AC Center Vent Hissing Even When Car Turned OFF

I got a refrigerant recharge because the AC was not working -- blowing warm air and the compressor was not even engaging because I was almost all out of R134a.

I took it to a mechanic and the machine showed I had only 50g of R134a in the system. So the mechanic filled it with the specified 740g of R134a and the AC worked great at the shop.

Immediately afterwards, I hear constant hissing from the center vents, even when the car is turned completely OFF.

Any ideas why I am getting this hissing sound EVEN when the car's ignition is OFF?

Thanks

I would not be concerned about some hissing when the car is first turned off because the low and high side pressures may be equalizing by the expansion valve. That should go away within 30 seconds or so. If you still hear it, you may have a leak. If you hear it from the center vent, it could be the evaporator. My dad needed that replaced in his E46. Not a fun job.

If you can take it to the shop, they may have an A/C sniffer that can detect refrigerant gasses. Harbor freight sells one for ~$50. If you stick it up near the vent and it goes wild, the evaporator is likely your issue.

I think you're correct about the evaporator having sprung a massive leak. No more refrigerant left, just after a day.

How much did your dad's evap replacement cost, parts and labor?

Have you guys ever heard of a freon recharge that actually worsens a small leak into a massive one in the evaporator? I'm wondering if my shop messed up the freon recharge process, even though it was all done by machine.

I know it's not well received, but before you go replacing the evaporator, you could give a shot at this stuff:

https://www.amazon.com/dp/B000CO9IBG

It's not the old stop leak crap that turned to sludge when exposed to air - it's an oil that swells the old, shrunken o-rings throughout the system. It's worked great for small leaks on both of my e46's.

Probably won't help with a leak that large, but worth a shot I'd say

Sent from my Pixel 3a using Tapatalk

I was thinking of doing that and using Stop Leak. But I feel the leak must be rather large because the freon was gone in less than a day.

Sorry for your troubles and that bummer of a job. That is an absolutely huge leak that the stop leak will do nothing for. In fact, then you just have the stuff gumming up your system.

My dad's car is an E46 M3, so probably not very different from our cars. He ended up paying around $2000-$2200 IIRC. That was from the dealer who originally quoted $3000, but our indy mechanic quoted $1800. The dealer wanted the job as you can see they came way down in price. Unfortunately, the whole dash needs to be disassembled.