Possible vaccum leak?

[NEK5]

Not sure if it is normal or not, but I notice when I have my truck running, and then i shut it off, it almost sounds like a release valve was pulled on an air compressor, except not as loud, like a PFFFFFfffffffffffffffffffff (decending letters as it gets quieter )..As soon as i kill the motor, you can here the hissing.....any ideas? The vaccum lines were all recently replace, not sure where it could be coming from....

 

[Can Can]

It's probably not a vacuum leak. All my trucks have a tendancy to let off what I call a "sigh" when I shut them off.

I think it has something to do with the vacuum-operated heater controls, but don't quote me on that.

 

[Confedneck79K30]

it's a vacuum release from the resevoirs or vac. bottles or whatever their technical terms are...

 

[southernspeed]

Yeah mine does that too. Just as the other guys say, vac stuff for heaters etc.

 

[4X4HIGH]

I vote for a bad brake booster. Disconnect the vacuum hose and plug it then start the engine and let it run for a mintue then shut it off and i'll bet your noise is gone.

 

[NEK5]

I vote for a bad brake booster. Disconnect the vacuum hose and plug it then start the engine and let it run for a mintue then shut it off and i'll bet your noise is gone.

In fear of asking a stupid question, care to elaborate?

 

[Mad-Dog]

that's normal bro, you got like 20" of vacuum built up in your power brake booster and any other vacuum amplifiers that may be hiding under the dash, when you shut off the engine the vacuum source stops and the stored vacuum leaks off.....hissssssssssssss

 

[4X4HIGH]

Your brake booster is suppose to store vacuum and has a one way valve in the hose going to the booster so that once the vacuum source is turned off the vacuum that has been stored will remian in the booster. This is so you have about two pedals worth of assisted brakes with the engine off. If the diaphram has a tear in it or if the booster halves aren't sealing correctly then once you shut the engine off you'll hear the hiss from the vacuum escaping.