Genuine Bosch overflow valve for P7100 Pump
This overflow valve regulates the primary pressure inside the injection pump. If this pressure is not within standards it can effect your timing, injector function and a number of other factors.
This can cause your engine to run poorly in many ways. Low RPM stumble, low power, excess smoke, hard starting, poor fuel mileage. Check your fuel pressure. It should be around 20psi at idle and rise to around 27psi when you rev it up without a load.
Be sure to check for other fuel flow restrictions, such as plugged primary filter (screen under your fuel heater), fuel filter, fuel lines etc.