Its a teachable moment. A lot of people still call all refrigerants Freon and that's OK with me, just as long as they don't go around calling every plain old table tennis game "ping-pong". That one really chaps my a---.

On my 2001 4.4i - the red cap is the high pressure side. But, you can't always go by the cap color as it is not standardized. I've been tinkering on a Mini Cooper S this week and both caps are black.

On the E53 4.4i, the low pressure valve is in the line between the engine block and the passenger strut tower. The high pressure valve is close to the radiator.

The good news is that they are not interchangeable sizes - For a 134A system, the high pressure quick connect is the larger one. The low pressure quick connect is the smaller one. You fill from the low pressure one.

The catch is that BMW made it hard to fill correctly - some systems can be filled by pressure and so a cheap fill kit with a pressure gauge will get you close enough if you know the pressure to look for (which depends on the outside temperature and humidity). For these cars, BMW specifies a fill by weight, but there is no way to know what is currently in the system unless you pump whatever is in there out (recover it, don't vent it) and then fill it to the correct weight. In theory, you can get a shop to do the recovery for you, and fill it yourself, but good luck finding one that will.

Its been a few years since I took my EPA test (I did), but I believe it remains legal to top off a system leaking up to 15% a year. May not be the greenest thing, but there is no requirement (and no such thing) as perfectly sealed system if you have rubber hoses and rubber o-rings (which they all do). The refrigerant will diffuse through the rubber at some slow rate, even when everything is working as designed.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles