AC mystery

So this weekend was the first warm weekend we have had. I turn on the AC to my 02 C4S and it is blowing warm. I took it to a shop this morning. It wasn't a Porsche shop but a reputable shop that I have dealt with before and never had an issue. Their shop's AC guy evacuates the system and sure enough it didn't have any freon. He ran a vacuum test and found the system held vacuum just fine. He also ran some dye in it just to be sure and found no leaks. Sure enough, he filled it with the proper amount of freon and the system runs great. On the one hand I am happy it was just a service and nothing else. On the other hand, WTF? How does a system that holds vacuum and works perfectly ever lose any freon let alone all of it? I have heard these cars will because they have such long hoses linking the compressor at the rear with the cabin and the evaporator at the front that they just "eat freon" because the hoses leach a tiny bit over their length as time passes. I have also heard that if you don't run the AC it will lose its freon for some reason. The previous owner of the car treated it as a garage queen weekend driver and may never have ran the AC for all I know. I have never believed either of these theories. Yet, I am at a loss to explain the missing freon. Maybe the shop messed up and there is a leak. I seriously doubt that though since they have every reason to find a leak so they can charge me to fix it and checking if a car is holding vacuum is not something even the worst tech is likely to screw up. Any theories on this? We will see how long the car holds its freon. I bet considering the known competence of the shop, it holds it for a long time. If it does and the shop really didn't miss a leak, how could this have happened?

AC refrigerant also contains oil to lubricate the compressor and o-rings so they don't dry out and leak. If you don't know when the ac was run last time, then anything is possible. I would just keep an eye on the ac and run it from time to time, even in the winter. If the system holds, then nothing more to worry about. Some leaks will only manifest itself under pressure but not under vacuum so that's also a possiblity.

It is a DD. So from now on the AC will be run frequently. The point about it only leaking under pressure is a good one. It might have a very small one that results in losing a can or so of freon every few months. Clearly it doesn't have any serious leaks or it wouldn't have held vacuum.

My '99 cab has to have a one to two pound Freon boost about every 6-8 months. It's been tested many times over the years with dye added each time and no leaks have ever been found. Oh well. I can live with it.

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The system holds only about 2lb of refrigerant total so if all that leaks out in 6-8 month, that's a pretty big leak. You should try a better shop Did they use sniffer as well? They may need both e.g., sniffer is good at detecting evaporator leak at its drain hole.

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IMHO, even one can every few months is a big leak (relatively). One can per 2 yrs I can accept. Also it takes only a very small leak to deplete all the refrigerant, which is at about 60 PSI at rest, depending on temperature. Kinda like water pressure from a faucet.

Is Freon refrigerant still being used in the US ? Its been banned for years here in Australia and in many countries in Europe. Well documented for destroying the ozone layer.....

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No, not freon. We have been using R134A in USA for many yrs now. Next is R1234yf.

I had the same issue - my charge would last several years, then would have to be recharged. The leak is too small to detect w/ dye but over time will eventually leak. My climate control module went out and needed to be replaced, and one of the 2 blower fans is also in need of replacement (and maybe it contributed to the CCM failure and freon leak?).

An AC that holds the charge just fine when used regularly can lose the charge if unused for a month or two. I don't know if it's dried-out seals or what, but I've just accepted it as truth. A cheap auto zone refill kit with pressure gauges will work just fine for recharging the 996's system...

