J S Machine

I have a hissing sound coming from my engine. Very loud and noticeable, and it doesnt really vary with RPMs. I'm thinking maybe an intake gasket, but what would cause that to go bad all the sudden?

tblu92

Sounds like a vacuum leak--best way to check is to get some brake-clean and spray it around the intake manifold and all the hoses near there including the throttle body---If a leak is present the engine will try to die--Brake clean will not leave a mess as it dry flashes clean--another option is to use starting fluid--which will make the RPM's flare UP--

69SSC5

i agree with the above poster... sounds like you have a vacume leak. I would use a Propane Torch (do not light) and go over the intake to see if it jumps your RPM, if and when it does you found your vacume leak... as far as intake gasket going bad all of the sudden not sure, its a pretty beafy gasket. try to narrow down your hissing sound with the propagne torch and post back what you found

J S Machine

wouldn't the engine have trouble running if it had any type of intake or vacuum leak? It doesn't seem to run any different and the gas mileage is about the same.

dgrant3830

Check the PCV valve's connection tube to the intake.. look for it to be hooked up and/or holes in it

runner140*

Use starter fluid.

69SSC5

depends how big of a vacum leak but in general yes the motor should run a little funny... usually idles irratic with a vacume leak. you say it runs fine, does it idle a little funny since the hissing started???
are you sure its not a tiny exhaust leak that your hearing, does the hissing go along with the RPMs or at least lower RPMs,,, like hisss, hisss, hisss, hisss, hisss or is it one constant Hissssssssssssssssssssssssssss

tblu92

Depends if the leak was pre MAF or after the MAF--after the MAF ( most likely) is un metered incoming air coming into the engine-- With pre MAF leaks-- the leaking air gets compnsated for via the ECM's fuel trims
Adjusts very quiclky
However an after MAF vacuum leak may take some leaning for the ECM to compensate for it----the larger the leak--the longer it takes--A huge leak may cause the engine to die--but it dies by triggering other safety tables---( like min. RPM ) etc

cdkcorvette7

I had the exact same symptoms a while back. Prominent hissing sound (I suspected a vacuum leak) but the car ran pretty normal. It turned out that the little vacuum supply line off of back of my intake manifold had been broken the last time I'd removed the manifold... It runs from the back of the manifold down underneath the battery compartment... Sometimes a leaky battery will drip acid onto the lines beneath it and cause this problem; pretty easy place to check. Do all of your AC modes work properly? If you had this problem your ac would be blowing air out of all the vents all the time irrespective of what mode you have it set in. http://forums.corvetteforum.com/c5-g...with-pics.html

If you don't have that issue I would start by checking the PCV line off the right side of the intake (mentioned above) since it's so easy to check.

If neither of those fixes your problem then with the posters above. Brake cleaner works very well for finding vacuum leaks...

GL

J S Machine

It is a constant hissssssssssssssssssssss.

The idle isn't really erratic. It seems like it always has to me.

I still haven't even checked it. Maybe I can go out there this afternoon. Been getting home from work too late every day to look at it.

rebelheart

Also check your air bridge for cracks or loose connections.I perfer to use brake clean to look for leaks as it leaves no residue and with the wand is easier to narrow down a leak over propane.

tblu92

Yeah--I'm always afraid that with propane if the car backfires i have a bomb in my hand !!! ( Sorry hank hill)---!!

69SSC5

If its a constant hisssssss then it sounds like u have a vacume leak either in the intake or before the intake. If u have a vacume gauge I would hook it up to the intake just to make sure do infact have a vacume leak.

celty

I know this is an old thread but i just recently got a GS 2019 and i hear the hiss hiss hiss sound on low RPMs....does that also mean i have a vacum leak??

leadfoot4

What's a "GS2019"??

celty

Sorry, Grand Sport 2019 LT1. I have heard that most LT1's make that hiss or ticking sound when pressing the gas pedal on low RPMs. I will still check it out next weekend at the dealer but just wondered if OP or anyone here found a solution to it :/

leadfoot4

A) the 2019s are out, already?
B) if it's indeed a 2019, it's under warranty. I'd let the dealer document the issue and fix it.

rob-cyn

I am having this same issue with a 99 I just bought. Car runs and idles fine but there’s a constant hiss coming from the top of the engine somewhere. Guess I will try the brake fluid trick. I can’t believe the car is not running bad as much noise as it’s making.

Ronnie_W

Brake cleaner, not fluid!

rob-cyn

hahahaha. Damn right. Spell check must have got me on that one