hissing, perfect AC

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Staff member
Dec 14, 2002
New Jersey
Hello, Recently Ive noticed something in most of our car AC systems that I have not remembered hearing/noticing before... If one turns on the AC with the vehicle on, fan blowing, etc., there is a notable hissing noise from the vents. It is the AC system, not a vacuum actuated air diverter or similar device - I know that for sure. It is a hissing sound, almost like a very muffled sound of evaporation, which obviously I'd assume to have occurring in a closed rankine cycle... but I never used to hear it, and I don't hear it in every vehicle or every time. The catch is, the static, resting pressures and the operating refrigerant pressures all seem to be fine... Any thoughts? Have I just not been observant enough, or does this indicate anything in our aging vehicles??? Thanks!
I think you're just observant. Either at, or just before the evaporator is a small orifice. This is where the refrigerant gas goes from high pressure to low pressure. This is called the "Joule-Thompson effect", and renders the fluid cold. Any moving, turbulent fluid generally makes a lot of noise.
OK, I am acceptimng that it is just the turbulent fluid going through a phase change... that can be noisy - think of water dropping on a hot plate... that is kind of like refrigerant going into the evaporator, right? I am aware of J-T effect/valves, and the cycle in general... Just odd that on multiple vehicles, within the last 6 months, I note it unlike ever before. Weird. Just wanted to verify that it didnt mean low lube oil, refrigerant, growth in the system or some other situation. Thanks!
Good question. You didn't notice the hiss on older vehicles. Maybe it's more compact automotive HVAC systems or quieter blowers now?
Maybe... but for example, I just noticed it this weekend on our 94 toyota previa, which has an original, never opened, AC system and 222k miles. Ive noticed it for the last 6 months or so in my 04 saab. I never notice it in my 91 BMW, which has been retrofitted to 134a and works great/cold. Sounds like it is nothing to worry about...
Some of the Chevy cars and trucks I've owned over the years made the hissing noise. A/C systems were great in those vehicles no problems at all. As long as the compressor is quiet and not short cycling, I wouldn't worry about it. JMO AD
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