The original electric fuel pump is capable of producing over 30 psi of fuel pressure. If you pinch closed or block the fuel return line, you should see the fuel
pressure rise from around 15 psi. to over 30 psi. The pressure in the system is automatically lowered to around 15 psi by allowing most of the fuel to return to the fuel tank via the return fitting on the fuel injection pump. This fitting is designed to perform as a basic fuel pressure regulator. The early 230SL injection pumps had a more complicated fuel regulator built into the return line. It had a spring and some moving parts.
Your engine will never run correctly with fuel pressure less than 10psi. If you have low fuel pressure, your problem could be the result of several factors;
...........non original electric fuel pump not up to specs.
...........original electric fuel pump, not working properly and not up to specs.
............fuel supply problem (sediment in tank clogging fuel flow into flower pot, clogged tank screen, clogged electric fuel pump screen, clogged main filter, no fuel tank venting, electric fuel pump hung too high)
............improper, non-functioning or missing fuel regulator on the return line of the injection pump allowing large volumes of unrestricted fuel to return to the fuel tank.
Now if you have low fuel pressure, try pinching closed or blocking the return line to the fuel tank then take a fuel pressure reading. Your fuel pressure should rise to over 30psi. If it does not, you will have to check all the items listed above.
I recommend using an original electric fuel pump or a rebuilt one. Very few aftermarket pumps work out. However there are some economical alternatives than can be used in case of emergency or in case originality is not an issue. Stick has used the later Bosch high pressure electric pump with success. Review his threads. Also the Carter #GP4601 HP will work and can be found easily and cost is around $100.00 US. In every case, the electric fuel pump must be mounted level with the bottom of the fuel tank since fuel must be "gravity fed" to the electric fuel pump.
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