Restoring the Mechanical Fuel Pump
by Bruce Adams
On internal combustion engines, fuel pumps are used to deliver fuel to the carburetor or fuel injection system. A mechanical fuel pump is typically used on a carburetor system, delivering fuel under low pressure. Fuel injection systems require high pressure and often use electric fuel pumps mounted inside the fuel tank.
Most carbureted engines used mechanical fuel pumps to move fuel from the tank into the fuel bowls of the carburetor. Our Mercedes-Benz mechanical fuel pumps are diaphragm pumps or positive displacement pumps that contain a pump chamber where volume is increased or decreased by the flexing of a diaphragm. Check valves are used to force the fuel to flow in one direction and are located at the inlet and outlet ports of the pump chamber.
Specific designs vary but, in the most common configuration, mechanical pumps are typically bolted onto the engine block. The engine’s timing chain drives the distributor which in turn has a cam lobe that operates the lever on the pump via a pushrod. The push-rod moves the diaphragm, increasing volume in the chamber and causing fuel to be drawn into the pump from the tank. A spring on the diaphragm then pushes the diaphragm back to top dead center, which squeezes the fuel in the pump chamber out through the outlet port and into the carburetor. The pressure with which fuel is moved from the pump is regulated by the force applied by the diaphragm spring. Low pressure from the fuel pump often is caused by a weak or broken spring or a defective (cracked) diaphragm, requiring replacement of the diaphragm or spring.
On carburetor-equipped cars, fuel level in the carburetor float bowl raises a float that closes the inlet valve (needle and seat) to the carburetor, preventing the fuel pump from pumping more fuel into the carburetor. At this point, remaining fuel inside the pump chamber is trapped, unable to exit through the inlet port or outlet port. The diaphragm will remain under pressure and during the subsequent rotation, the eccentric will pull the diaphragm back to bottom dead center, where it will remain until the inlet valve to the carburetor re-opens.
As engines moved away from carburetors and towards fuel injection, mechanical fuel pumps were replaced with electric fuel pumps because fuel injection systems operate more efficiently. Electric fuel pumps are generally located in the fuel tank, in order to use the fuel in the tank to cool the pump and to ensure a steady supply of fuel.
In our workshop we often add a low-pressure electrical fuel pump as a method of priming the carburetors. We purchase a 1.5 psi 12-volt low pressure in-line pump from Pierburg and splice it into the main fuel line just after the tank. It is wired either to a single momentary toggle switch mounted under the dash or wired to the choke cable. I prefer the choke connection, as it is a switch that also operates an instrument indicator (a white light) on the dash. To start the car, pulling the choke out activates the electric fuel pump and primes the float bowls. After the car is running, pushing the choke cable completely in turns off the auxiliary pump. For safety reasons and for proper operation, it is important to operate the electric pump only when priming the carburetors.
On a 190SL, the Solex Carburetors require 2 to 3 pounds per square inch (psi) of pressure for proper operation. Make sure you check your M-B service manual for the recommended pressure to match your carburetor. Low or high fuel pressure outside the recommended range will create carburetor adjustment issues. After installing a new fuel pump, or rebuilding your current one, make sure you use a psi measurement gauge or measure the amount of fuel pumped during a specific period of time to monitor the fuel pressure to the bowls. Be careful if installing an aftermarket pump to ensure the pressure is not excessive.
We have also seen aftermarket fuel lines sold as replacements. These lines from the pump to the carburetor should be 8mm; however, aftermarket lines are typically 5/16 inches, causing excess fuel supply. In general, aftermarket parts save the restorer money, but if not evaluated correctly, create additional problems. We recommend using M-B parts if available.
The restoration process is shown in the pictures on these two pages. Be sure to pack the hollow space in the pump and joining flange with axle grease when assembling. Once the fuel pump is rebuilt, measure the psi to the carburetors. If the pressure is a little on the high side, you can shim the fuel pump away from the engine block to reduce the pressure. Consult the Service Manual for specifics; a free-play measurement of 0.4-.0.5 mm is required in the distance between the tappet (push-rod) of the fuel feed pump at the beginning of its power stroke and the BDC of the drive cam.
Mechanical Fuel Pump before restoration
Disassembling the pump. Note that the screen is missing from this pump resulting in sludge inside the housing.
Disassembled Fuel Pump
Components plated and ready for reassembly
Mercedes-Benz rebuild kit
Current M-B cost $250
Rebuilt fuel pump ready for installation
Bruce L. Adams, is a member of the Triangle Section, and has owned and operated B.L. Adams, LLC, specializing in restoration of the 190SL model Mercedes-Benz, for 31 years. For more information, visit www.bruceadams190sl.com.