Air conditioning conundrum

TAABVW

A2OC Donor
Having had the engine out of my project A2 and done items such as: 6 speed MYP, cam belt kit, clutch and DMF, oil chain tensioner. I had the AC recently re-gassed. A nitrogen pressure test was performed followed by a vacuum test which the system passed. The system was fill and initially worked. 2 days later all the gas was gone. I revisited the air con shop and they pointed out a few potential leaks which led me to replacing the pressure switch (for which there was also a fault code), the large oring in the connection to the pump and the radiator/condenser. There were no obvious leaks elsewhere indeed even in these areas any potential leak was not very obvious. I had it pressure and vacuum tested again and refilled. Exactly the same result, no pressure after a couple of days. The pump (one time) clutch is good.

I would have thought with the fluorescent dye that is added that a leak big enough to empty the system in 72 hours would spray the dye everywhere and the leak would be clear to see. But there is no obvious signs of the leak. The fact that the system also hold pressure and vaccuum on test has left be completely stumped.

Any suggestions from the wise or experience?

Trevor
 
Having had the engine out of my project A2 and done items such as: 6 speed MYP, cam belt kit, clutch and DMF, oil chain tensioner. I had the AC recently re-gassed. A nitrogen pressure test was performed followed by a vacuum test which the system passed. The system was fill and initially worked. 2 days later all the gas was gone. I revisited the air con shop and they pointed out a few potential leaks which led me to replacing the pressure switch (for which there was also a fault code), the large oring in the connection to the pump and the radiator/condenser. There were no obvious leaks elsewhere indeed even in these areas any potential leak was not very obvious. I had it pressure and vacuum tested again and refilled. Exactly the same result, no pressure after a couple of days. The pump (one time) clutch is good.

I would have thought with the fluorescent dye that is added that a leak big enough to empty the system in 72 hours would spray the dye everywhere and the leak would be clear to see. But there is no obvious signs of the leak. The fact that the system also hold pressure and vaccuum on test has left be completely stumped.

Any suggestions from the wise or experience?

Trevor
There is no clutch on the A2 AC compressor. It is belt driven, by engine, full time. When AC is off (ECON) the flow is around 5%, when AC is on, flow is regulated, depending on demand.
When the pressure was tested the etc, the control valve would be 95% closed, and the internals of the compressor may well hold pressure, when stationary. I'd at, least examine the compressor for leakage. Perhaps the AC shop could advise you, and measure pressure with the engine, and compressor running?
Mac.
 
How long did they leave it on the vacuum test? Most places only do 30 minutes which is not really sufficient.
The leak could be the evaporator which is inside the air-con unit within the car, have they checked this to see if there is any evidence of dye? As you say, if all of the gas leaks within 2 days then it should be fairly obvious where it is escaping from. I assume that you are not being charged for the work they are doing?
 
It is worth changing the Schrader valves in the high and low pressure charging ports. This can be the source of the leak although the other areas still need to be checked. The dye in the system will glow brightly under black light.
 
There is no clutch on the A2 AC compressor. It is belt driven, by engine, full time. When AC is off (ECON) the flow is around 5%, when AC is on, flow is regulated, depending on demand.
When the pressure was tested the etc, the control valve would be 95% closed, and the internals of the compressor may well hold pressure, when stationary. I'd at, least examine the compressor for leakage. Perhaps the AC shop could advise you, and measure pressure with the engine, and compressor running?
Mac.
Yes the Ac pump is a continually variable displacement pump but there is what is sometimes called a "one time clutch" in the pulley which is designed to break or "let go" in the event of the pump seizing. This item is fine and not broken.

There is no apparent leakage from the compressor. Are you suggesting that with the control valve 95% closed the pressure or vaccuum test might not reach the whole system? This would go a long way to explaining how it can pass a leak test and then leak.

After filling, with the AC running the pressures for low and high side can be read off the dials on the filling machine and they seemed pretty stable albeit only viewed for a few minutes.

Trevor
 
How long did they leave it on the vacuum test? Most places only do 30 minutes which is not really sufficient.
The leak could be the evaporator which is inside the air-con unit within the car, have they checked this to see if there is any evidence of dye? As you say, if all of the gas leaks within 2 days then it should be fairly obvious where it is escaping from. I assume that you are not being charged for the work they are doing?
Yes, left it on vacuum test for only about 20 minutes. How is that not sufficient time? I would have thought any leak would be apparent on a vacuum test within 30 seconds let alone 30 minutes. Clearly AC systems are a weak point in my A2 knowledge and would love to understand why or how a system might hold vacuum for 30 minutes but still leak.

