2013 F70 intake noise at idle

[evman2023]

I’ve got a 2013 F70 with 180 hours, and I’ve noticed it makes a noise at idle. It’s kind a of knocking, and I have located it coming out of the intake. I recently used a 2011 version, and it didn’t have this noise, so now I wonder why mine has it. It’s sounds a bit like an air pump and continues for a second after I shut the engine off. I’ve used a metal rod to “sound” the block and head, and can’t hear anything, so I don’t think it’s internal. Engine runs perfect, has been serviced by me, including valve clearances. the noise isn’t noticeable once reved slightly. I think it’s been there since I bought the boat a year ago. Is this normal, does anyone have any ideas what it is?

I’ve uploaded a video here https://youtube.com/shorts/UAZNE1WWKG4?feature=share

[99yam40]

I would think a leak down test would show if an intake valve was not closing off completely

[evman2023]

I would think a leak down test would show if an intake valve was not closing off completely

I don’t have a tester or compressor. If I check the compression and all cylinders are the same, is a leak down test still needed, or is that only going to tell me where I’m losing compression if low?

[99yam40]

test with what you have and see.
figuring out if one cylinder is lower than others might help

[zenoahphobic]

I don’t have a tester or compressor. If I check the compression and all cylinders are the same, is a leak down test still needed, or is that only going to tell me where I’m losing compression if low?

It certainly is a sound in sync with the engine rotation as it runs on that little bit after switch off.
Pressure in cylinder (leakdown) is the simplest. Next would be putting a bit of oil one cylinder at a time to hear a change in sound. Oil in intake will also tell you but not which cylinder. Oil may help lubricate the valve guides if sticking.
Next would be checking valve clearances again (see manual).
All tests won’t necessarily fix the issue though, in the end the head may need to come off to get to see the condition of the engine and the valves and possibly to remove carbon deposits and/or clean/ relap valve(s) seats.
I would get an “expert” to listen to the engine to determine whether it is OK to continue running this engine without attention, seeing you say it runs fine with more power applied.
Good that you have noticed and seek advice.

[evman2023]

I checked the valve clearances before posting, and they were fine (I have the service manual). I haven’t had a chance to get the compression tester yet, but I removed all the spark plugs and the sound was exactly the same when turned over. So it’s not noise from compression. Could it still be valve related? Maybe I need to take a closer look at the valves with the rocker cover off as I turn the crank?

[zenoahphobic]

I checked the valve clearances before posting, and they were fine (I have the service manual). I haven’t had a chance to get the compression tester yet, but I removed all the spark plugs and the sound was exactly the same when turned over. So it’s not noise from compression. Could it still be valve related? Maybe I need to take a closer look at the valves with the rocker cover off as I turn the crank?

Yes, your last line.
From where I’m listening it seems a groan from internal moving air, although bafflingly that sound still “exactly” without spark plugs in. Can’t see (hear) it as rubbing or bearing noise; what else could it be coming from the intake?

[evman2023]

Well compression seems fine:
1 190
2 190
3 188
4 190

Sound is still continues for a second after stopping cranking with no spark plugs and I’ve realized it makes it whether the throttle butterfly valve is closed or open. I’ve taken on the hoses attached to the silencer, no change. Really odd.

[evman2023]

Update: think I found the issue. Turns out I needed to block off one of the hose leading to the vapor shut off valve and the noise disappears! Initially I just disconnected it. Listing to the valve with a stethoscope and it’s making the noise. Should I replace it?

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[99yam40]

I am not sure what a vapor shutoff valve is or what makes it work, so I am of no help