There is a group of M103 owners having this very problem on the forum. Keep us posted. My Dad has "our" 1990 190E 2.6 back, and it's been doing that for some time. Since the engine runs normally, doesn't use any additional oil, makes no valvetrain noise at hot idle, and sends oil to the top end at hot idle with the gauge reading zero pressure, I dismissed it.

OTOH, I thought maybe the pump is going south, so I told Pop to keep an eye on it.

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John Shellenberg
1998 C230 "Black Betty" 240K

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