Yes I fear that the leak could be at the evaporator which I believe is a dash out job to replace. Does anyone know whether the connections for this are the ones visible from the engine bay behind the engine at the top of the fire wall or whether there are more connection within the dash? If the evaporator were leaking would I smell it in the cabin?

I have done the work myself. The AC shop didn't charge me for the second re-gas which I was pleasantly surprised with.

Trevor
 
It is worth changing the Schrader valves in the high and low pressure charging ports. This can be the source of the leak although the other areas still need to be checked. The dye in the system will glow brightly under black light.
Good shout. I suggested to the AC man the filling valves could be the source of the leak. He checked they still had the seals in the caps and looked for signs of dye but found nothing.

Trevor
 
Yes the Ac pump is a continually variable displacement pump but there is what is sometimes called a "one time clutch" in the pulley which is designed to break or "let go" in the event of the pump seizing. This item is fine and not broken.

There is no apparent leakage from the compressor. Are you suggesting that with the control valve 95% closed the pressure or vaccuum test might not reach the whole system? This would go a long way to explaining how it can pass a leak test and then leak.

After filling, with the AC running the pressures for low and high side can be read off the dials on the filling machine and they seemed pretty stable albeit only viewed for a few minutes.

Trevor
I don't have enough knowledge about the compressor internals to be sure, but I as you say, a leak at the compressor fits the symptoms. Although evaporator is more common.
This suggestion might be nonsense, but if you remove the filter, try a uv light look for leaks.
Mac.
 
I have tried looking for leaks with a UV light to no avail. It has occurred to me that my search might be more successful in the dark.
If it were leaking at the compressor then I think I would see dye at the compressor as it would have to be leaking externally else it wouldn't lose pressure.

I think you mean the cabin filter? that's a good idea. Can I see the cabin evaporator in there?

Trevor
 
Yes the Ac pump is a continually variable displacement pump but there is what is sometimes called a "one time clutch" in the pulley which is designed to break or "let go" in the event of the pump seizing. This item is fine and not broken.

There is no apparent leakage from the compressor. Are you suggesting that with the control valve 95% closed the pressure or vaccuum test might not reach the whole system? This would go a long way to explaining how it can pass a leak test and then leak.

After filling, with the AC running the pressures for low and high side can be read off the dials on the filling machine and they seemed pretty stable albeit only viewed for a few minutes.

Trevor
The leak test vacs the whole system and holds vacuum on the whole system. Possibly not the vales thought as advised by Audifan.

I had this very same problem, I did have a leak, a damaged O ring, but the vac tester was faulty and not detecting it.
 
Yes I agree it must be leaking unless by some bizarre effect of quantum physics my AC fluid is spontaneously de-materialising. My experience of this in my garage, when a tool or part spontaneously de-materialises (and it definitely does), is that about half an hour later the offending tool or part tends to re-materialise though sometimes in the different place!

Probably will turn out to be an oring, I hope so and not the evaporator, just thought with the dye it would be more obvious. I think I'll try looking in the dark tonight......with a UV light!

Trevor
 
Yes I agree it must be leaking unless by some bizarre effect of quantum physics my AC fluid is spontaneously de-materialising. My experience of this in my garage, when a tool or part spontaneously de-materialises (and it definitely does), is that about half an hour later the offending tool or part tends to re-materialise though sometimes in the different place!

Probably will turn out to be an oring, I hope so and not the evaporator, just thought with the dye it would be more obvious. I think I'll try looking in the dark tonight......with a UV light!

Trevor
A vac test is not always the best way to test for a leak in a pressure system. The vacuum can pull two surfaces together and seal the leak. There must be test a pressure leak, as you say, so the vac test must be concealing it.
Have a read of the attached SSP, HVAC starts at page 54.
Mac.
 

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A vac test is not always the best way to test for a leak in a pressure system. The vacuum can pull two surfaces together and seal the leak. There must be test a pressure leak, as you say, so the vac test must be concealing it.
Have a read of the attached SSP, HVAC starts at page 54.
Mac.
Good point. And thanks for the link.

Trevor
 
Even though he "checked the cap seals" I would still replace the Schrader valves as the O rings flatten with time. Any leak that is dumping all the refrigerant will be clearly visible under black light as a fluorescent green colour. Look carefully at the condenser and dryer and pipes connecting them.
 
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