I have this loud hiss coming from my plenum/TB area. It is not a vaccum leak because the car idles at 750 in neutral and sits there with little to no surges. The upper intake gaskets and the TB gasket have been replaced with factory GM ones with the ridges. All of the vaccum lines have been checked with no leaks. One of the small hoses was disconnected and the idle instantly rose so like I said above there cannot be a leak. I am just really curious what is making that sound. It sounds like a leak but itsnt a leak. Any ideas would be greatly appreciated.

BTW the car is a '89 305 5 speed.

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"I'm not dumb, I just have an amazing command of thoroughly useless
information."
Bill Waterson creator of Calvin & Hobbes.

 

Might want to check and make sure that the hose on the AIR is tight and secure where it heads to the cat.

 

That is on as tight as it can be. I also have checked all the way around the engine and it can only be coming from the area that I expained right around the TB/Plenum area.

 

Have you checked the EGR solenoid?

 

If you cannot associate the noise w/ a problem then i'd assume what you are hearing is normal intake noise. If you take the ducting off the front of the TB you will be deafened by the noise that air going through the TB just at idle makes. Ok, not deafened, but it is loud. Some of that noise makes it through the ducting, etc and is prolly what you are hearing.
...ed

 

That's what I was thinking. I know that on my car if I juice the motor by hand that I can hear the IAC valve leting air in when it comes back down to idle speed.
bsa

 

This just happened to me and it turned out to be the alternator. Try unplugging the plug that plugs into it and see if you still hear the noise. You can also take a large screw driver touch it to alternator and put your ear on the other end to find where the noise is coming from.

 

Ed and bsa I was thinking the same thing that you said. I just wanted to be backed up by someone that is more knowledgeable than I am. I will also check the alt. and egr solenoid just to make sure it isnt either of those. I am almost positive that it isnt the alt since it is only about a year or so old and I havent put but 8,000 miles or so on it. Thanks for the replys.

 

all else fails use carburattor cleaner or compressed air to spray the area in question it will surge if it is leaking don't rule out a vaccum leak had a small one comng from the intake base and my idle was rock solid as well at least until i just the new chip in that is

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87 trans am 350,L98 aluminum heads,LT4 hot cam,slp runners,slp 1 5/8 headers,3in.y-pipe,edelbrock base,hi flo cat,air foil,afpr,as&m ported plenum,t-5 tranny w/centerforce clutch and a 3.27 9bolt(11.9in. brakes),ed wright's fastchip,relocated iat sensor,tb bypass,accel supercoil,hollowed maf,slp 3in. catback,a/c delete.
oh!no!i think my maf is going!!!!!!!!!!!!

14.10@97mph w/2.01 60'

slp catback is finally on!!!
fastchip prom is in.OH!MAN!
FLOWMASTER S.U.C.K.S