A/C system passed the sealing test but actually leaked all the refrigerant!

[Tony_1992]

This is so weird!
Succinct version:

I replaced the compressor, did the sealing test(result is good) and added enough R134a, then A/C is normal for 3 to 4 days.
But after 10 days not turning A/C on, refrigerant leaks to almost empty level.

Verbose version:

The last time I replaced the A/C compressor, I did the sealing test following the instruction. I used a vacuum pump to suck all air out of the A/C system, and connected a manifold gauge set to see if it can hold the vacuum for some time. The result is satisfactory. The system can hold a vacuum about 30 psi for more than one hour. That should indicate the A/C system is perfectly sealed, at least theoretically. After that I added 28 oz R134a refrigerant to the system and the A/C worked as expected, super cooling.

This week the temperature in Atlanta is about 70 degree and there's no need to use A/C. So I just drove the car without A/C for about a week. But today when I wanted to get some cooling air, the A/C didn't work at all! After inspection I found that the compressor clutch is not engaged, not turning when the A/C light is on.

At first I think the culprit behind is relay or fuse, but the possibility was eliminated after voltage and resistance examination. Finally I found that the pressure in the low side is extremely low, nearly nothing left inside the A/C system, which means the whole system has a leaking somewhere.

That is beyond my comprehension. Why could it pass the sealing test while actually leaking refrigerant? Does anyone of you guys have any idea what might happen to my car?
Thank you so much!

 

[Tony_1992]

Also, a weird phenomenon(when the A/C works normally) is that when the car is accelerating, such as starting after red traffic light, a slight hissing noise will come out from the central vent, sounds like behind the stereo radio unit. Does that indicate leaking in evaporator core?

 

[Kumba]

What you are hearing is the gas in the liquid line swirling around and whistling through the system. Just like how air trapped in plumbing lines makes a loud pop/hissing sound when you turn on a faucet.

Now why it's doing that is another question. Usually a non-stop hissing/gurgle sound points to low refrigerant levels. It can also be a worn out compressor or a clog in the system somewhere. About the only way you can test is with a proper set of gauges to read high and low side pressures while it's operating.

Given your explanation, you definitely have a leak somewhere. A nice long slow leak. That's usually going to be a failing o-ring somewhere. If you are sure your compressor isn't leaking from the front seals (a pretty common thing with age), then I would try to open up all of the system you can easily get to and replace all the o-rings along with the receiver/drier. Have the system purged then filled again and see if the problem goes away.

 

[Tony_1992]

What you are hearing is the gas in the liquid line swirling around and whistling through the system. Just like how air trapped in plumbing lines makes a loud pop/hissing sound when you turn on a faucet.

Now why it's doing that is another question. Usually a non-stop hissing/gurgle sound points to low refrigerant levels. It can also be a worn out compressor or a clog in the system somewhere. About the only way you can test is with a proper set of gauges to read high and low side pressures while it's operating.

Given your explanation, you definitely have a leak somewhere. A nice long slow leak. That's usually going to be a failing o-ring somewhere. If you are sure your compressor isn't leaking from the front seals (a pretty common thing with age), then I would try to open up all of the system you can easily get to and replace all the o-rings along with the receiver/drier. Have the system purged then filled again and see if the problem goes away.

Thank you for your help. Yes you are right, there must be leaking somewhere. The hissing noise didn't show up in the first 3 or 4 days of adding refrigerant, perhaps because the leaking is very slow and so the refrigerant level is not that low to cause hissing sound. I'm preparing to buy a uv dye to see where the leak is. The hardest part is the expansion core, which is hidden behind the instrumental panel. Do you have any good idea how to inspect the expansion core, without taking all parts off?

 

[joegr]

You didn't do any "sealing test," you just fooled yourself. Your test only verified that there isn't a gross leak.
A few points. It's not 30 PSI of vacuum, it's 30 inches of mercury of vacuum. You didn't have a micron gauge, so you could not read low vacuum accurately enough to know if you made it all the way down to a good vacuum or if it held. Anywhere the system has been leaking for a while, there will be a dirty/oily stain. The factory refrigerant charge included UV dye, so that may fool you when you go looking for a leak. There are electronic leak detectors. Maybe you could rent one.
It is the evaporator that is in the dash. The only way to get to it is to remove the whole dash.

 

[Kumba]

It is the evaporator that is in the dash. The only way to get to it is to remove the whole dash.

Isn't the TXV bolted to the evaporator? So essentially you have to pull the evaporator to get to it?

 

[joegr]

Isn't the TXV bolted to the evaporator? So essentially you have to pull the evaporator to get to it?

Correct.

 

[Kumba]

Correct.

Thought so. Now if only getting it out was easier. I'm pretty sure I got some broken actuators in there somewhere so I've gotta bite the bullet and might as well do it all.