Hello,
Fuel pressure in the system is around 15 psi. However the electric fuel pumps are capable of over 30 psi. The fuel return system helps reduce the fuel delivery pressure to the prescribed 13 to 16 psi. Each injection pump has a "by pass valve" which allows the over-abundant fuel to return to the tank. These by pass valves act as pressure regulators for the fuel delivery system. The 230 SLs actually have a spring and valve in their by-pass fitting and the later cars had a by pass fitting with a small orifice or a check valve with no orifice. All three types of by-pass fittings serve as regulators to maintain the prescribed pressure in the fuel by-pass system and allow the over abundant fuel to return to the fuel tank.
The latest version uses the by-pass fitting with "y" connection at the fuel tank, after the electric fuel pump, which allows some fuel to return directly back to the fuel return line . This system uses the by-pass fitting with no orifice. If the car has no "y" and no by-pass hose at the tank, the injection pump should have the pressure fitting with the small orifice.
The early spring and valve by-pass fittings on the 230SLs can become stuck open or shut after long periods of storage. This can cause reduced or cause excessive fuel pressure in the fuel delivery system.Take these apart to fix them or to check them. Having the wrong version fuel by-pass fitting on the later systems (280SLs) can also cause incorrect fuel delivery pressure depending on whether you also have the "y" by-pass installed.
Why a by-pass at all? The continual circulation of cool fuel from the fuel tank helps keep the injection pump and its fuel cool, preventing vapor lock. If the fuel return becomes inoperative, or if the fuel return line becomes blocked, the fuel will not circulate and may become hot enough to vapor lock causing difficult hot starting and roughness at idle. In addition the fuel delivery may rise to over 30 psi, as excessive fuel and pressure from the electric fuel pump cannot be relieved. In this instance an rich running condition can be caused by excessive fuel delivery pressure.
Checking the engine tuning items, linkages and the fuel delivery pressure on these engines should be done before adjusting the complex injection pumps.
Joe Alexander (ja17)
Blacklick, Ohio
