pressure hissing sound after turning engine off???

[warrenb5]

OKay

Audi A4 1.8SE 1999.

After a good drive and engine is warm as soon as you turn engine off and get out you can hear a hissing / pressure sound.

I have done various fluid checks and it is not losing water.
I have checked the oil and no signs of head gasket failure, drives perfectly fine.

Its been doing this for months and I cant get to the bottom of it, any clues???

 

[boost-addict]

Check the elbow from the manifold to the bulk head at the rear. This holds vacuum after the enigine turns off and is the pipe work for the brake servo. I had to change mine as i got the same hissing as your describing.

 

[B5 BIKER]

High pressure switch for the air-con??

 

[Rhodesy]

Air-con I reckon too

 

[mojocvh]

Same as last two guys.

 

[warrenb5]

what, do you mean air con switch??
If so does anyone know where its located on the car, I can have a look.
Thanks in advance.

 

[B5 BIKER]

Not a 100% sure where it is on the A4, but on my A6 it was on the gas pipe behind the headlight.
You might be aswell to get an air on specialist to check and sort as the system will need re-gassing and you don't want be letting out more cfc's into the air than you need

 

[Broken Byzan]

if the sound is coming from up under the scuttle then it's the aircon slowly depreassurising after having been run, it's normal

 

[warrenb5]

Yep under the scuttle, sounds for about a minute after a really long run, so yes sounds as if de-pressurizing

 

[Chriscooke]

Mines always done this. It sounded just like the noise my other car made from the rad expansion bottle when the headgasket was on the way out but seems fine so far, and this thread has given me some relief