Replacing The Fuel Pump In Your Mercedes Ml500: Step-By-Step Guide

If you're looking to replace the fuel pump in your Mercedes-Benz ML500, you'll need to set aside a fair amount of time and prepare your workspace carefully. The process involves removing the rear seats, disconnecting electrical connections and fuel lines, and carefully removing and replacing the fuel pump and filter. It's important to relieve the pressure in the fuel lines before disconnecting them, and you should also be prepared for some fuel spillage.

You will need a range of tools for the job, including various socket wrenches, a sharp wood chisel, and possibly a specialised tool for disconnecting the fuel lines. It's also important to have the correct replacement parts, and some sources recommend upgrading to a larger capacity fuel filter and pressure regulator.

The whole process can take several hours, so it's important to be methodical and patient.

Diagnosing the problem

Diagnosing fuel pump issues can be a complex process, but there are several tests you can perform to identify the problem. Here is a step-by-step guide to diagnosing fuel pump problems in your Mercedes-Benz ML500:

Hearing Test

Turn the ignition to position II without starting the car. Listen carefully near the fuel tank, usually located near the rear seat. The fuel pump should run for several seconds, building pressure for the engine. If you don't hear the fuel pump running, it may indicate a problem.

Check the Fuel Pressure

Locate the fuel test port on the fuel rail. It is recommended to start from one of the fuel injectors and follow the fuel rail. Once located, remove the Schrader valve cap and connect a fuel pressure tester. With the ignition turned on, the fuel pressure should build up even before cranking the engine. Start the engine and get a fuel pressure reading. The normal fuel pressure range is between 54-61 PSI. If the fuel pressure is outside this range, it could indicate a problem with the fuel pump.

Check the Voltage at the Fuel Pump

If the fuel pressure test indicates no fuel pressure, don't rush to replace the fuel pump. Instead, perform the following tests to ensure the fuel pump is receiving power:

• Check the fuel pump relay. You can test the relay or swap it with a relay from a different system to determine if it's functioning correctly.
• Check the voltage at the fuel pump. Remove the electrical connector and metal cover to access the top of the fuel pump. Use a digital multimeter to check the voltage at the two main lines (black and red) going into the fuel pump. With the ignition turned on, you should get a reading of around 11-13 volts. If you don't get this reading, it could indicate a bad fuse, relay, or an issue with the Engine Control Unit (ECU).

Common Symptoms of Fuel Pump Issues

Several symptoms can indicate fuel pump problems in your Mercedes-Benz ML500:

• Engine cranks but doesn't start.
• Irregular engine function and delayed starting.
• Vehicle slows down or comes to a halt without braking.
• Lack of acceleration or vehicle stalling when pressing the gas pedal.
• Jerking during acceleration from a stop when hitting the gas pedal hard.

Additional Troubleshooting Tips

• Always perform tests to confirm a defective part before replacing it.
• If you have a check engine light on, scan the fault codes using a diagnostic scanner to assist in your diagnosis.
• If the fuel pump is not the problem, consider other potential issues such as a faulty MAF (mass air flow) sensor, which can cause similar symptoms.

Researching the fix

Step 1: Diagnosing the problem

Before you start to fix the problem, you need to make sure that the fuel pump is the issue. In my case, the car lost the ability to accelerate in gear. It would start and idle fine and you could accelerate the engine in Park or Neutral, but not "under load." It turned out that the fuel pump was partially working but not when the engine made a significant demand for fuel. I also had both fuel trim error codes: P0170 (Bank 1) and P0173 (Bank 2). In addition, during the live view, my OBD2 code reader captured very low Short Term Fuel Trim STFT percentages (as low as 2.3%).

Step 2: Research

• Wolfgang's MB Page: M-Class Fuel Pump and Sender - this post has lots of great photos and a great overview of the ML320 fuel system.
• Wolfgang's MB Page: Fuel Filter Install - this post has fuel pump and filter replacement steps and photos.
• Mercedes service bulletin - this bulletin describes the need to upgrade to a large-capacity fuel filter and pressure regulator (these 4 pages were absolutely critical).

Step 3: Ordering parts

I purchased all the parts from GermanStar in Arizona. They delivered almost immediately and I paid:

• Pump - $320 (the dealer wanted $517)
• Filter - $75 (the dealer wanted $156)
• 3 fuel lines - $75 (the dealer wanted $136)

Step 4: Prepare for the work

It's true that you will spend 3 times the time preparing to do the replacement, so don't get in a hurry. Here are the tools you will need:

