Missing the 166 :(
Join Date: Jun 2007
Location: United Kingdom
If you can hear a hissing noise then, generally, the car is low on refrigerant. What you can hear is gas going through the metering valve / tube and the evaporator. With a fully correctly charged system liquid refrigerant should be entering the valve and changing state (to a gas) within the evaporator, not before it enters.
Charging a system with gas and dye to check for a leak isn't really the best way to check for leaks, it should be the last resort. Before you pay anyone to service your A/C system ask if they will undertake nitrogen or better still nitrogen Helium mix pressure testing. If they suggest they will just charge a system with dye to find leaks then walk away. Such folks are not following the Mobile Air conditioning "Best Practice" guidance. Nitrogen testing is the best way to find leaks and Nitrogen helium is even better due to the smaller molecular composition, it is able to to find really tiny leaks.
With regard any leaks from the test point valves. Those valves are simple and cheap and are probably one of the weakest links of any A/C system. They should always have sealed caps and anyone servicing A/C systems should always check the points before signing off the work and giving you a bill.
TIP - If you have a system that has lost all its gas or that has been empty for some time, then you should always ensure the dryer cartridge is replaced and the system held at vacuum for at least 30 minutes too, preferably longer. This is to ("Burn off") extract all moisture from the system. Moisture left in a system will cause a freshly charged system to loose charge pressure even if the system has no leaks!
Oh for the record I am "F-Gas" certified technician and also (after an error by my boss) have a level 3 mobile A/C certificate too.
2005 Facelift 156 JTS