2000 2.5L merc efi hissing sound

I was just out cleaning the boat up for winter. Ran the motor in a barrel without the cowl and noticed a hissing sound. Sounds like a vacuum leak to me, never noticed anything while using the boat this year. Motor ran great just last Saturday. While searching I found that a lot of people had issues with the oil injection check valves. The previous owner removed the oil injection, I imagine that the check valves would have been removed too.

My only experience with chasing vacuum leaks is on cars. This leak sounds bad enough that it would cause running issues, at least on a car. Any ideas on where I should start looking?

Thanks

Spray it down with a starting fluid or sumthin and c if it revs up any. Look for cracked broken hose, and round intake manifold etc

Great suggestion Merc! Also check your pulse pump I have had them work loose and where the pressure comes out of the block it will hiss and spit. Gasket in the back could be blown out as well. Just a thought

Ok. I'll spray a little carb cleaner tomorrow and let you know. Thanks

The acu can hold back small leaks that would have a carb running high.

Check the map tube if it has it.

Propane is better just like a car, ether starting spray is ok if a small torch isn’t available

So I went out this morning and started it up. Noise wasn't there. I thought maybe it would once it warmed up, but nothing. I did use some propane then carb cleaner to find a leak, but there was no change. Admittedly I don't know much about outboards, but I would imagine that as clearly as I heard it yesterday that if it was a cracked hose or bad connection it wouldn't have completely gone away. Is there some valve that could work intermittently that could be the culprit? Would those crankcase pressure valves still be there if the oil injection was removed?

Also the motor is an EFI.

Could have been a stuck injection well no that would run bad.

It is injected ? Carbs often had a temp valve. It could be noisy

check the oil pump spot is caped properly. It can hiss from down in the gear area.

Could it it have been the garden hose if you’re running it at home.

Check the butterflies intake noise noise can be loud. A stuck choke plate or such.

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Originally Posted by claypoolxx

So I went out this morning and started it up. Noise wasn't there. I thought maybe it would once it warmed up, but nothing. I did use some propane then carb cleaner to find a leak, but there was no change. Admittedly I don't know much about outboards, but I would imagine that as clearly as I heard it yesterday that if it was a cracked hose or bad connection it wouldn't have completely gone away. Is there some valve that could work intermittently that could be the culprit? Would those crankcase pressure valves still be there if the oil injection was removed?

Also the motor is an EFI.

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Did you check the pulse pump clay? Mine would do the same thing. The pump will move an the hole in the block are out of line...it would hiss and or spit. Sometime the hole would line up and it sounded perfectly normal. Good luck!

Was out there again this morning. Checked the pulse pump. Didn't seem overly loose, but gave the bolts about an 1/8 of a turn fairly easy.

Started it up and at first there was no noise. After a minute or so it was back. Took some propane and move it around but didn't make a difference. It sounds like its coming from around the fuel filter area.

There is a small hose the connects the intake to the back of the computer that didn't have a zip tie and was pretty easy to move. I put a zip tie on it and haven't heard the noise since. Don't know if it was a coincidence or what. Guess I'll find out this spring. Does anyone know what that hose is for?

That’s the map sensor hose.

Fuel filter can also suck air

Do EFIs have the thermal air valve the carbureted motors have? They usually take about a minute to open/close.

GET a 2' piece of garden hose and when you hear the noise, put the hose up to your ear and comb the suspect area with the hose,u should be able to pin point the source or get very close. works like a amplifier for engine noises.

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Originally Posted by Fishwhisper

Do EFIs have the thermal air valve the carbureted motors have? They usually take about a minute to open/close.

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They do not.