2003 50 MPI loud hissingsuction what is it

"I have a 5.0 mpi that runs gr

"I have a 5.0 mpi that runs great, but makes a very loud hissing noise at the air filter (I can definitely still hear it with the engine hatch closed...I had it looked at and a mechanic said that MPI's suck a lot of air and that is all it is (he didn't seem that bright)...well I saw an identical boat this wk end at the lake and had a listen...the very same MPI is very quiet. Do I have an intake leak (gasket) maybe throttle body gasket? What else might it be, and am I damaging my engine by running it. The best description is a very loud suction noise that varies with the rpm's (example: as I stop the boat and put it in neutral, the hissing is not as loud as when the engine is revving...you can hear the suction dissipate as the engine slows, but it never stops and is annoying and loud.

Any info. would be greatly appreciated"

"Interesting challenge. Sounds

"Interesting challenge. Sounds like an air leak. When did it start making the hissing noise? My first thought is that an air leak of that size would cause it to run bad at idle... but depending on the location of the leak and the type of fuel injection, it may not make it run bad, but perhaps idle fast. Any running problems or high idle? check to make sure all air duct work is in place and vac lines are either plugged or in place. Is the engine hatch insulation hanging in such a way as to interfere with air flow?"

"Thanks for the reply...by say

"Thanks for the reply...by saying it is running fine I mean I have no problem starting, it doesn't die, and it opens up and hits a very fast top end with no problem. The engine doesn't idle fast, and the hissing is very pronounced when the air filter cover is removed. The engine hatch does not interfere in any way. I am very confused, I have only had the boat a couple months and when the mechanic said that all MPI's suck air like that, I took his word for it...he is the expert right?! Anyway, I was in for a rude awakening when I heard another guy's engine run with the hatch up, very smooth, very quiet, no hissing or loud suction. I thought maybe a dirty filter but that wasn't the case either. I am sure it is affecting performance somehow (either gas, top end, or overall rpm's) but I have had no problems that are obvious...it is just annoying and it worries me."

Wish i could be of more help.

Wish i could be of more help. Consider getting alternate opinions or... turning up the radio and enjoying the boat.

"You're probably hearing t

"You're probably hearing the air rushing into the IAC (Idle Air Control) inlet. If it's still under warranty, take it in and have them reset the IAC and see if this fixes it. It may be more closed than normal but if it's closed too far, your idle will be affected negatively. If it sounds like it's from the hose attached to the PCV valve, yours may be stuck. Check that out, too.

Post a photo of your flame arrestor if you can."

Did it always make this noise?

Did it always make this noise? What is the top rpm?

"I don't think so (sou

"I don't think so (sounds odd) I really started noticing it a couple weeks after I bought the boat. The bottom line is it does not sound normal at all. Very loud suction that resonates through the hull when engine hatch is closed. It is clearly suction, I just don't know what is causing such a loud high pressue type suction. I need help...the last mechanic just said it was normal, I don't buy it"

"I may get in trouble for this

"I may get in trouble for this, but, you can lightly spray carb cleaner along the air intake path at each join. It doesn't take a lot, just spray all around each stage of the air intake while the engine is running. When the engine speeds up, there is your leak."

"Phillip- nothing wrong with d

"Phillip- nothing wrong with doing that, it's a common way to find vacuum leaks but the caution I would add is that some carb cleaners (NAPA in the blue can) will remove paint. For that matter, spraying water on it will get the same results with the addition of the steam coming out the exhaust.

The IAC can and does make this noise but if there's no increase in RPM at idle (it should be at 700 RPM), it's nothing to worry about. It does sound strange when compared to a carb but the idle air goes through a small opening. Again, if you can show a photo of your flame arrestor and the throttle body, maybe that will shed some light on this. One thing that could cause it, as I mentioned before is that the IAC may need to be reset. Unfortuantely, it's not something that should be done manually since it can damage the gear or teeth on the pintle.

When I was working at a couple of dealerships, I did hear some motors make this sound. It is a strange sound but none of the boats that did it ever had any problems. It could be as simple as a burr on the casting, causing turbulance when the air has to make a sharp turn.

Out of curiosity, is there any difference as far as sound deadening material placement between your boat and your friends' boat?"

