We’ve recently did a leak test on some ac coils with suspected formicary corrosion, but definite leaks, and the helium leak detection was only good to narrow down the leak to a few square inches. The submersion test got it to the exact vane 2/2 times. More leaks were located, but only two precise locations were verified without removing the vanes.
Soapy water spray didn’t really work for our application. I’m guessing it’s because the aluminum vanes are difficult to get a water seal on with just spray.
I think we only have the capability to run up to more than 1k psi. I can ask about the precise pressure used if you care. If you need more than that to find the leak, then it’s probably not leaking in a car ac system either.
Overall, pulling the ac components out of the car for leak testing is pretty far down the list for most people I think. But if it comes to that, full submersion definitely works.