fuel pump

[barry230sl]

Our '65 230SL has been diagnosed with a shot fuel pump.  It has a pierburg in it but the mechanic nor I can locate a replacement so he is recommending a new "original" pump at $800 plus installation.  Does anyone know if this is the only/correct option.
Thanks for any help,
Barry

[ja17]

Hello,


Most owner's prefere originallity and correctness. However if economics are the major issue you can improvise. This can also be a temporary or emergency procedure.

Most electric fuel pumps do not have adequate volume or pressure. You must have both.

The small Bosch fuel pump used on the EFI (electronic fuel injected) engines will work fine as it is. It has nearly the same specs as the original and is smaller and simpler. Mercedes used these pumps on many models (1972-1974).

Later Bosch electric fuel pumps used on nearly all modern MBs with the CSI (constant fuel injection, fuel distributor etc.) has much too much pressure. However these pumps are quite plentiful in most salvage yards and can be had for little money. These fuel pumps can be used if a bypass is used to bleed some of the pressure back to the fuel tank. Not exactly sure but this is my understanding of the procedure.... add a "t" on the pressure side of the pump and a "t" in the return line to the fuel tank. Connect the two "Ts" with a small hose (size may be critical). The result is a corrected pressure and volume of fuel to your mechanical fuel injection pump on your W113.  Next check the pressure and volume of fuel available at your injection pump to make sure it is to specs (.8 to 1.1 bar (bar = 14.7 psi), volume should be around 1 liter (min) every 15 sec.  with return line to tank blocked.

All these electric fuel pumps are gravity feed and should be mounted with their intake, even or lower then the bottom of your fuel tank or they will not pick up fuel.

[Witt]

....how about installing an adjustable in-line fuel pressure regulator. I have done that on a few other cars, non Mercedes, with good results. You can dial in the desired pressure at will!
Just a thought.

CHEERS !
WITT !

[ja17]

Hello,

As long as the fuel volume stays in specs a regulator may work on one of these later pumps. The W113 system is designed to work at less than 20 psi.  New Bosch electric fuel  pumps can produce nearly 100 psi ! The W113 system cannot handle this pressure. Things will rupture. So a regulator would have to be able to handle this kind of pressure also. Any lines between the electric fuel pump and pressure regulator would also have to be able to handle the higher pressure. AOn the other hand, a by-pass lowers the pressure in the entire system by creating a giant leak back to the fuel tank.

The original W113 electric fuel pump will also produce about 40 psi pressure if the return line is blocked off ! This return line,  in the same manner,  "creates a giant leak" back to the fuel tank, lowering the pressure in the system to around 11 to 15 psi.

[barry230sl]

Thanks for all the help.  I guess we will let the mechanic go ahead with the replacement.

[Andres G]

Hello Barry,

If you do not mind... could you please tell us what the symptoms on your car were before you got the diagnostic?

I am finding a couple of occurences on my 250 SL that lead me to believe my pump is about to go. Mainly, I've experienced long cranking periods and sometimes refusal to start for some time. Also, I've felt the engine skip/misfire under wide open throttle acceleration when going over 5000 RPMs, particularly in higher gears 3dr and 4th.

Anyway, please let us know.

Thanks,
Andres G

[jszeman1]

I purchased two original early fuel pumps on ebay or thru the forum and had them rebuilt.  Total cost 250-300 including the pump and they work perfectly.  The second one is just in case.  All is still original.