"There is no difference in the

"There is no difference in the boats at all...his engine was MUCH quieter. I listened to it with the engine hatch up. Mine just makes a very loud HISSING/SUCTION (that is my best description!) I believe the RPM's are fine, I will look next weekened, but when I start the boat it is very calm and doesn't seem to be running hard at all. The hissing is strange, when i put the boat in gear and increase the throttle the hissing gets louder until I can't hear it due to the engine and thru-hull exhaust. When I come to a stop and the engine is slowing, the hissing remains at standstill and then gradually goes to a much less strong noise/hiss. As the rpm's drop the hiss eventually drops, but it is never gone. It is more than an annoyance, it does not sound right. Very strong, loud hiss."

"If it's somewhat hollow s

"If it's somewhat hollow sounding, I know what that sounds like. If you have any contacts at a Mercruiser dealership or, have them reset the IAC. It's something they can do with a diagnostic computer and as I said before, it's not soemthing that should be done manually. If that part breaks, the motor won't run right.

Look at the ECM and maek note of the manufacterer- Delphi or Motorola. It may say Mercruiser on it but they don't make these controllers. If it says Delphi on it, a GM dealer may be able to service it but OTOH, they usually don't have the marine programs. If the dealer can service GM Performance Parts crate motors, they should be able to check it out. The problem with this is that neither place is going to do it free and you'll want to ask what they charge for this kind of thing.

Where are you located and where is the boat? I may have some connections who could possibly help."

You said something that got my

You said something that got my attention. What type of exhaust is on the boat?

"He said something about the e

"He said something about the exhaust going through the hull and now that you mention it, there could be a loose clamp or bad tube if the motor overheated. However, he also said the noise is coming from the area directly around the flame arrestor or near the throttle body."

"Everyone...thank you for ever

"Everyone...thank you for everything so far...here are some answers to your questions.

Jim - I live in Dallas, TX and the boat is in Lewisville, TX. There are plenty of places to have the boat serviced, but these days you pay three different people a LOT of money to get something fixed. So I am leary of just dropping it off at a dealership etc...

Exhaust - The exhaust is thru-hull...The sound is coming from the vicinity of the air filter underneath the plastic cover (I apologize I am somewhat of a novice) I am not sure what the flame arrestor is...I am very new to marine engines and am not a mechanic. It really does sound like the filter is just sucking air really hard, and it sounds constricted.

You mentioned something about a "hollow" sound...the hiss is very distinct, there is nothing hollow about it. It sounds like when you put your hand over all but one part of the suction hose on a vaccum cleaner....just louder.
And loses some of it's sound once the rpm's have been down for a few seconds. For example my friend said "i don't hear it now" I revved the engine in neutral to get the rpm's going, it immediately re-appeared and then slowly settles back into a more "regular" air suction sound."

"The flame arrestor is just th

"The flame arrestor is just the marine equivalent of an air filter, the Coast Guard has regs that say there can't be any spark pruduced by any electrical devices and if the motor backfires through the intake, the flame must be suppressed in order to avoid explosions. The reason for this is that there is usually some kind of combustable material or fumes in the bilge, also the reason for the bilge blower.

If you want to check this out, remove the flame arrestor and see if the sound goes away. If it does, look for bent fins on the arrestor or if it looks clogged. It may also be that the arrestor isn't on all the way and air is bypassing it, making this noise. I would still like to see the arrestor, just for reference.

If the sound goes away at idle, gets worse with the throttle opening and quiets down again after going back to idle, it still sounds like an IAC noise. There's a function called "IAC follower" that the ECM controls and if the IAC valve is restricted or faulty, it can be sluggish and the noise will be produced when the IAC opens more. If you have a mid '90s GM truck, you can sub the IAC from that to see if anything changes."

"Thanks Jim,

I will send a

"Thanks Jim,

I will send a picture of the fa and try the things you suggested...If you hang in there with me I will have pictures for you on Friday...Can't get out there until then. Please send me a list of items you would like to see, I will take pics of any parts of the engine/exhaust that you want for reference...by the way do you have any connections around Dallas?"

"Your flame arrestor is basica

"Your flame arrestor is basically the air filter. This may sound silly, but is the flame arrestor on securely? Also, check the condition of the flame arrestor. Good luck."

