Brand new ac leaks

[dv53]

Hi,
I replaced an ac system on a 96 Cadillac Seville STS. I had a certified GM mechanic oversee my work. The unit blows ice cold, but starts losing refrigerant after about 6 weeks. The only thing I didn't replace was the evaporator as it's a bear. However, it always holds a vacuum which would indicate no leaks. I've refilled it three times and have left it for up to two hrs and the needles are still pegged at negative zero when I come back. Any ideas what's going on? Thanks!

[Cornbinder89]

You've got a leak, and you might need a sniffer/dye/blacklight to find it. Could be the service valves, could be the compressor seal. Either you or your GM mechanic are going to have to follow the trail.

[dv53]

You've got a leak, and you might need a sniffer/dye/blacklight to find it. Could be the service valves, could be the compressor seal. Either you or your GM mechanic are going to have to follow the trail.

I'm aware of the leak, but curious why the vacuum holds. Doesn't make sense as that's the procedure to check for leaks on an empty system before filling. Thanks!

[Tom Greenleaf]

? dv53: "Curious why vacuum holds yet leaks"

Understand that we live in "pressure" such that a vacuum is about 14.7 PSI of air held out isn't enough for tire pressure if reversed. That is a standard that it's OK to proceed not proof of no leaks.

When refrigerant is inside and only refrigerant the game changes. It's remarkably sensitive to temps and pressures to what "state" it's in. Refrigerant will also leak where outside air wouldn't - smaller molecule.

Cornbinder89 hit this spot on (no shock there) the sniffer, dye, UV = blacklight) are the usual way to find them it must be filled and work if only a short time evidence will show. Hard to find or see absolutely anywhere it's contained.

The common spots are where anything moves or said the cap(s) for ports very common they get messed with in checking, service if needed.

Good luck some are intermittent, refuse to show evidence. If so have to keep on trying if not DIY costly to just find then a fix that lasts to prove it's done. That's a lot because some are hard,

[Cusser]

I'd think a leak in the evaporator would show itself with the vacuum-holding test.

I'd look first at the service valves, their caps, and the compressor seal. Did you add UV dye? I'd be surprised if a certified GM mechanic would fail to add dye, GM added dye from factory. Does certified GM mechanic have an electronic leak sniffer?

[Tom Greenleaf]

Sniffer would find it if just pressured up and engine off. Shops may not like them as if area has spilled assorted stuff will pick up that and sprays so take it outside have to learn habits of each sniffer wind can louse mine up,

[Mikel]

I'm aware of the leak, but curious why the vacuum holds. Doesn't make sense as that's the procedure to check for leaks on an empty system before filling. Thanks!

Seals and o-rings can behave differently depending on which direction the pressure differential is. Might hold vacuum and leak under pressure. Or viceversa. Sometimes o-rings or seals will leak at some pressures but not at others.

Could try putting some UV dye in the system next time you charge it. Should make leaks quite visible.

[Tom Greenleaf]

Seals are two ways can hold vacuum but not much higher pressure or vs verse. About all of A/C you wouldn't move on to charge if plain vacuum didn't hold is the best we do or another gas if elusive to find.

Dye is factory fill on some do or don't need UV light. Many other tests i a blowout was fixed note dye is everywhere will fade away in some time.

Leaks TMK remain the Home reason for A/C problems BTW if ignored for slow ones can cause lots more to fix/replace so deserve attention. Many can be intermittent is a snag for about anything that could leak and quit it so evidence a helpful tool or looking for oily mess that isn't something else,