Usually when an oil pressure gauge is pegged and will not move it is due to a bad ground. They are not easy to find or fix. Usually I end up actually pulling the gauge off the instrument cluster electrical board and cleaning the contacts that ground it to the cluster board. It is not fun but it can be done.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red