Your 129 has 2 fuel pumps with check valves along with a fuel accumulator. The setup is located just forward of the rear wheel, passenger side underbody, under a black protective cover.

You can test to see if both pumps are operating by pulling the MAS relay and checking amp draw between big pins 1 and 2. Proper specs for 2 pumps is 6 to 7.5 amps.

Not sure if the early 3.0 104 engine has an OVP or not, but I suspect it does. Most likely located behind battery on passenger side.

Another suspect could be the fuel pressure regulator or old fuel filter.

GL

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi