2005 Subaru - pressures fine at idle, really high at higher RPMs

[whitney]

Hi everyone, have been searching this forum for ideas but thought I would register and see if I can get some thoughts:

2005 Subaru WRX (Actually a Saab 9-2x, but it's a WRX under the skin). AC has been working fine for years but recently does a lot of short-cycling when driving >2000 rpms. Put the gauges on tonight to see what's going on.

- At idle, ~30psi low, ~225 high at 85F ambient. Sounds about right, correct?
- Revving past 2000 rpm, low drops to ~20psi and high shoots to ~400psi, then one or the other limit switches cuts off the compressor
- (From reading most low switches are around 20psi and high is about 420psi)
- While driving this results in short-cycling like every 20 seconds
- There's a bit of a chirp coming from the compressor when this happens, but figured it may have to do with the high pressures

With a limited understanding of how the AC compressor actually manages higher RPMs, I have to believe I either have a problem with

(1) The AC compressor itself - whatever inside manages or regulates pumping when it's spinning faster
or
(2) an obstruction in the system that causes high high side pressures as the compressor is pumping more volume/

Thoughts? Any advice is appreciated

[JohnHere]

Has any work ever been done to the system, especially recently? Compressor replaced? If not, I recommend you begin by recovering the refrigerant that's in it now (or having it recovered), evacuating it well, and recharging the system exactly to specs: 16 ounces net weight of R-134a, and 4.9 fluid ounces of PAG-100—or whatever the under-hood decal says (the decal always takes precedence).

[whitney]

Thanks for the advice. I think I sorted it out, and you were on the right track -- it was overcharged. Rookie mistake! Others learn from my bad

Turns out I was (1) trying to set pressures at the wrong RPM - idle. And (2) I was using a pressure table that had me trying to achieve too high low side pressures - 30-40psi

Broke out the FSM and found that I should be using 1500rpm, low side 18-28psi and high 212-242psi. I got the idle up and brought the pressures down, and viola, no more short cycling. Will do another test Sunday when the temps will top out in the upper 90's to be sure.

td;dr: RTFM!

[JohnHere]

Pressure tables and some FSM's can be deceiving, which is why many pros don't use them. For example, at a low-side pressure of 18 PSI, the evaporator would turn into a block of ice in no time, preventing any airflow.

Bear in mind that pressures are great for diagnosing a system but can't tell you how much refrigerant a system holds at any given time.

Weighing-in the refrigerant from a well-held vacuum is the only correct way to charge or re-charge a system, especially one like yours that holds only one pound of refrigerant. The margin of error is thus very small, so just an ounce or two "off" will negatively affect performance.