I would expect about 250 psi on the high side with 80 F external temp, although if the aux fan is running it may be lower. Mine usually kicks in at 300 psi and runs until the pressure drops to 250 or so.

However, if the sight glass is clear and it's blowing cold a idle, you are fine!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!