Have an 03 Protege DX, non-turbo. Lately I've noticed a hissing sound coming from under the hood right after shutting the engine of. Could this be related to the fuel injection? A leaking regulator valve perhaps? Car seems to start and run OK. Once in a while idles a little rough. Thanks.

 

it is probably just the a/c condenser blowing off the pressure when you turn off the car.

 

have you check ed cooling system? when car is turned off , the pressure starts to buld in cooling system. water/antifreeze could be returning to overflow reservor or ......... leaking on to a hot engine !! lift the hood and find the noise , soon!

 

could be a vacum leak they will hiss for about 4 second after you shut the motor off

 

Don't think it is AC, as does it when I'm not using it. No water leak or coolant overflow either. Probably vacuum leak. I hear the sound when engine is running with hood up, but can't tell where it is coming from. Will have to have checked out when I get drive belts replaced. Thanks all.

 

Get a 2 ft.piece of tubing and improvise a stethoscope so you can pinpoint the noise.
The duration of it suggests it might be the pollution control evap loop returning vapor to the charcoal cannister. There might not even be a leak at all, just the "heavy breathing" of the valve. If you look on the underside of the hood you should see the "map" of the evap loop plumbing.

 

Hey folks,
Been a long time. Still have the same symptom, but think it must be normal. My mechanic discovered a cracked intake hose that I replaced, but it had nothing to do with this issue. Car performs fine, so I'm thinking this is normal. Just a hissing valve somewhere. Might try the tubing trick if curiosity gets the best of me. If anyone has any other suggestions or insight, would be interested in reading it. Thanks.

 

I have the same problem with my 03 Protege. The hissing noise comes from the front left side of the engine, somewhere by the ac compressor.