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Ac not blowing cold when driving on highway. 2006 scion XA with a fresh r134a recharge. Ac blows cold idling and around town, but when driving on highway (>50mph) ac blows totally warm. Cools down within 1 minute after slowing down. Any thoughts?

I just put it on a set of manifold gauges. I checked the pressures at idle and when revving high for a few minutes to simulate highway driving when the problem occurs.

Starting pressures

  • (idle) 35psi (L) 200psi (H)
  • Max pressures (when revving to ~3k rpm for ~2min) 62psi (L) 245psi (H)
  • Min pressures (when revving to ~3k rpm for ~2min) 23psi (L) 165psi (H)

These are slightly higher than the manufacturer spec I pulled from the shop manual I have.

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  • Welcome to Motor Vehicle Maintenance & Repair! Commented Sep 26, 2022 at 21:11
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    Did you have your high-side/low-side pressures checked? That will usually help identify the cause. Could be low refrigerant, clogged orifice, or just about anything else at this point.
    – jwh20
    Commented Sep 26, 2022 at 21:22
  • I just put it on a set of manifold gauges. I checked the pressures at idle and when revving high for a few minutes to simulate highway driving when the problem occurs. Starting pressures (idle) 35psi (L) 200psi (H) Max pressures (when revving to ~3k rpm for ~2min) 62psi (L) 245psi (H) Min pressures (when revving to ~3k rpm for ~2min) 23psi (L) 165psi (H) these are slightly higher than the manufacturer spec I pulled from the shop manual I have.
    – John
    Commented Sep 27, 2022 at 21:22
  • Do you know the exact amount of r134a in your system? Second guessing can be either too much or not enough. Idle rpm pressures mean nothing; do you drive at idle when using ac? If using service manuals then there should be specs to follow when measuring low/high side pressures relative to engine rpm. Some models can be measured at idle while one model specifies 2k rpm. Under most operating conditions for r134a, low side should remain between 28-35 psi, corresponding to evap temps around 35F-42F regardless of rpm. High side will vary in proportion to engine rpm and shouldn't exceed 300 psi.
    – F Dryer
    Commented Sep 28, 2022 at 21:33
  • Initially 0.98lb. I checked the sticker under the hood and the max R134A fill is listed as 0.90lb so I had the shop discharge it to 0.90lb and retested the pressures. In the retest I noticed some differences in pressures. Idle 35/185PSI L/H 3krpm 25/255PSI L/H In the first test with .98lb the pressure rose on the high side then crashed to 165 and the low side fell to 23psi then rose to 62 as the high side fell which roughly corresponded to when the AC gave out. In the second test the pressures held constant and the AC didn't give out, but it still gives out on highway.
    – John
    Commented Sep 29, 2022 at 21:34

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