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Discussion Starter #1
I'm mostly just trying to figure out if this is normal, as I don't recall hearing it before but I've never really paid attention to it before.

When you shut your car off is there a kind of hissing noise, sort of sounds like air being sucked through a tank or something.?

like say if you were to put an air tank under a small vacuum, and then open it up, the sound of air being sucked into it until the pressure is equalized.

is this a normal sound when the engine is shut off? is it pressure being relieved from somewhere? or do I actually have a problem?

its the 4.6L, and the noise IS coming from the engine bay area, it is not coming from anywhere else on the car.
 

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Pedal Faster
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Its the AC system pressure leveling off, probably. (Just a guess because it's summer.)
 

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Discussion Starter #3
I don't use my A/C much, only high speed driving really, highway and such. so it hasn't been running when this is happening.....thats my best guess as to why it's not the A/C...... that doesn't mean i'm right though :)
 

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I heard the same thing, it was the ac in mine, and I never run a/c its a waste of hp and gas
 

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If the outside air temperature is 50 degrees F (10 C) or higher the a/c compressor will run if the climate control is set to anything other than panel or floor. IE, defrost and panel/floor and a/c.
 

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Discussion Starter #6
I haven't used def or any other panel control, they have been off, I sure haven't been running the heat. I keep the windows down and the climate controls set to off.
 

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A bad brake booster will sometimes do that; there's a check valve that keeps it from losing vacuum when the engine turns off. It could also be anything else on the other side of the check valve, or the valve itself.

You should be able to hear a bad booster by listening carefully with the engine running; putting you foot on the brakes should make a hissing noise, if it's bad. The engine may also stumble.
 

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Discussion Starter #9
so just to be clear on this, there shouldn't be any kind of hissing noise when I shut my car off??? This is not a normal sound and because I'm hearing it, there is possibly a vacuum leak ??

I really just needed to know if this is normal or not, and it would seem that it's NOT a normal sound. I should also add, it sounds like it might be coming from the passenger side somewhere, But I'll check the booster today.


now for that A/C thing.... wtf.... why does the compressor need to run if your using def, or something in temps above 10*C? curiosity here, since I haven't been using my A/C or Def. or anything else, so I know my A/C isn't running.
 

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Nope, no noises should be heard. What I'd do is check the a/c clutch or touch one of the a/c lines by the firewall with the engine on just to be sure the a/c isn't running. Once you've ruled that out for sure, I'd check over the vacuum lines.

The logic behind keeping the evaporator cold at temps over 10C in defrost is that it will dry the incoming air. For the panel+floor setting, it gives you the option to direct cold air at both your feet/the back seats rather than through just the dash vents like normal a/c mode. Again, the only modes the a/c compressor should NOT be running in are panel and floor.
 

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Hissing for how long? Up to two seconds is completely normal as the pressure in the cylinders bleeds off.
I'll agree with you as well. A SLIGHT hiss for a couple seconds is normal. Many cars that I have owned has done this(slight, almost inaudiable). If it sounds like the air brakes on a 18 wheeler, then you have a problem. If you leave the vent system on ANYTHING other than off, you will also hear this. Remember as said before that the AC will be functioning if you have it set to defrost(or defrost/floor).
 

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Discussion Starter #14
Hissing for how long? Up to two seconds is completely normal as the pressure in the cylinders bleeds off.
maybe this is what it is then, I've just never paid attention to it before.

Maybe I'll see if I can get a video so it can be heard by all :)

I'll try and get one up for later.
 

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Whatever cylinder that is on the compression stroke when you shut the engine down is bleeding down. This is more pronounced on this engine compared to a pushrod motor because there is no lifter valley volume.

Mitch
 

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Discussion Starter #16
I never knew that....never actually thought about it.

Where does the pressure bleed off at? being that compression is important, how can it bleed off when the engine shuts down, without bleeding off when it's running?
 

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Discussion Starter #18
is that every engine?? even new ones? that means that even moving fast, there is a small amount of compression being lost that way, minute enough that it doesn't make a difference I'm sure.

I had honestly never thought of that, I guess I just assumed that the compression cylinder stayed compressed.
 
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