What causes an air leak sound when the engine is off?

[CheeseHelmet]

Last night when I got home and was walking by the back of my P5 I heard a psssssssssssssssssst sound that gradually quieted down. It sounded like when you let air out of a tire by sticking a screwdriver in the valve.

Let's play .... IDENTIFY. THAT. SOUND!!!

 

[FlyinHawaiian]

Think of your engine as a pressure vessel. The intake being under negative pressure and the exhaust under positive pressure. Once you shut your car off, the pressure in the system needs to equalize, specifically on the intake side. That's what you're hearing...

 

[CheeseHelmet]

I've never heard it before though, it lasted a good 20-30 seconds. I'll be sure to listen for it now

Could that be the only cause of the sound?

 

[Flash5]

the only hissing sounds i have ever heard has come from my intake but that only lasts maybe 2-3 seconds, not sure what could be making a 30 second hissing noise...

 

[Matt V]

The stock intake also makes the same noise that an aftermarket intake does.

 

[03 mp5]

That noise could also be from the ac compressor like if you are useing the ac or defrost i have heard the same sound on my car i thought i ran something over but then poped the hood and thats where it was comin from.

 

[i12drivemyMP5]

The stock intake also makes the same noise that an aftermarket intake does.

The stock one just conceals it more. Betting OP just changed out intakes recently, lol.....

 

[8.5MS3]

most likely the ac compressor. its probably refrigerant passing thru the expansion valve while the system equilizes. my previous car did this, it i could actually make that happen by playing with the fan and temp dials after the car was off

 

[CheeseHelmet]

My A/C *should* be off, I was running with the windows down and the A/C and fan buttons on "off". Keep the ideas coming, this is amusing

 

[i12drivemyMP5]

It's just the compression or something disappating after turning the motor off. The oem intake suppresses it and aftermarket intakes amplify it. Do you have oem or aftermarket intake? I never really noticed it until getting an aftermarket intake. Does it whether I had AC on or not.

 

[CheeseHelmet]

It's just the compression or something disappating after turning the motor off. The oem intake suppresses it and aftermarket intakes amplify it. Do you have oem or aftermarket intake? I never really noticed it until getting an aftermarket intake. Does it whether I had AC on or not.

Nope, everything's stock on my car. I'll check the air intake to make sure everything's properly put together though.

 

[P-Funk!]

Brake booster leaking down? (bad check valve)

Gas tank pressure leaking down (bad gas cap?)

The last air in your spare tire? (Least likely)

 

[CheeseHelmet]

Brake booster leaking down? (bad check valve)

I like it! I just changed out most of my rear brake parts and bled the entire system (perhaps I broke something). I'll inspect the check valve tonight or tomorrow (I'z at school all day).

 

[P-Funk!]

No - none of the maint you did would affect that.

If you seafoam'ed and used the brake booster vacuum line - then, maybe...

Inspect brake vacuum lines for cracks and leaks - and listen at it after shutdown (and be sure it is not the intake)...

 

[Donas64]

Purging of your super secret NOS tank?

 

[FlyinHawaiian]

Purging of your super secret NOS tank?

Win

 

[CheeseHelmet]

I checked the air intake, the vacuum hose, my spare tire, and my super secret NOS tank... It all looks good and the sound didn't come back after a short ride so it's a mystery for the time being.

It's a real shame though, I was prepared to award the person who correctly identified the sound with a genuine chunk of rust from the bottom of my car ;_;

 

[P-Funk!]

Don't throw out that rust yet! I am not done!

uhm.



uh....


Okay, yeah.
Throw it out.

 

[P-Funk!]

Fuel pump bleeding down pressure.

Is it hard to start - takes longer?

Gas cap/tank pressure

Shut it off and quickly loosen the gas cap while it is hissing - does it suddenly stop?

 

[CheeseHelmet]

Fuel pump bleeding down pressure.

Is it hard to start - takes longer?

Gas cap/tank pressure

Shut it off and quickly loosen the gas cap while it is hissing - does it suddenly stop?

It isn't hard to start at all. And next time I hear it I'll make sure to take off the gas cap to see if that makes it stop.