Use shop air to find AC leak?

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Wife’s 2007 Pacifica has a leak in the AC system. I put gauges on it yesterday and there’s no pressure at all, so it’s completely empty.

I’ve read mixed reactions to the idea of hooking up shop air to the system to pressurize and listen for leaks (or use soapy water). The criticism is that this introduces moisture into the system. Others push back by saying a big leak means the system already has regular ‘atmosphere‘ air in it, and you’re going to pull a vacuum anyway once the repair is made.

Any thoughts on this?

And if I don’t DIY the diagnosis, is taking it to a pro for them to use nitrogen the only other option?
 
Did it lose it all at once, or slowly? Either way, empty i'd take it to a shop for nitrogen. Using shop air might not lubricate it. If you lost refrigerant, you also lost oil.

That way you do not vent excess refrigerant to the atmosphere. Plus you will probably need a new accumulator/receiver-dryer anyway.
 
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I'd try to push 2oz of UV dye and a pound of Freon it. System will have to kick on to circulate the dye so you might need more than can of freon.

First inspect the condenser for damage and all the connection points for any wetness.

I would not pressurize with shop air.
 
Did you already look for any obvious signs of a leak? Where the Freon got out, some oil must’ve gotten out, where oil got out dirt should’ve been attracted to it. We used to easily find Chrysler condensers leaking at the (bottom) corners usually.
 
Most shops use shop air if they even pressure test it at all. Most don't. You can add a dryer which would be a very good idea.

Or bring it to me. I cobbled this nitrogen pressure tester up many years ago.
 

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We found a leak using UV dye, very clear using a UV flashlight.
Obviously need some refrigerant in it but also had an electronic sniffer to double check the whole system.
 
I literally used a bicycle pump to find the AC leak in my 2001 Honda Civic. I pumped air through the pump into a dryer with desiccant then to the car. The psi of 40 or so quickly revealed the AC line that was leaking. That was over a month ago and it’s still working fine. Soap and water made the leak obvious.
 
You can boil off any moisture in the system by pulling a vacuum for a good hour. That’s if you have a vacuum pump.

I’ll be honest with you, when most AC repairs are done we fill the system, locate the leak. Repair it and the moisture that’s already in your system is most likely staying there. It’s not like a garage is going to leave their AC machine on that car, pulling vacuum for two hours to boil off moisture. It’s out the door and the next car comes in. The receiver dryer will absorb some of it.
 
I used shop pressure and soapy water to find a leak on my truck's condenser while it was removed from the system. Vacuuming the system down and isolating, just tells you there's a leak. It's very difficult to locate that leak with vacuum. Pressurizing with the freon is the best solution. In the case with my truck, there was a service bulletin on the condenser and the area was not leaking enough to detect with a one hour isolation of vacuum.

I am in the process of diagnosing and repairing the neighbor's car AC, so this thread caught my eye. I have had a vacuum pump and a few sets of gauges and many other AC tools over the past 30 years. Just for my own DIY stuff. First in R12, and now just the R134.

It started with a request to just add some freon. It's turning into full leak diagnosis and repair.

Neighbor asked if I could help with his wife's AC. I accepted. I'm not big on doing other people's vehicle AC service, but he just asked if I'd "add some freon". I put the gauges on and it was fairly low, so I added 4 oz of R134. Pressures came up, but were still lower than necessary. So, 4 more ounces of R134. It worked ok, but not great during the test drive. I'd say it was marginally acceptable. OAT was 81F and the vent temps were in the 50s. I told him I suspected the drier was in need of changing and since the low side was lower than specified for the high side pressure, I suspected the orifice/expansion valve needed cleaning or replacing.

The drier came in last night. But meanwhile, the system has leaked down again. So, leak diagnosis is on the schedule today. I have a sniffer as well as a dye injector and black light. I'm not looking forward to this.
 
Did it lose it all at once, or slowly? Either way, empty i'd take it to a shop for nitrogen. Using shop air might not lubricate it. If you lost refrigerant, you also lost oil.

That way you do not vent excess refrigerant to the atmosphere. Plus you will probably need a new accumulator/receiver-dryer anyway.

You don't need to run the system with the shop air, so lubrication isn't an issue
 
You can boil off any moisture in the system by pulling a vacuum for a good hour. That’s if you have a vacuum pump.

I’ll be honest with you, when most AC repairs are done we fill the system, locate the leak. Repair it and the moisture that’s already in your system is most likely staying there. It’s not like a garage is going to leave their AC machine on that car, pulling vacuum for two hours to boil off moisture. It’s out the door and the next car comes in. The receiver dryer will absorb some of it.
For a regular shade tree mechanic you can borrow a vacuum pump from the parts store. Make sure it has vacuum oil. Next door neighbor fixed my 2008 Miata I gave him that had 2 “O” rings located directly on the AC pump that flattened out. He used compressed air to find the leak and we installed the vacuum pump to dry and went for a long lunch.

If I ran a shop I might put dry nitrogen in but if you use soapy water even under pressure moisture can get in. (believe or not) You can use a mechanic stethoscope to listen.
 
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I used dye, an Amazon UV light and a small can of 134a to find my Passat TDI's a/c leak. Talking to a/c mechanic friends they thought it was most likely the high side. Sure enough the bottom on the condenser showed the dye (and oil and dirt). The bottom row was pretty corroded after 9 years and 233k miles. Condenser cost ~$275 from VW, UV light was ~$25, I had HF vac pump from previous repairs ($75?), 134a was ~$35. Took me a few hours and probably saved $500 to $750. The weather is cooling off, if you have the time, tools and aptitude go for it. Good luck, John
 
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