Throttle down - sounds like a blow off valve!

YYZRC

Any idea why my 6.2 has a hiss noise when throttling back (no load)?

https://youtube.com/shorts/U67Zm0T-puo?feature=share

PickleRick

Usually a hissing noise is associate with a vaccum leak.  


Hopefully a vaccum hose has popped off.  At idle someoene who vapes can find it pretty easily if they can blow smoke over the top sides of the motor

YYZRC

No vapers here. Would a vacuum leak make the sound you hear in the video - only when throttling down?

PickleRick

My cell phone has crappy audio so I cant make it out 100% but your description of a hiss is the noise an air leak would make (naturally aspirated engines make the most vaccum at idle) 

I bet you'll find a little hose that has popped off. 

Aqua_Aura

It does sound like vacuum. It seems like the sound is still there at idle as well. Haven't messed with a carbureted engine in a long time but I believe when the throttle is closed it creates more vacuum and that's why you hear it louder on throttling down. 

Willhound

Yep, take the spark arrestor off ( watch that little nut or washer doesn't go down the throttle body) and check for a smaller hose that just pushed on to a nipple near the base of the throttle body/carb. Makes that exact noise.

YYZRC

I don't see where a hose connects as per the parts diagram?

davidbrooks

Not sure about the carborated models but on the EFI ones if you have a vacuum hose disconnected you will also notice a difference at idle.  It happened to me once and i thought i had a bad MAP sensor.  Turned out to just be a disconnected hose.  

YYZRC

Idle is perfect - smooth and identical RPMs to the starboard motor.

davidbrooks

What about an exhaust leak?

Aqua_Aura

Exhaust leak shouldn't sound that smooth. 

According to some old post on another site there is only 1 vacuum line that goes to the cool fuel module. 

YYZRC

I checked the vac line to the cool fuel (there's a tee in that line as well) and it all looked ok.  Will check it again this weekend.

rasbury

Make the hose is not cracked or something...

YYZRC

I think I found the source of the sound. Is it possible that it’s the IAC valve? Maybe the filter is ajar. Will check tomorrow. 

YYZRC

https://youtu.be/RXoCCMxX3Ug

aero3113

Big difference 

mattie

I was gonna suggest that the IAC might be plugged. 

Liberty44140

Wow! I’d heard that without it was a noticeable hiss but that is more than noticeable. Nice cheap and easy fix!