Sounds like the monovalve is stuck open. With pressures like that you should be able to blow <60˚ in those ambient conditions with all the doors to the car open. The cold return line is your clue here, if the monovalve isn't sealing off, you're trying to cool the heater core.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)