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Technical Hissing Sound: AC

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Technical Hissing Sound: AC

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SebastianBerko

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Hi everyone,

I own a Fiat Punto mk2 from 2002 CVT which made a hissing sound whenever I turned on the air con.
In June 2020, I had the system re-gassed, re-oiled, UV dye added + some “leak sealer”. Back then I was informed the system was tight. (which I was confused about because the system was completely empty. It must have escaped somehow, right?)
Now, however, no cold air coming from AC again.

I’m not sure how long the UV dye lasts, probably washed away by now, but last night I checked the system with a UV light and couldn’t seem to find any trouble areas. Of course, I couldn’t see the evaporator as it is buried deep within the dash.

The hissing sound is exactly the same as this guys: https://youtu.be/nPqA7dLhaVo

I fear a new evaporator is needed, but the evap is located in a cosy interior place, so it would be the last item to fail I suppose.
Before I take apart the whole dash apart I just wanted to hear some second opinions.

Would you advice to just have en re-gassed again or would anyone have another way of checking out the evap?
 
Last edited:
SebastianBerko said:
Hi everyone,

I own a Fiat Punto mk2 from 2002 CVT which made a hissing sound whenever I turned on the air con.
In June 2020, I had the system re-gassed, re-oiled, UV dye added + some “leak sealer”. Back then I was informed the system was tight. (which I was confused about because the system was completely empty. It must have escaped somehow, right?)
Now, however, no cold air coming from AC again.

I’m not sure how long the UV dye lasts, probably washed away by now, but last night I checked the system with a UV light and couldn’t seem to find any trouble areas. Of course, I couldn’t see the evaporator as it is buried deep within the dash.

The hissing sound is exactly the same as this guys: https://youtu.be/nPqA7dLhaVo

I fear a new evaporator is needed, but the evap is located in a cosy interior place, so it would be the last item to fail I suppose.
Before I take apart the whole dash apart I just wanted to hear some second opinions.

Would you advice to just have en re-gassed again or would anyone have another way of checking out the evap?
Click to expand...

Make a video of the sound coming from the compressor. There is an electomagnetic clutch on the belt end of compressor. The clutch should click solidly and create metal on metal contact. That noise in the video does sound like metal lightly sliding against metal while it rotates
 
Update for anyone reading this in the future and finding it useful:

I had the system refilled with 550g+oil and the hissing sound is now gone.
The system seemed tight and held under vacuum load.
There was still 30g in the system, so if there is a leak it somehow didn’t leak all of it.
My working theory is that the last place underfilled the system (Punto mk2 takes between 525-575) which caused the hissing sound and perhaps that there is a tiny tiny leak somewhere.
I’ve borrowed an electronic leak detector (the beeping kind) and been around the joints etc and couldn’t get it to go off. I also bought a UV light+yellow glasses and similarly couldn’t find any particular problem area.

So case closed for now. Hoping it lasts for the summer here in Gib.

Thanks for the responses and time taking to watch the videos!! I appreciate it.
 
That hissing sound is made when there is just a little quantity of refrigerant gas in the system, about 20%. And the sound is produced by the wrong pressure in the systeme, around the passing valve high pressure/low pressure. SebastianBerko at yours, something happened that made the gas get lost, there must have been an opening. Did they put contrast substance in the system with the gas and oil so you could see with the UV light and glasses if there's a leak, or the refill was made only with freon+oil?
It is usual that the orings from the end of pipes to be damaged and there to be the leak. If your system still looses gas, do a visual check. You can see blueish/greenish traces where the gas gets out of the system. If the hissing sound comes back, definitely there is a leak, if not, something was done wrong at your first refill.
 
Vaccum testing isn't the best for leak finding as it tends to close up the joints where it can be leaking from
Nitrogen testing is a lot better but tends to be only done by specialised AC engineers
 
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