Just because an A/C system holds vacuum doesn't mean it won't leak when you put refrigerant in. You are supposed to pressure test the A/C system with nitrogen at ~100psi pressure. That is about what the rest pressure of the system is when off. When running, the high side can see pressures up to 300psi. You can have a sizable leak in a system that holds vacuum. Refrigerant hose slowly leaking refrigerant should not be a problem with barrier hose designed for R134a. This happens on old R12 non-barrier hoses leaking refrigerant, primarily in R134a converted cars since R134a molecules are smaller than R12. It is usually easy to tell if a hose has been leaking this way as it will be covered in refrigerant oil, usually the high side hose(s). Most of that length from the back of the car to the front is aluminum A/C pipe, but the 996 does have multiple condensers and Porsche seems to have added hoses before the condensers (not common, but probably due to the long lines and chassis flex) so there are more hoses and crimps in the system than other cars. A system that needs 2 lbs of refrigerant added every 6 - 8 months as a huge leak and if an A/C tech cannot find it, better find another tech. Some compressor designs have carbon shaft seals that will begin to leak slowly after they are several years old if they are not run for several months. It is possible the 996 has such a compressor shaft seal design. A system that continually has refrigerant added to it over the years will continue to lose refrigerant oil until the point where there isn't enough to properly lubricate the compressor. At that point, the compressor will start to seize up and will fry the A/C clutch. You can add refrigerant oil when you add refrigerant, but you never know how much has been lost and if you don't put in enough, compressor dies. If you put in too much, you compromise the efficiency of the system. Hopefully the 996 does not have the same stupid design evaporator as the 928 does where they used epoxy to bond the evaporator "U" pipes to the evaporator core. This design is guaranteed to fail after a certain number of years, depending on usage. However, you never know since they also decided to use epoxy on the 996 to bond coolant lines (WTF??). Usually difficult to detect leaks are in the evaporator and are unfortunately the most painful to fix in most cars. Dan

Its my understanding that when the defrost is selected, it will operate the A/C compressor to remove moisture from the circulated air when passing through the evaporator.The "O"ring on the compressor needs stay lubricated to help keep it sealed. Letting it become dry from inactivity can be a source of a refrigerate leak. Gotta run the A/C or defrost a couple times a month.

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You may be right. A/C is all they do and it's the shop my indie uses as he doesn't do A/C. They say they've sniffed it and added dye each time, but no leaks have been found. I could be wrong about the amount of Freon. After about 6-8 months, I can tell it's not cooling correctly, so they top it off. They normally say it was a pound or so low. It did seem like it lasted longer when I had them to evacuate and recharge it.

I picked it up yesterday and the way the shop explained it was that on really hot days your coolent expands and the pressure in the system goes up significantly. Sometimes you get a leak that just won't show up until you get a hot day and the resulting pressure increase. I am fine until it gets hot and may even be then depending on how small the leak is. If it does get hot and I lose cool, they will then run some die through it and hopefully find the leak.

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Makes sense. As someone mentioned in a previous post, the high side can reach a few hundred PSI, compared to vacuum, which is ~14 PSI.

Yes, the pressure in a refrigerant system at rest depends on ambient temperature and the temperature of the components (PV=nRT, well kinda). However, if you leak test with Nitrogen, you can set the pressure to whatever you want. Dan

John, You have a leak that should be traceable. Pulling a vacuum to check leaks will only tell you if you have a 'gross' leak, -30 inches of mercury is about -14 psi. The charged system static pressure, at rest (system has been off for a while), ranges between, say 70 psi (70f) to 124 psi (100f). A modern day leak detector can find a leak at those static pressures as small as .25 oz per year easily. The issue typically is where to look, and unless the technician is familiar with your model car he may not take the time to trace it. Checking for leaks while the system is operating does tend find leaks more easily on the high side of the system. UV tracer dye in the refrigerant oil 'sometimes' shows up provided you can access (see) the leak point. The o-ring elastomers used today do not dry up. And, the post 1989 year models primarily use aluminum tubing vs. the earlier years having 40 feet of rubber hose. There is no need to run the system often (these aren't old chevy's with cork valve cover gaskets). You can run a system once year. Take it apart a year later and the elastomers will still be resilient and the compressor pistons/cylinders and bearings will be wet with oil; it does not evaporate nor disappear into the twilight. Unless you are familiar with gauges, system pressures and theory, 'topping off' a system is a crap shoot. Without pulling a complete vacuum and properly charging the system you could be mixing refrigerant with air and getting false pressure readings as well as an under performing system. Have someone experienced find the leak; the evap core on your model is a common place to start.