Air being pulled in to an A/C system??

[dasinc]

I've been in the A/C business for 40 years and have never seen this. 2013 Ford Fiesta came in with a leaking condenser. Replaced the condenser and recharged the system. Working great and pressures where they should be. Vehicle came in a few weeks later with a complaint of not cooling. Hooked up gauges and the system was equalized at approx 200lbs. Reclaimed the refrigerant and the machine would vent a lot of air and system was actually a little low. Recharged to spec and pressures suggested an issue with the expansion valve. Replaced the valve and recharged. Working great and pressures equalized at around 100 lbs. Vehicle is back today with equalized pressure at 200 lbs again. Reclaimed and again got a lot of air venting. In my eyes the only way to pull air in is for the system to run in a vacuum and have a one way low side leak. Has anyone experienced an issue like this? Makes no sense. Thanks for any input.

[dasinc]

After looking at the vehicle I have a theory. Comment if you think I'm nuts. lol. We found a very slight leak at the compressor manifold. This compressor has a manifold that is similar to the denso compressors. 4 bolts with a large oring seal. Also it's not holding a strong vacuum. Is it possible that when the system loses enough refrigerant and runs in a vacuum the lip seal on the compressor is weak and sucks in some air. When shut off the pressure in the compressor allows the lip seal to seal better against the shaft. We are going to pull the compressor and remove the hub and do some bench testing with vacuum and pressure. I'll post results. Again, comments are appreciated.

[JohnHere]

You're thinking-through any and all possible causes, and I would as well. But let's look at it in simpler terms. In my mind, the only other possible way that situation can happen is if someone else (for whatever reason) introduces compressed air into the system after you have it running correctly when it leaves your shop.

[dasinc]

I had those same thoughts first time through so we installed anti tamper caps. Plus in working with this gentleman I trust him. You an usually tell what people are like. Thanks for the comment.

[Tim]

If the system was somehow sucking in outside air into the closed system? Eventully the pressure would become to high and blow a line, pop-off valve or something along those lines. Something seems fishy.

[dasinc]

Tim. I agree but the system does have a pressure transducer that will shut down the compressor somewhere around 400 pounds but there is no low pressure protection on the low side. We're going to put a compressor in it next week and see if the issue happens again. Seems to take about three weeks to a month for it to quit cooling. May not know for sure until next year being as winter is close and I'm in Minnesota. Once the old compressor is off we are going to run a couple tests on it. This is why I posted. Makes no sense at all and goes against any principle that I know. I'm very comfortable that no one is messing with the vehicle. Would just like to figure it out from a knowledge standpoint. I appreciate the input.

[Tim]

Tim. I agree but the system does have a pressure transducer that will shut down the compressor somewhere around 400 pounds but there is no low pressure protection on the low side. We're going to put a compressor in it next week and see if the issue happens again. Seems to take about three weeks to a month for it to quit cooling. May not know for sure until next year being as winter is close and I'm in Minnesota. Once the old compressor is off we are going to run a couple tests on it. This is why I posted. Makes no sense at all and goes against any principle that I know. I'm very comfortable that no one is messing with the vehicle. Would just like to figure it out from a knowledge standpoint. I appreciate the input.

This is undoubtedly an odd set of circumstances. If I remember, I check with the compressor shop next week and see if they have any ideas.