I checked the fuel pressure on my 240 before changing the pump spring, and it was about 10 psi. After changing the pump spring I did stretch the relief spring checking it with a gauge to see what I was doing. 21 psi under load now. I do not see a need for an electric pump in addition to the lift pump. The lift pump seems to hold pressure just fine. As Forced has said one or the other.

For the gauge I drilled and tapped the banjo bolt that the fuel enters the IP. 1/8 pipe thread is the size I used. Make real sure no filling/chips etc are left in it. Of course do this off the car. I really would not do anything without starting with a gauge.

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1977 240D turbo