"Jim,

I just read more of y

"Jim,

I just read more of your post about the IAC...if that is the problem what kind of issues am I looking at if I continue running the boat? Am I doing damage to the engine? What are the risks, and how much am I looking at to fix it if it is the IAC?

Thanks for all the info."

1

1

"If the IAC is open too far at

"If the IAC is open too far at high RPM under load, it could run lean, which isn't a good thing but even if you're at WOTand the excessively open IAC wants to make the RPM go higher, you still have a rev limiter and this will kick in. If you never go to WOT, I wouldn't worry about it unless it sticks and the idle speed is over 1000 RPM. Then, the IAC needs replacing or the circuit needs to be checked out. The circuit can be checked with a multi-meter, which makes it more user-friendly. If you call a GM dealer, tell them that you need the price for an IAC on a 5.0L GM truck motor. I would compare that price with a Mercruiser dealer and get their part number. Occasionally, they have proprietary parts but I'm not aware that this is one of those. Maybe you can see the OEM part number on it, if it's not painted black."

The IAC has nothing to do with

The IAC has nothing to do with the operation of the engine over about a 1000rpm. It is just what it's name says it is (idle air control).

Has anyone checked the throttle position sensor?

If the engine is running too much back pressure from the exhuast it can make this type of sound.

The thing that bothers me about it being a back up is it still runs well. If there is a exhuast back up the engine should have a lack of power and not reach top rpm.

If it had a vacuum leak it would run like crap as well. It would miss at idle (if it would idle at all) that you would have too notice it.

I do agree it should not be making this noise.
It will be best not to use it until you find the problem.
You need a factory trained mechanic that has done the course on your system and the equipment to work on it.
Except nothing less.

"Yes, the IAC does very little

"Yes, the IAC does very little over 1000 but as it follows the throttle, it has opened more by the time it gets to 1000RPM and as he said, the noise is still there when he goes back to idle and then gradually quiets down.

I doubt that the TPS would contribute to this- if it was bad, acceleration and/or deceleration would be sluggish. The MAP runs the show after 2% throttle, anyway, except on hard acceleration/deceleration.

I thought about restricted exhaust too, but since it runs well, I kind of ruled it out. It could still be an exhaust leak, though. If there's a definite cycle to the sound, I would be looking for that and it would be nice to have a way to record the sound and access it here (maybe in a yahoo profile or something like that).

The vacuum leak would raise the idle speed and if it was bad enough, the IAC wouldn't be able to get the idle speed down to normal and since it varies with engine speed, I still lean toward the IAC, but again, I'm not where the boat is. It wouldn't necessarily miss at idle- ever take a PCV valve off? It runs fine, just a lot faster since it's an uncontrolled vacuum leak. If the leak is ahead of all intake runners, the vacuum leak would act just like opening the throttle. If it was on an intake runner of one cylinder on the intake manifold, I could see this being an issue. Still needs to have a vacuum test and someone has a mechanic's stethoscope, it would be easier to find the source if it's other than the IAC. I also think the PCV hose should be checked for leaks or cracks- a cracked PCV hose would make this sound, too.

I am a factory trained tech- I just don't do this professionally now because the dealer I worked for had an ugly management meltdown and since I really prefer to work on tournament ski boats (Mastercraft, Malibu, etc) and there aren't any dealers for these near me now, I changed directions a few years ago. Like I said, I'm not in the same place as the boat or it would be a lot easier to diagnose this.

t lembke- you may be able to find a better dealer through Mercruiser. They have contact phone numbers on their website and if you ask for a dealer near you based on CSI (customer satisfaction index) numbers, you might come to a quicker solution."

"Great string of posts. IAC mi

"Great string of posts. IAC might be working fine... sounds like a hose missing on the upstream side of the IAC, if there is one on this set up. Air leak below the throttle fly and or IAC would result in a fast idle.

In terms of affecting performance (per JIM N's comments), IAC will have little authority above idle and no ability to lean the mixture. Speed Density fuel injection (which uses MAP, TPS and Air temp and known volumetric efficiencey to calculate air flow and fuel needs) should compensate for the extra air as MAP sensor value would be different. In other words, not going to hurt the engine it if you use it... assuming it is IAC related.

