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Lets do this in english and stay out of the bar.
Unfortunatly this is not an easy or clear cut question to answer but maybe I can help shed some light on this. I worked in HVAC/R during the 90s putting myself through school and I was looking for the same answers. OK Here goes
At 75F you should be around 77PSIg with the system off and the car cold meaning it hasnt been run yet. R12 tracks somewhat close with temp so you can use this as a means of judging the charge as a starting point.
Now factor a running car with a hot condenser running at 95f with a dewpoint around 70F, a condenser running at 150F with the aux fan running. Ok 105F will be about 125PSI high side (take notes chris from the dealer). The factory cutout switch was adjusted to open just around the evap freezing temp or around 28-30 PSIg and this is an ideal situation (100psi diff)
I found a satisfactory charge on most CCOT systems (volvos) to be 25-35PSI low side with a cutoff at 22 psi and restart at 40 PSI on the road(moving). Trying to charge in a driveway doesnt get the ram air going through the condenser thus making it hotter. Bringing the engine up to say 2500 RPM will help if your fan clutch is good.
My method of charging. Run the low side hose through a plug in the firewall and bring it inside. Attach as needed and while you have someone drive the car note the pressure and timing of the rise/fall. Also use a known accurate thermometer in a dash vent and run the fan high in the fresh air mode (not recirc) Your goal is the lowest temp and 5 to 8 seconds of compresser run per cycle. Short running means you are low on gas or a possable restriction in the orifice tube, long cycle means overcharge and you will need to purge and reclaim. If the compresser runs constantly in the driveway at 2500 RPM you are overcharged. If it long cycles you are close. If you can connect to the high side (careful here) get your system to around 100 PSI diffrence between high and low, word of warning, do NOT open or crack the high side valve connected to a tap-a-can it explode.
A/C systems loose efficiency quickly when slightly overcharged, much faster than being undercharged.
What you are describing indicates you may have overcharged the system but do the tests to confirm this. The elevated temps at idle and cold highway seem to point in this direction at least for the moment. Now it being "just OK" at highway leads me to a possable leak. Snoop it out for a leak, holding an EPA card you should have or have access to an electronic leak detector. Bet you the compresser seal is leaking while under elevated pressures and settles down when the pressure drops.
Let me know how it goes.
Badge988
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