what could be hissing? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tim21391 · 07-09-2010, 05:58 AM

i noticed on my truck that there is a hissing under the dash of the passenger side..you cant hear it while its running but as soon as you shut the truck off it goes on for about 2-3 seconds then dies off... truck runs fine but what could it be?

rsavage · 07-09-2010, 06:13 AM

If you have AC, you could have a vacuum line off the control. There is also a vacuum dashpot that operates the vent on the passenger side if you have AC. The line for that could be off or the diaphragm leaking. Just a couple of guesses.

cdowns · 07-09-2010, 07:34 AM

there could b a pinhole in your heater core

caminokid · 07-09-2010, 10:17 AM

I had gotten a 71 with air. It had been sitting awhile. I heard hissing while the truck was off. The next morning I found out what it was.....There was a snake laying in the seat when I opened the door!

Check your vac lines under the dash.

tim21391 · 07-09-2010, 06:22 PM

so with this vac leak.. does it take off your engine horsepower?.. since my ac dont work.. do i really need to worry about it?

oldblue1968chevy · 07-09-2010, 06:45 PM

mine did the same thing, but it was a bad booster 100$ later it stopped better and no more hissing lol

rsavage · 07-09-2010, 06:53 PM

A vacuum leak, if that is what it is, can effect your engine idle and ignition advance response. If severe and if you have power brakes or the leak is in the PB diaphragm, it could be dangerous. Start by checking what vacuum you are pulling with a vacuum gauge. Maybe one of the auto parts chains could loan you one? If it looks like a vacuum leak start tracing all your vacuum lines looking for breaks/cracks.

ERASER5 · 07-09-2010, 11:39 PM

I looked at your build thread. I say vac leak under the dash. You could just plug off the line before it goes into the cab, and then listen for the hiss, then you will know for sure. The vacuum operates a door in the cowl, a vent door, and a heater shut off. There may be more that I am not aware of.

Oh yea, as stated above, a vacuum leak can be very bad. Your brakes could be affected, and under the worst conditions, a very lean burn can melt a hole in your pistons. Bad day indeed!

07-09-2010, 05:58 AM	#1
tim21391 Registered User Join Date: Jul 2007 Location: Salina, Kansas Posts: 1,998	what could be hissing? i noticed on my truck that there is a hissing under the dash of the passenger side..you cant hear it while its running but as soon as you shut the truck off it goes on for about 2-3 seconds then dies off... truck runs fine but what could it be? __________________ 1972 C-10...402/400..flowmaster 40 series 1967 Mustang 347 stroker/C-4 1966 Mustang 289/3-speed 2013 Mustang V6/6speed w/300 ponies I may be 23, but i sure do knows 67-72s are sexy! Its not MPG its smiles per gallon! build started 11/25/08 build thread: http://67-72chevytrucks.com/vboard/s....php?p=2993796

07-09-2010, 06:13 AM	#2
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: what could be hissing? If you have AC, you could have a vacuum line off the control. There is also a vacuum dashpot that operates the vent on the passenger side if you have AC. The line for that could be off or the diaphragm leaking. Just a couple of guesses. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7

07-09-2010, 07:34 AM	#3
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: what could be hissing? there could b a pinhole in your heater core __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

07-09-2010, 10:17 AM	#4
caminokid Chevy nut Join Date: Jun 2007 Location: Springfield, Illinois Posts: 2,266	Re: what could be hissing? I had gotten a 71 with air. It had been sitting awhile. I heard hissing while the truck was off. The next morning I found out what it was.....There was a snake laying in the seat when I opened the door! Check your vac lines under the dash. __________________ 84 Chevy Custom Deluxe 1/2 Ton Named Fade To Black II "Remember kids there is NO replacement for displacement!"

07-09-2010, 06:22 PM	#5
tim21391 Registered User Join Date: Jul 2007 Location: Salina, Kansas Posts: 1,998	Re: what could be hissing? so with this vac leak.. does it take off your engine horsepower?.. since my ac dont work.. do i really need to worry about it? __________________ 1972 C-10...402/400..flowmaster 40 series 1967 Mustang 347 stroker/C-4 1966 Mustang 289/3-speed 2013 Mustang V6/6speed w/300 ponies I may be 23, but i sure do knows 67-72s are sexy! Its not MPG its smiles per gallon! build started 11/25/08 build thread: http://67-72chevytrucks.com/vboard/s....php?p=2993796

07-09-2010, 06:45 PM	#6
oldblue1968chevy Grandpa in the rustmobile... Join Date: Nov 2008 Location: Spokane WA/Viola TN Posts: 11,422	Re: what could be hissing? mine did the same thing, but it was a bad booster 100$ later it stopped better and no more hissing lol __________________ John Goose-1968 C10 355,9.32-1CR, Vortec Heads ,262 voodoo, 3.73:1 3OTT (HS ride/beater/farm truck) http://67-72chevytrucks.com/vboard/s...d.php?t=317684 Grams 53-1953 Chevrolet Belair http://67-72chevytrucks.com/vboard/s...=1#post4327784 1969 Chevy C10 Shortbed 4.5/6?" Frame off resto http://67-72chevytrucks.com/vboard/s...d.php?t=548136 1999 Toyota Tacoma 4x4

07-09-2010, 06:53 PM	#7
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: what could be hissing? A vacuum leak, if that is what it is, can effect your engine idle and ignition advance response. If severe and if you have power brakes or the leak is in the PB diaphragm, it could be dangerous. Start by checking what vacuum you are pulling with a vacuum gauge. Maybe one of the auto parts chains could loan you one? If it looks like a vacuum leak start tracing all your vacuum lines looking for breaks/cracks. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7

07-09-2010, 11:39 PM	#8
ERASER5 Registered User Join Date: Jun 2006 Location: Indiana Posts: 4,859	Re: what could be hissing? I looked at your build thread. I say vac leak under the dash. You could just plug off the line before it goes into the cab, and then listen for the hiss, then you will know for sure. The vacuum operates a door in the cowl, a vent door, and a heater shut off. There may be more that I am not aware of. Oh yea, as stated above, a vacuum leak can be very bad. Your brakes could be affected, and under the worst conditions, a very lean burn can melt a hole in your pistons. Bad day indeed! __________________ '70 GMC C1500 LWB Power disc brakes. WooHoo! Posi 6 Lug Dana 60 Last edited by ERASER5; 07-09-2010 at 11:44 PM.



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