Intake leak????
 

I have a 2000 z272 with a 454 MPI and at cruising speeds, the engine is producing a whistling sound. It almost sounds like a belt squeal. However if you give it full throttle, or reduce throttle, the whistle goes away. I am thinking it's an intake or vacuum leak. I have inspected the top end of the motor and I can't see anything wrong. Does any org have any advice?

Re: Intake leak????
 

Try using a can of non-chlorinated brake cleaner around the intake.
Fire it up and just spray conservatively around the intake.
If you got a vacuum or intake leak you'll find it.
Also check and make sure you don't have a idler or tensioner pulley going bad.

Re: Intake leak????
 

That's a good idea. Thank you. I did check the belt and all the pulleys. It all looked fine. I'll try that and see what I come up with.

Re: Intake leak????
 

Don't mean to step on your wrench turning skills:wink_thumbup: , but
did you check the pulleys with the belt off? Alot of time it's hard to tell unless you can physically spin the bearing in the pulley by hand. Alternator bearings and even the P.S. bearings can raise he-- too.

Keep us posted:)

Re: Intake leak????
 

Oh no sir you can step away hahahaha. Yes I pulled the belt off, checked for wobble and anything that felt like it was dragging. I put a new alternator and water pump on, for other reasons and nothing changed. The noise only happens user a load at cruising speeds. When I have it out of the water or in neutral I cannot make the whistle. If the brake cleaner doesn't work, my next option is to pull the seat out and pull everything I can apart and put it in the water.

Re: Intake leak????
 

My 7.4 MPI was similarly whistling but it was due to the flame arrestor. I pulled it off and cleaned with brake cleaner but it still whistled. Turns out the aluminum fins were oxidized...needed to be replaced.

Re: Intake leak????
 

Hmmm ,
Gimble bearing , u joints maybe.
I think your right , you're going to have to pinpoint the area a little more.

Re: Intake leak????
 

Hmmm ,
that's interesting Steve. Gotta remember to check for that in the future.

Re: Intake leak????
 

Steve, that was my first move. I cleaned the flame arrestor with brake clean and it didn't stop it. So I figured it wasn't that. I was going to replace it, but it's like 200 bucks. I think I'll pull it off and run the boat to be sure. Did you find a cheaper one?

Re: Intake leak????
 

I did the same, clean with brake cleaner but closer inspection showed that it was corroded. Probably due to salt air...not sure if that is your problem.
The stock Mercruiser flame arrestor can be replaced with a higher flowing unit from K&N for much less than half the cost....win-win! 59-5004 - K&N Marine Products, Marine Flame Arrestor direct from K&N

Re: Intake leak????
 

That is extremely helpful. So that K&N flame arrestor/filter will fit in place of the stock flame arrestor?

Re: Intake leak????
 

Take off the plastic cover and the flame areastor attaches to the round throttle body with a simple hose clamp.

Re: Intake leak????
 

I have the K&N model. It still whistles

Re: Intake leak????
 

That sound is normal. That is a big motor and it requires a lot of air (and fuel!). If you were to modify your pickup trucks air intake to a straight pipe and filter it would sound the same while operating under load. That whistle sound is air moving through the throttle body. I kinda like that sound...

Re: Intake leak????
 

I forgot to mention why I used a pickup truck as a comparison: To meet emissions standards they have to operate under a certain decibel limit therefore they put the sound chambers on the factory plumbing. Without those chambers the motor can actually be heard breathing!

Re: Intake leak????
 

To be clear, my 7.4 MPI does not whistle at cruising speed...it only whistles on deceleration when coming off plane.

Re: Intake leak????
 

So I am revisiting this problem. I installed the filter that you recommended and the whistling did not stop. I even ran the boat with nothing on the throttle body at all. I am running low on ideas. I have narrowed it down to the throttle body I am pretty sure. Im thinking that the air moving through it is creating turbulence and causing the whistle. Is there a throttle body spacer that would work to see if that may be causing the issue?

Re: Intake leak????
 

Mine whistled the same way. It's air rushing Over and thru the throttle body. Normal. You could always run a straight hose to yur engine bay vent