vacuum leak??
Thread Starter
Burning Brakes
Joined: Jan 2004
Posts: 933
Likes: 0
From: Inland Empire CA
vacuum leak??
Well I finished putting a regular LS1 intake on my car... but it doesn't run quite right... there's a noticeable hissing noise... it changes with the rpm. Like if I blip the throttle it will get really sharp then quiet down. And when I initially started the car the rpms went up to about 2k then it went down to 400 or something and almost died but it recovered. One time the rpms jumped up on me by about 500 without giving it gas. Also when the car is shut off you can hear a loud suck - like the sound an intake has but considerable louder... So am I right vacuum leak - most likely the port gaskets? I just want some confirmation. Thanks.
I didn't use new intake gaskets because well... they're so damn expensive. I looks like I may be getting some after all.
I didn't use new intake gaskets because well... they're so damn expensive. I looks like I may be getting some after all.
Last edited by WickedC4; Jan 22, 2007 at 02:09 AM. Reason: spelling/grammar
Melting Slicks
Joined: Sep 2006
Posts: 2,041
Likes: 4
From: Rockford Illinois
Well I finished putting a regular LS1 intake on my car... but it doesn't run quite right... there's a noticeable hissing noise... it changes with the rpm. Like if I blip the throttle it will get really sharp then quiet down. And when I initially started the car the rpms went up to about 2k then it went down to 400 or something and almost died but it recovered. One time the rpms jumped up on me by about 500 without giving it gas. Also when the car is shut off you can hear a loud suck - like the sound an intake has but considerable louder... So am I right vacuum leak - most likely the port gaskets? I just want some confirmation. Thanks.
I didn't use new intake gaskets because well... they're so damn expensive. I looks like I may be getting some after all.
I didn't use new intake gaskets because well... they're so damn expensive. I looks like I may be getting some after all.
It is a vacuum leak. Stick your hand at the very back behing the intake. There is a port right next to the brake booster vacuum line that should have a small vacumm line there. The line could have gotten behind the intake. Once you find it your problems will stop.
The gaskets are reusable, unless they are split I would leave them. Otherwise GM sells them for $50 a set.
Thread Starter
Burning Brakes
Joined: Jan 2004
Posts: 933
Likes: 0
From: Inland Empire CA
I'll double check the small vacuum line, but I remember putting it on nice and tight. The service manual says not to reuse them... I suppose some people have problems some don't.
Team Owner
Joined: Mar 1999
Posts: 22,025
Likes: 192
From: At the beach in little Rhody
Gene Culley will have them cheaper, but they obviously take longer to get than from a local dealer. You could spray with brake cleaner and locate the leak, but if you didn't thoroughly clean the intake valley and heads you could wash in some grit and sand, which wouldn't be too good. Also check your two vacuum ports up front, the driver's side for the EVAP line and the passenger side for the PCV line. Did you torque it in sequence? I think it is 89 lb-in, but would need to verify as that is from memory.
Thread Starter
Burning Brakes
Joined: Jan 2004
Posts: 933
Likes: 0
From: Inland Empire CA
Yes it was torqued according to the manual, first pass 45 in lbs then second pass 89 in lbs in the correct sequence... although bolts 8 & 9, the rear ones, you just have guess because of the clearance. When I get a chance I'll double check things before I go ripping the intake off again. Thanks.
Thread Starter
Burning Brakes
Joined: Jan 2004
Posts: 933
Likes: 0
From: Inland Empire CA
Well I think the culprit may have been a torn MAP gasket. I put on new intake gaskets as too and it runs really well for not having a supercharger on it. There is still a slight hissing noise.... but I don't think it is a vacuum leak. The car runs really strong and has absolutely no knock retard at WOT. Thanks for the input guys. 
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