve and or any hoses are are lose. See if the big hose from the top of the valve cover is connected to the throttle body or intake manifold.

If there is nothing obviously disconnected, I suggest a close look at the mounting hardware of your throttle body, intake manifold and anything in the "suck" area for loose or missing mounting hardware, paint chips wear.

Hope you find the problem. Take a good look and it may find you
 
Fun Times

Fun Times

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Re: Mercruiser 350 Mag MPI, 5.7L sucking air

dmm192 said:
I have a 2003 Mercruiser 350 Mag MPI engine in a 2004 Crownline. Out of the blue, I had trouble starting the engine and once the engine was running the engine starting making a loud sucking noise. The day before this noise started, I filled up with medium grade gas at the marina where I normally buy gas. I thought maybe the gas clogged the fuel filters, so I changed both the inline and the water separating filters - no help. The boat will still run and got up onto plane with 7 people aboard, but it was very sluggish from 2900 rpm to 3200 rpm. Once on plane, RPMs topped out as expected. But, this 'sucking noise' is loud and must be robbing me of performance. I removed the flame arrestor and the noise is definitely coming from inside the air intake.

I read other posts about checking carb sealing surfaces, etc.. but none seem applicable.

Any ideas?
Click to expand...
Kind of sounds like the IAC motor at the throttle body is sticky. You could try buying some throttle body cleaner and spraying the hole inside of the throttle boby and all the holes you can find on the inside of the body and see if that helps, and if not you may need a new [IAC]. Idle Air Control.
 
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dubs283

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Jul 27, 2005
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5,346
Re: Mercruiser 350 Mag MPI, 5.7L sucking air

there is a muffler located in the throttle body that most likely got sucked through. this happens quite often, usually when younger people pump the trottle rapidly to get the sweet sweet engine noise to impress thier friends. the muffler is just a piece of foam about a one inch cube. with the engine running and the flame arrestor off look down the throttle body and you will see a 1/2" (approx) hole on the aft side. place your finger over the hole and you will notice a definite change in the sucking noise pitch. you can get the muffler piece at any mercrusier dealer, it is rather cheap. just stuff the foam piece in the hole and the noise will be drastically reduced. it does not hurt the motor to run without the muffler, just noisy and annoying
 
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