Can the engine function without the primary pup operating?

[rutger kohler]

I was surpised to recently see, a suggestion that a pagoda engine would still function if the primary pump was off, but the petrol/gas tank was full? I should have thought the relatively small increase in static head, by having a full tank would be easily overcome by the friction losses in the lines to the injector pump, let alone the pressure drop across the petrol/gas filter etc?

[Paul & Dolly]

Hi Rutger,

I suspect the car will not start....

My 250SL handbook (Page 38) says - Re Starting

Set Transmission selector to N or P
Ignition key to position to position 2 (driving) - red generator lamp should light up
then AFTER waiting a few seconds turn ignition key to Starting.

I have always understood the waiting was to allow the primary fuel pump to pressurise
the fuel system to the mechanical Injector pump.

I may be wrong as I know there are other starting aids in the system.
 
When I follow these instructions the car usually starts
When I forget it usually does not immediately start !

Keep well

Paul

[Rolf-Dieter ✝︎]

Hi Paul,

What you are doing seams very logical. Once the primary pump pressurizes the system the secondary pump can supply fuel to the injectors for a quick engine start.

Hi Rutger,

I don't think starting our engines without the (PFP) primary fuel pump (located near the fuel tank) is possible. Last fall I had to be towed due to a burned out fuse for the PFP. I may be wrong, however, the PFP is there for a good reason.

[Benz Dr.]

The fuel pump on our cars is actually a charge pump as it does not supply the primary or running pressure needed for proper engine function. On later cars ( 107, 126, etc. ) the fuel pump supplies primary fuel pressure and there isn't any other systems to boost fuel pressure.  Regardless of these main differences, you need fuel pressure to start the engine via a cold start valve or you won't be going anywhere.