I have an '87 300DT sedan, 603.960 engine, 143500 miles. Reading thru my electronic copy of the W124 Service Library concerning the fuel primer pump, it lists the minimum pressures it should be able to achieve at idle and at a certain rpm.
Measured at the supply line between the cartridge fuel filter and the inlet to the injection pump, doing the conversion from bar to psi, it looks to indicate that the primer pump should obtain a minimum pressure of 4 1/2 psi at idle.
I have an underhood fuel pressure gauge installed (Mr Gasket 15 psi from Advance Auto) in the line specified in the FSM. Initially idling cold, it registers 6 psi. After driving 20 miles to work, idling when hot it registers 5 psi.
Mu understanding is the 4 1/2 psi listed in the FSM is a go/no go for replacement at a service visit.
My question is, with the hot idle reading being only 1/2 psi above the minimum for replacement, is this pump long since past it's prime (no pun intended) and due for replacement? IE, is there enough pressure difference between this pump and a new one that I would see any benefit from replacing it? Anyone have any idea what the pressure reading at idle on a healthy primer pump would be?
Pump looks to be original with over 143K on it.