Vannin' Home Forums Chevy Troubleshooting Ac blowing off

So the ac in my 94' Chevy g20 is warm. So I figured I'd fill up with some 134a and get it back to cool. I start to fill and it gets to half way on the pressure gauge for the refrigerant, (no idea what pressure exactly but around 40) I get to a certain point and the car with stutter and it sounds like every fan turns off and then I can hear what I'm assuming is the refrigerant blowing off and then everything goes to normal. The pressure on the gauge drops to zero then goes back to what it was before I started filling which was almost at the "green" about 25 psi. This is repeatable. What could be causing this?

What do you mean by "refrigerant blowing off"? Is it venting to the atmosphere???
If So. You may have a restriction somewhere, such as a clogged orifice. This would cause the suction to drop and (if you keep adding refrigerant) the discharge pressure to skyrocket. If you have this condition be very careful. The relief doesn't open till about 500psi!!! At that pressure all kinds of bad things can happen! At that pressure the compressor would cause the engine to "stutter" because it becomes very hard to pull. If a hose blows off, or the condenser or lines rupture, you could be seriously injured.
Generally with a clogged orifice the low pressure switch on the suction accumulator will cycle the compressor clutch off. If that's defective or as been by-passed then pressure can run uncontrolled. If excess refrigerant is added or air/non-condensables are in the system pressures will be higher as well.
I suspect that by hooking a refrigerant can to the low side you are raising the suction pressure, which causes the low pressure switch to close, which runs the compressor until the relief blows off! Very very dangerous!
Don't have much to go on here so I would suggest you take it to an AC shop to be checked out!
DIY is not always the best plan.

The AC shop may charge you some for their expertise but it'll be a lot cheaper than a visit to the ER........ just saying.

Alright so the venting is no longer occurring, I wanted to repeat it and see if it was actually refrigerant venting and what noises were occurring. The motor itself isn't idling down. The ac compressor is just turning on and off. I don't know why it sounded the way it did when I first filled it. But now my ac works fine and I have no problems