Overcharged system produces cold air... correctly charged blows hot air

[kevboost]

I am stumped right now. I have a 2015 Nissan Sentra SV. I hooked up gauges to low side and high side. Low side was too high, about 60. High side was correct pressure. I recovered some of the refrigerant to the correct psi based on the ambient temperatures. The air was blowing hot. Then i added it back to 60 and now its cold again. Anyone know why this would be the case? I have the max ac on and its still not getting as cold as i'd like.

[JohnHere]

Pressures are an effective diagnostic tool but can't tell us how much refrigerant a system contains. The only way to do that is to weigh the charge into a system that's under a well-held vacuum.

According to the specs I have, your car takes 17.6 ounces net weight of R-134a and 3.0 fluid ounces of PAG-46.

[kevboost]

Weigh it on a scale? I rented one of those vacuums from autozone. How would I weigh the refrigerant already in the system? And why does every video on the internet use the pressure gauge on the can to fill it?

[Tim]

I've never seen a pressure gauge on anything other than those Wonder cans they sell at parts stores. Real Guage set, cylinder of refrigerant, and a scale. As for how much a system has in it. Professional machines can reclaim the refrigerant and show how much was removed. I've yet to see a formula calculating the proper amount of refrigerant in a system by pressure.

[JohnHere]

How would I weigh the refrigerant already in the system?

You would have to recover whatever refrigerant is already in the system (if any) using a professional RRR (Recover, Recycle, Recharge) machine, which will tell you the weight of the refrigerant removed, along with the fluid ounces of oil removed. Then, you would recharge the system according to the exact weight-of-charge the system calls for while adding back the same amount of oil. Yes, very accurate refrigerant scales are also used together with a Manifold Gauge Set and refrigerant source—e.g., a 30-pound cylinder—to properly recharge a system. Charging or recharging by weight is the only correct way to do the job. Pressures alone simply cannot tell you how much refrigerant a system contains.

And why does every video on the internet use the pressure gauge on the can to fill it?

With all due respect, you came on this Forum, which comprises a small group of trained MVAC professionals, retired pros, and knowledgeable others, for advice on how to properly recharge your system. We offered you our best opinion about what should be done to address your issue. Just because you see videos on the Internet doesn't mean it's the correct way to charge a system. But you already found that out. Go to any professional MVAC shop in your area and ask about charging using one of those Miracles In A Can and see what kind of answer you get. We can only offer you our best professional advice. What you do with that information is up to you.