Check to make sure that oil cap isn't dis-lodged or improperly seated, it might even be the rubber gasket under the cap going bad allowing too much pressure to escape through your oil fill orifice. One way to check would be to have someone sit in the vehicle while its running, and you unscrew and screw the cap back on to check for any changes in pressure and or rise in engine idle. If possible remove the cap all together and place your hand over the oil fill orifice to plug it then see if oil pressure rises.

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93 300 D 2.5 turbo