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454 MPI Hissing/Whistling Sound

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persuaderkurt

Member
I have a 1996 218 Persuader and I love the Persuader style so much I went and bought a 1999 234 Persuader. I am very happy with the condition of the 1999 234 boat and motor with one exception. The 1999 454 MPI which looks brand new with only 22 hours showing on the ECM makes a rather annoying and louder than expected hissing/whistling noise when I get on it. I took it to a certified merc mechanic and took him for a ride in it. He said the noise is normal for that year engine and was from the air intake. I know only the very basics of engines and do my own basic service and thats it. Can anyone tell me if there is anything I can do to cut down on this noise. Maybe a high performance air intake manifold or throttle body service. I am just guessing. Please help.
 
Are you sure their is no supercharger under the hatch roflrofl.gifSomething does not sound correct to me. Maybe somebody else can reply. A vacuum leak will do that but it should run rough
 
There is no supercharger. The hissing/whistling sound is loud enough for me to think it is not normal. I can't imagine Mercruiser or any decent manufacturer would allow a sound as loud as this to be normal. I originally thought the sound was the serpintine belt and had it tightened. The serpintine belt needed tightening, but did not eliminate the whistling/hissing.

I thought it may be a bearing going on the alternator or something else on the serpintine belt, but as I listened to it closer, it does sound like a sucking sound. It sounds like a sucking sound with a whistle or hiss depending on what the RPMs are and how hard I am jamming the throttle. You can not hear it just by reving the engine in neutral. You have to be accelerating with a load or cruising at a few thousand rpm. Other than this sound the engine runs like a dream. Is is possible there is something in or around the air intake that may be causing this sound. This sound is clearly the predominant sound I hear over any other sound the engine makes. Any ideas? I am thinking about taking it to another mechanic but do not want to go on some kind of wild goose chase. These goose chases can get expensive.
 
thats funny believe it or not my bmw does the same thing i sent it back to the dealership several times and kept getting the same answer its a normal thing im thinking if it really bothers you you might have to take stuff apart and look for any rough edges
 
Just because a guy is a certified merc mechanic does not mean he knows how to work on a boat or trouble shoot. Something is wrong I feel the way you describe the sound you here. It could be a assery bearing or maybe your power steering pump. Also I herd a dry belt make that sound, did you put a new belt on.
 
Thanks for the input guys. I do believe something is wrong. After reading your posts I think it could be something as simple as air rushing over a sharp edge or something more involved like a bearing on something or maybe the power steering system.

I do know what you mean about mechanics. Some are good and some not so good. I think I will take the boat to another mechanic later this fall or wait until the spring. I will take it for a ride with the mechanic to see what he says.

My main concern is having something fail with the wife in the boat. If that happens, I am dead meat. I also would like to quiet it down as well because I am going to have to explain it to anybody who ends up riding in the boat. It is such a nice boat and engine and I will hate to have to explain or make excuses for something so lame.

I will keep you all posted with any results. If anyone else has any other thoughts I would sure like to read them.
 
Big Red....I did put a new serpentine belt on. I do think the old serpentine belt was adding to the noise somehow. Thanks for your interest and I do enjoy reading your posts.
 
On your MPI there is a small seal at the base of the MAP sensor if the sensor is loose or the seal is bad it can cause a vacuum leak and whistle. The sensor is right behind the water neck housing on the manifold. or it could be the AIT sensor it the plenum intake.
or baybe throttle body gasket leak.
It sounds like a vacum leak to me and these are some places to look
 
There is a "muffler" pretty much a piece of foam in the throttle body assembly, it sits in front of the idle air control valve, it may be missing or got sucked thru the motor. This will make the whisling sound you describe. Really not a big deal if this is the case.
 
The sensors you have on your MPI is a MAP,TPS, IAT and IAC I would check the seal on all of them, I have seen stuck PCV valves create a whistle. I believe it's something simple.
 
Guys,

Thanks so much for your input. You guys gave me a good list to talk over with the mechanic. I will more or less give him a checklist on things to "check".

I do tend to think it is not something serious. I just want to address it before it potentially leaves me in a bad situation.

I have one last question. If it was some kind of a vacuum leak, wouldn't I see some kind of a perfromance problem?

One thing I did fail to mention and I don't know if it will give any further indication on the problem, is that the first time I start the engine (cold) it idles for about 2-3 seconds and then stalls. It does not do this when I start the engine warm.
 
Stalling on cold start up can be a number of things from a sticky IAC motor, faulty IAT sensor ect. Have your mechanic hook it up on a ddt or computer and record the cold start. This way you can see whats happening with all the sensors and provide a better diagnostic start.
 
I have a hardin / K&N intake on my 454 mpi mag and yeah - you can hear it. Not much at idle, but like you say as soon as you're on it. Especially sitting on the driver's side of the rear bench. I always thought it was 7.4L of displacement sucking air though that K&N. It just sounds too much like a big airflow to me.

During my summer of injector cleaning, my mechanic and I checked A LOT of stuff including vacuum pressure which was right on and very steady. Swapped MAP sensors (noise was there before and after) also.

Now with all of that said, I still have an issue- after I idle up a river for a while it will stall. It will always restart, rev up a little and die. Takes a few tries to get her going again. If I hear it start to stumble before it stalls, I have about 5 sec. to pop it in neutral and hold 2000 rpms for a few seconds and then I'm good for another 10 min of idling. Not a big deal, but doesn't help me to get the wife to take the kids out solo on the boat while I'm at work.

I have aftermarket injectors so I figured they were just loading up from idling, but the way it starts, revs and dies makes me think an IAC - or a sensor tricking the IAC. MK- they put it on a ddt (as you suggested) and didn't see anything out of the ordinary so I'm not real sure what it is. I'd like to get a set of stock MPI injectors, get them cleaned and try those out.

In any case, I'd guess it just might be normal - or at least as normal as mine runs!
 
Needafunboat, pull your number 8 plug and see if its flooding out. Especially if you have a VST tank. If it is, clean your VST, should solve the problem.
 
A small vacum leak can cause a hiss, if it is not big enough for the ECM to be thrown of parameters and the ECM can adjust to a certain extent and may not show up and set off a trouble code. A ddt tool will not show what a Diacom or scanner pro will. The things happy and I suggest are easy and cheap things to re-seal and check you can do it in a few hours one day. If none of that fixes it it could be the characteristics of your engine.
 
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MkHammer,

I love your websit and your DART engines. Maybe I can save up some $$ and just fix this whistling noise with a brand new MkHammer DART engine. If it came to that I would feel quite comfortable buying from you. You are obviously extremely knowlegeable and your pricing seems quite fair. Thanks for all you posts in this thread and others. I have learned a lot from reading them.
 
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