I general, it would have to be a major air leak to make that kind of harmonic noise... which would cause a fast idle condition or a lean miss if located on one intake runner. Bring on those pics!!

cheers,
D"

"They don't use an IAT in

"They don't use an IAT in most marine and I can't think of one where they do. Indmar is including a catalytic convertor and 02 sensor for the '07 models but still aren't using the IAT- they still use the ECT. When I was at Mastercraft training in 2000, one of the guys I went with took his boat down and we installed an IAT, then they did a recal. Better hole shot, better mid range torque and better fuel efficiency. Still don't know why they don't use it. He did a lot of wakeboarding at the time and the recal was application specific, though.

Re: harmonic noise- if you're referring to "definite cycle to the sound", that was a bad description. I meant the sound made by a single cylinder exhaust leak. Kind of a tf tf tf tf tf sound that increases in frequency with RPM. Come to think of it, I would check the spark plugs to see if they're all tight."

"they can probably use coolant

"they can probably use coolant temp as a proxy... or none at all. After all, fuel injection strategy can be real basic (no cat, O2, MAT or even MAP) or real sophisticated (multiple heated pre and post cat O2 sensors, MAP together with a mass flow sensor) depending on emissions strategy. This is car stuff though.

If we haven’t already, we should be seeing closed loop (O2 sensors) and catalysts on boats and motorcycles as the EPA pressure ramps up. Hopefully not."

"Ok guys...you have been a tre

"Ok guys...you have been a tremendous help, however in a lot of your technical talk I got lost. Please if you would shoot me a list of the top things to check out to try an remedy the situation...I have no problem spending money to get it fixed right, but I want to rule out anything that I can do on my own so when I take it to a mechanic I can say it's not a,b,c,or d.

When I get out there Friday, I will take pictures of the F.A., exhaust, and other parts of the engine as needed. Please send a list of the things I should do FIRST!

Thanks a million,

tl"

"An engine with a major vacuum

"An engine with a major vacuum leak on the intake side is not going to idle smooth at 6 or 700 rpm.
(On a vehicle, if you were to pull the vacuum line for the brake booster off the engine will die if it is at idle)(open air leak) If you had the engine running faster and pulled that line, the noise at the end of the line would be close to what this sounds like by what you have said.(a vacuum cleaner)
But the engine would not idle no matter what with the vacuum line removed. You could control the air entering the line and make the engine get close to idle. But then is when the engine will try to idle higher because of the extra air entering the intake.

This sounds like a controled leak / restriction that is not connected to the air / fuel mixture entering the engine. An open leak lets more air in leaning out the fuel mix.
This sounds like it can not get air into the engine at some other point. This air does not add to the air / fuel mix. Its not extra air entering, it is a lack of air if anything.

Like trying to pull air into the crankcase with a blocked off air intake to the crankcase.

This is if the engine does have a pcv system.

Most boats have free vented crankcases because there is little or no intake vacuum at wide open throttle to keep the pcv valve open.

If this engine does have a pcv system installed, remove the cap from the valve cover for adding engine oil to make sure air is entering the crankcase.
If this is not the problem I'd just take it in for service.

To keep running it is stupid when you have no idea what the problem is.

If you have no idea what is wrong, how do you know it will not damage something if you keep running it?"

"What's the difference bet

"What's the difference between a vacuum leak and opening the throttle? Not much if it's just after the throttle plate. When it's on one cylinder, it's a problem This is an injected motor, not carbed and if it had a major vacuum leak, the IAC wouldn't be able to get it to idle at 700 even if it was fully closed.

This one does have a PCV and it would probably be a little funky if that was clogged. As I said, I think it would be a good idea to check anytihing in the vacuum system.

Taking it in before running it again is the prudent thing to do. I'll come up with a list for the dealer when I get back."

"Jim,

Thanks for the insigh

"Jim,

Thanks for the insight...any chance to come up with the list of things to go through?"

Have we defined it to be a vac

Have we defined it to be a vac. leak ?
How about a Vac gauge ? How many in' of vac
do you have at idle. I usally take a pair of needle nose plyers and start clamping down on Different line next to the engine if it is a bad vac. line the engune will improve as soon as you clamp the bad line..