AC Low Pressure Gradually Increases

[Kallas]

I have a 2017 Hyundai Tucson. Recently I've noticed that the AC starts cold (not as cold as I'd expect) and then gradually loses cooling power, to the point where it's pretty much ambient air being blown. I did the following:

1. I made sure that the refrigerant was topped up (this had no impact on the above issue).
2. I connected a set of AC gauges to the low and high pressure lines. Initially the LP was around 40psi and the HP was at 150psi (ambient conditions were around 80F). What I noticed is that the LP gradually climbed to 70psi, yet the HP stayed at 150psi.

Any ideas or suggestions as to what could be causing this?

Thanks in advance!

[Mark86]

How did you top off the system? Did you evacuate and recharge a known weight or did you use a can with the gauge?

[Kallas]

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[Kallas]

How did you top off the system? Did you evacuate and recharge a known weight or did you use a can with the gauge?

I used a can with a gauge.

[JohnHere]

Mark86 wrote: ↑Sat Mar 25, 2023 12:35 am
How did you top off the system? Did you evacuate and recharge a known weight or did you use a can with the gauge?
I used a can with a gauge.

I think it's probably just low on refrigerant due to a slow leak.

"Topping up" a system doesn't work because the pressures alone don't tell us how much refrigerant the system contains and how much to add. The only way to ensure the charge is correct is to recover the refrigerant that's in there now, evacuate the system well, and recharge to the manufacturer's specs using an accurate scale or a so-called RRR machine that professional A/C shops use. The system will also need to be thoroughly leak-checked and repaired.

I believe your vehicle also uses a variable-displacement compressor. In such a system, the pressures can seem to be out of specs, when in reality, they're not. Again, the only way to ensure a correct charge is to recover/evacuate/recharge to specifications by weight.

Read the fine print on the can you used to determine whether the refrigerant contains any additives—and specifically, sealer. The latter is bad news for an A/C system since it tends to plug-up critical components, resulting in problems you didn't have before that are very expensive to repair. Additionally, a professional shop will usually decline to work on a system with sealer in it because sealer will damage their RRR machine.

Let's hope that the can you used doesn't contain any sealer.