I need some help tracking down a slow leak in the a/c system of my 2009 Clubman S. I just re-installed the engine, and while it was out I replaced all the a/c lines in the engine bay. I usually pressurize the system to 100psi with nitrogen and let it sit overnight before filling with r134a.

This time it leaked down overnight, so I re-pressurized it and tested all the connections with soapy water but could not find any leaks. I injected with UV dye, pressurized with nitrogen and ran the engine for a bit, and then looked for the dye. No detectable leaks. I bought a 134a sniffer, filled the system to 80 psi with freon, and still can't find any leaks with the sniffer, but it is slowly leaking down. I even probed behind the dash to see if the evaporator might be leaking.

Could the compressor have an internal leak that is being missed by the sniffer and dye? I'm running out of options and would appreciate any suggestions/guesses/incantations that might help me track down the leak. I have a replacement compressor and condensor and my next step will be to swap those in.

 

Was the overnight drop in ambient temperature taken into account(?) when ascertaining whether the corresponding drop in idle nitrogen charge was excessive?

Also, from the description UV dye and an 80 psi charge of 134a was injected for leak detection purposes, was a vacuum pulled prior to that and did it hold?

 

Good points.
The overnight temp wasn't a consideration, because it dropped from 100psi nitrogen to 0psi overnight.
I did pull a vacuum before injecting dye. No, the vacuum didn't hold. It drops to 0 after several hours. So I charged it up, ran the car with a/c for 5 minutes, then looked immediately for leaks. Can't find any with dye, soapy water, or chemical sniffer.

 

Did you eventually locate the leak? If so, was it the evaporator?