Problem solved!
We checked the pressure wit another manometer and got the same indication.
I removed the relief valve for checking and discovered it vas open because of a piece of dirt blocking the ball.
Fortunately, my friend didn't drive the car like this!
I cleaned it and readjusted the spring: now, 1.5 bar at idle and 3.5 to 4 bars @ 2500rpm when hot, that's far better!
Thank you very much Geoff for your help
