Okay, I really am counting on you guys for the answer.

I have a 2002 Merc 500 EFI that runs great, all is well according to the scan tool, oil temp and pressure are perfect, all injectors check out fine and a compression check showed all is well.

However there is a very big sucking sound coming from this motor.

I have replaced the Map Air Sensor, Intake air temp sensor and the IAC motor. Actually have just tried another IAC motor. I took the throttle body off if that is the right description, the part with the butterflies that the air filter hooks to and doubled checked the o-ring gasket between it and the plenum and all was well.

This is my port engine, and the starboard engine does not do this. Funny thing, but my best friend has 500EFI's also and he is having the EXACT same problem with the same engine. We have both messed with this for a month now and cannot locate the problem.

I had a buddy to tell me that he actually thought it was a bearing noise from the alternator instead of a sucking sound, so I ran the engine without the belt and it is not coming from an accessory. Very clearly an air type sucking sound.

Does anyone know where or what we need to be looking at to find the source of this problem.

Your help is much appreciated

Dock Holiday & Steel Outlawed

Dock,
have u tried an auto stethescope on bearing areas, ie. alt, ps pump,
Or if you really think it is a sucking noise, I would try a can of berryman's carb cleaner. simply spray carb cleaner with the pinpoint nozzle in the vicinity (around injectors) of the noise a little at a time while keeping a close eye on your engine rpm. if RMP raises in that area where your squirting, then track down to see if there is a hole in a gasket or ripped airhorn bringing in unmetered air. Keep the fumes away from the intake as it may give you false rpm increases.

Oh, if you try this technique, you may want to have a fire extin handy.

worth a try and good luck,
gociggie

Where do you live? What weather conditions are you running the motor in?

that noise is the money leaving your wallet everytime you start it up.

PCV valve hose ???

http://www.mercruiserparts.com/Show_...And+Components

item # 9

Probably more air going past IAC valve than normal. This makes a big sucking sound, just like you describe. If there is some carbon around throttle blades or throttle blades are set more closed than normal, the IAC will open further and thus more air going thru idle speed passage and thus more 'sucking' noise.

Also, adding a different flame arrestor can make this noise more pronounced.

Lastly, if you are not used to listening to your motors with the hatch up and you are doing so now (where they are normally closed) you will here a ton more noise.

That's a good idea with the carb cleaner. Would starting fluid do the same thing?

Yeah that's me, I'm the friend with the same problem. It is getting very annoying

What does the scan tool say the IAC position (at idle) is on that engine compared to the other engine?

If it' s high, then there may be a performance issue that is forcing the ECM to open the IAC a lot to maintain the desired idle speed. If it's low there may be a vacuum leak.

Dave

North Carolina

70 to 90 degree temps, high humidity.

SB,

I did have carbon build up around the throttle blades and cleaned all of that off. I compared the opening to that of the other engine and they are the same. Also played with that adjustment a little and saw no difference. Both engines are idling at the same RPM.

I do have after market flame arresters on the boat, but they have been on it for three years now. Again, the Port engine is as quiet as can be. I have heard they type of noise before when I had a bad intake valve on an engine, but this engine runs great and the compression test is perfect.

I'm going to look closer at the IAC and compare the reading to the other engine, then look at the o-rings around the injectors.

Thanks for all your help.

Dock