1979 Mercedes 450SL Fuel Pump Replacement: The Complete Guide to Restoring Your Classic

If you own a 1979 Mercedes-Benz 450SL and the fuel pump is failing, the most important thing to know is that replacing it is a straightforward intermediate-level DIY job that can save you hundreds of dollars in labor costs. The fuel pump is located under the car on the right side, below the fuel-filling inlet near the fuel tank. The entire assembly, which includes the fuel pump, filter, and accumulator, sits behind the rear passenger tire. You can complete this replacement in a weekend with basic hand tools, but you must prioritize safety, as working with fuel systems carries a real risk of fire.

Symptoms of a Failing Fuel Pump

Before you start pulling parts off your 1979 450SL, make sure you actually have a bad fuel pump. These are the most common warning signs:

  • Engine sputters or stalls while driving, especially under load or during acceleration
  • Difficulty starting the engine, particularly when the engine is warm (hot start issues)
  • Loss of power during acceleration or when climbing hills
  • Decreased fuel efficiency that you notice at the pump
  • A loud whining noise coming from the fuel tank area
  • Visible fuel leaks, especially around the electrical connector gasket on the pump

If you ignore these symptoms, you risk complete engine failure and being stranded. The fuel pump is the heart of your car's fuel delivery system, responsible for drawing fuel from the tank and delivering it to the engine at the correct pressure.

Where Is the Fuel Pump Located?

For the 1979 Mercedes 450SL, the fuel pump is in a specific spot. It sits under the car on the right-hand side, below the fuel-filling inlet, near the fuel tank. The fuel pump, fuel filter, and fuel accumulator are all mounted together in a cluster behind the rear passenger tire. You will need to jack up the car and place it on jack stands to access the pump.

Tools and Parts You Will Need

Gather everything before you start. This avoids frustration and keeps the job moving.

Tools:

  • 8mm, 10mm, and 13mm socket wrench with a 6-inch extension
  • 7mm, 8mm, and 17mm open-end wrench
  • Flat-face and Phillips head screwdriver
  • Floor jack and jack stands
  • Fuel line disconnect tool
  • Torque wrench
  • Safety glasses and gloves

Parts:

  • New fuel pump – Buy one specifically for your 1979 450SL. The Bosch OEM equivalent part number is 0020919701. The pump for 1978-1980 models is a Walbro GCL605. A remanufactured electric fuel pump that fits the 450SL is also available.
  • Optional but recommended: New fuel filter, new accumulator, new rubber mounts, new fuel hoses

Supplies:

  • Fuel-resistant container to catch spilled fuel
  • Rags or absorbent pads
  • Fire extinguisher
  • Pen and paper or a camera to document the disassembly process

Before you start, disconnect the negative terminal of the battery to prevent electrical sparks. Work in a well-ventilated area to avoid inhaling fuel vapors, and have a fire extinguisher nearby.

Step-by-Step Fuel Pump Replacement

Step 1: Access the Fuel Pump

Jack up the car and place it securely on jack stands. Wear safety glasses to protect your eyes from rust and dirt particles falling from the undercarriage. Locate the plastic protective shield under the car on the right side. Using a 13mm socket, loosen and remove the two bolts holding the plastic protective shield.

Step 2: Remove the Old Fuel Pump

Disconnect the electrical terminals. Using an 8mm open-end wrench, loosen and remove the two nuts securing the electrical terminals on the fuel pump.

Remove the clamp. Using an 8mm socket, loosen and remove the two nuts securing the clamp that holds the fuel pump in place.

Disconnect the fuel lines. Use a 17mm open-end wrench to loosen and remove the fuel line from the rear of the fuel pump. Make sure your fuel receptacle is in place to catch any leaking fuel. Use a flat-face screwdriver to loosen the hose clamp that secures the fuel suction dampener. Then, using a 17mm open-end wrench, loosen and remove the fuel line that feeds the fuel pump from the front. If the fuel lines are old and brittle, they may start to leak as soon as you touch them, so be prepared to replace them.

Remove the pump. The fuel pump is now free to remove. Pull up on the fuel pump lightly to remove it. Pour any fuel remaining in the pump into a container for recycling.

Step 3: Prepare the Mounting Surface

Clean away any grease or gunk around the edges of the mounting surface. Leftover grease and grime will keep the new fuel pump from attaching correctly. Install new gaskets on the fuel pump mounting surface before installing the new fuel pump.

Step 4: Install the New Fuel Pump

Compare the old and new pumps to make sure they are identical. Carefully insert the new fuel pump assembly into the bracket. Make sure the new fuel pump's protective sleeve is centered in the mounting bracket to prevent corrosion. Bolt the fuel pump assembly in place with the bolts or screws you removed earlier. Tighten them to the torque specifications in your service manual.

Reconnect the fuel lines. Reattach the two fuel lines to the new fuel pump. Use new copper and aluminum sealing rings on the banjo bolts to prevent leaks. Reconnect the electrical connectors.

Reinstall the protective shield using the 13mm socket.

Step 5: Test the System

Reconnect the negative battery terminal. Turn the key to the "on" position (but do not start the engine) to allow the fuel pump to prime the system. Check for any fuel leaks around the connections. Turn the key and pump the gas pedal a few times. It may take several tries to get the fuel flowing through the new fuel pump. Start the engine and let it idle. Check for leaks again. Take the car for a short test drive to confirm the pump is working correctly.

Important Safety Tips

  • Disconnect the battery negative cable before any work to minimize the possibility of sparks
  • Use a fire extinguisher within reach
  • Work in a well-ventilated area to avoid inhaling fuel vapors
  • Wear safety glasses when working under the car
  • Use a padded pair of vise grips to squeeze off the fuel line coming from the gas tank to keep your tank from draining when you disconnect the fuel filter

Notes on Fuel Pump Compatibility

If you are choosing between brands, the original Bosch fuel pump (part 0010917101) is expensive, but a Bosch equivalent (69438) is significantly cheaper and should work fine as long as it provides the correct pressure and flow rate. The 1979 450SL's K-Jetronic fuel injection system requires a pump that delivers approximately 5 bar of pressure. Most aftermarket pumps designed for this model will meet that specification.

Why You Should Also Replace the Fuel Filter and Hoses

Since you are already accessing the fuel system, replacing the fuel filter is a wise preventative measure. The old hoses that run from the filter to the pump and from the pump to the accumulator are likely old and brittle. As soon as you touch them, they will start to leak. It is best to order replacement hoses before you start the job and do the whole thing at one time. If you have been having hot start issues (the car starts fine when cold, but you have to crank it for several seconds when starting warm), replace the fuel accumulator as well.

Choose KEMSO for Your Fuel Pump Replacement

When it comes time to buy a replacement fuel pump for your 1979 Mercedes 450SL, consider KEMSO. KEMSO fuel pumps are high-performance, OEM replacement parts designed to match or exceed original factory specifications. Every KEMSO pump is built to handle modern fuels, including ethanol-blended gasoline, and each pump is individually tested for flow and performance before it leaves the factory. Best of all, KEMSO backs every fuel pump with a lifetime warranty, giving you peace of mind that your investment is protected for as long as you own the vehicle.

Visit KEMSO today to find the perfect fuel pump for your 1979 Mercedes 450SL: https://www.kemsoracing.com/


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