230 sl fuel system question

[merrill]

hello all,
quick question about the fuel system on a 230 sl.
I have always been told to let the fuel pump run a few seconds before starting the car. this was to let the pump get the pressure up.
is this correct and if not is there a valve in the fuel system i need to replace?

[rwmastel]

Yes this is correct, let it run a few seconds.  You could do some tests over time to see how much time your system needs.  Try no prime, then prime for 3 seconds, then 6, then 9.  See what's best.  It doesn't hurt anything to let it prime too long as the fuel system is a closed circut, returning excess fuel back to the fuel tank.

There are some check valves in the injection pump, but I don't know if they are designed to hold fuel pressure for a few minutes, hours, or days.  The check valve design changed, not sure exactly when.  Maybe when it went from using a seperate oil resivoir with dip stick to using a supply line from the engine?

Have you checked your oil level in your fuel injection pump?
www.sl113.org/forums/index.php?topic=2972

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[George Davis]

Merrill and all,

there is one check valve in the fuel system at the injection pump outlet, but the system is not designed to hold pressure when the fuel pump is off.  In the 230, 250 and early 280 SLs, the check valve actually has a hole in it to allow vapor bubbles to pass out of the system to prevent vapor lock.  The later 280 SLs have a different vapor bypass system and the check valve has no hole, but the system still won't hold pressure as the fuel pump is not designed to hold pressure either, so it just bleeds back through the fuel pump.  I let the pump run for several seconds before starting, although I really doubt it takes more than a second for the system to pressurize.  I just like to listen for the whine of the pump to assure myself it's running!

I pulled the fuel pump fuse one day and then cranked the engine, thinking it wouldn't run.  To my surprise, it fired up instantly and idled just fine.  I even revved it a bit, no problem.  Seems the fuel pump isn't super critical, at least not under those circumstances (stationary, no load).  The engine had been started earlier so it wasn't a "real" cold start, as in the first start after sitting all night.  A real cold start would probably be difficult without pressure.

George Davis
'69 280 SL Euro manual