• Torx E12 socket wrench
• 16MM socket wrench
• Wood chisel
• Vice grips

Step 5: Understand the process

• Remove the 2/3 rear seat (just behind the driver's seat).
• Remove the side floor trim piece (it pulls up in the back and the inside, then you pull it forward).
• Remove the trim piece that covers the bolts on the back side of the rear seats (you need to partially collapse both seats to see, then pop up this trim piece).
• Remove all the torx bolts and 16-MM nuts from the 2/3 seat.
• Remove the front bolts and nuts first, then partially collapse the seat and remove the back bolts and nuts.
• Move the driver's and front passenger seats all the way forward.
• Tilt the 2/3rd seat up and lean it against the driver's seat.
• Lift the carpet up and slide it out from the door sill trim pieces.
• Pull it toward the driver's seat and use a bungee cord to hold the carpet out of your way.
• You will see a circular cover about 7 inches in diameter. It is held in place with body sealant.
• Remove the cover with a sharp wood chisel.
• Start your engine and, while it's running, remove the electrical connection to the fuel pump. In about 5 seconds, your engine will die, and you will no longer have pressure in your fuel lines.
• Disconnect the fuel lines from the pump. Mercedes has a special tool for this, but you can also just pinch the rear fuel connector and lift it up.
• Remove the left rear wheel and fender well.
• Block all the other three tires with wood.
• After removing the wheel, use a 10mm socket to remove the bolts holding the fender well - be gentle, they are plastic.
• If you have the fender accents installed, you will need to remove the accent before you can remove the fencer well - use a T25 Torx screwdriver for this removal.
• Remove the old filter. It sits on the chassis, just forward of the rear axle - you can see it clearly if you lay on the ground.
• There is a clamp that you can loosen with a slotted screwdriver.
• When it is loose, pinch the fuel lines and disconnect them from the filter.
• The small vent line can be removed by unsnapping the tiny metal clamp. Use a small screwdriver to lift the cover of the clamp and it will snap open.
• Go back inside the car and remove the plastic retainer cap on the fuel pump. You can rig a tool for this by using a scrap piece of wood that fits between the cap's ridges and then use vice grips to grip the wood and twist counter-clockwise.
• Remove the cap and get a large bucket ready.
• Lift the fuel pump out of the gas tank, tilting it towards the front of the car to get the pump and the pickup line out. Be ready with rags as gas will spill.
• Place the pump into the bucket.
• Before placing the new pump into the gas tank, notice that the black pickup extension should point directly towards the front of the car, and the fuel level float should point toward the right side of the car.
• Insert the black pickup extension into the tank first, followed by the tan fuel level float.
• When these two items are in the tank, lower the pump into the tank.
• On the top of the pump, you will see a small triangle. It is critical that this triangle points directly toward the right side of the car. If it is off, the float

Ordering parts

Before ordering the parts, it is important to check the make and model of your Mercedes-Benz ML500, as well as the Vehicle Identification Number (VIN). This information will help ensure that you order the correct parts for your specific vehicle.

The fuel pump, fuel filter, and fuel lines are the main components that will need to be replaced. It is also recommended to replace the pressure regulator and upgrade to a large-capacity fuel filter if your vehicle has not had this update already.

You can purchase these parts from various online retailers, such as Amazon, Advance Auto Parts, and CarParts.com, or from your local Mercedes-Benz dealership. When ordering, make sure to specify that the parts are for the Mercedes-Benz ML500 model.

• Fuel pump
• Fuel filter
• Fuel lines (3)
• Pressure regulator (if needed)
• Fuel pump lock ring
• Fuel pump module assembly (if needed)
• Fuel pressure regulator (if needed)
• Fuel filter pliers (optional)

When ordering, pay attention to the part numbers and specifications to ensure compatibility with your vehicle. For example, if the last six digits of your VIN are less than 265965, you will need an upgraded fuel filter and fuel lines.

Additionally, some special tools may be required for the job, such as the Mercedes-Benz fuel line pliers or a Torx socket set. These can also be purchased online or from an automotive tool supplier.

It is recommended to compare prices and read reviews before placing your order. Some retailers offer discounts or free shipping for certain items, so it is worth checking multiple sources to get the best deal.

Once you have gathered all the necessary parts and tools, you can proceed with the fuel pump replacement procedure. Remember to set aside adequate time for the job, as it may take several hours to complete.

Disconnecting the fuel lines

Step 1: Prepare the Vehicle

Before beginning any work, ensure you have worn appropriate safety gear, including gloves and eye protection. Park the car on a level surface, engage the parking brake, and block the wheels to prevent accidental movement. It is also recommended to work in a well-ventilated area due to the risk of inhaling fuel vapors.

Step 2: Access the Fuel Pump

Locate the rear seats and carefully remove the flaps and bottom covers. This step requires caution as the tabs holding the covers can be easily broken. Once the covers are removed, unscrew the rear seats and disconnect the SRS cables.

Step 3: Expose the Fuel Pump

Slide the 60% seat towards the 40% seat to access the driver's side. Fold the floor mat to expose a round cap covering the access hole to the fuel pump. This cap is usually attached with body sealant, so carefully lift it with a sharp wood chisel.

Step 4: Disconnect the Electrical Connection

Unplug the electrical cable connected to the fuel pump. This is the power source for the pump.

Step 5: Relieve Pressure in the Fuel Lines

Start the engine and let it run for 5-10 seconds until it stalls and shuts off. This step is crucial to relieve the pressure in the fuel lines, making them safer to work with.

Step 6: Disconnect the Fuel Lines

Caution: Fuel will be present in the lines, so place towels or rags around the work area to absorb any spills or drips. Wear gloves to protect your hands.

The fuel lines are connected to the pump with locking tabs. To disconnect, push the line and press the tabs on each side with your fingers. There are typically three fuel lines: the feed line (center), and two other lines. The feed line is pressurized and will have some fuel remaining, so collect the fuel in a cup immediately after disconnecting.

By following these steps, you will successfully disconnect the fuel lines from the fuel pump of your Mercedes-Benz ML500, allowing you to proceed with the rest of the fuel pump replacement procedure. Remember to work cautiously and refer to a qualified mechanic if you have any doubts or concerns.

Removing the old pump and installing the new one

To remove the old pump, first, remove the rear seat bottom covers. There are no screws, just tabs. Be careful when releasing the tabs as they break easily. Next, unscrew the rear seats. There are two hex nuts and several torx bolts. Disconnect the SRS cables; there is one for each seat.

Now, remove 40% of the rear passenger-side seat from the car. The 60% seat is very heavy, so just slide it towards the 40% seat so that you can access the driver's side. Fold the floor mat (driver-side corner) and you will see a round cap covering the access hole. This cap is attached to the body by sealant, so carefully remove the seal and lift the cap.

Take a picture to document the alignment of the pump connections. Then, unplug the cable, which is the power line to the pump. Start the engine and wait 5-10 seconds until the engine runs out of fuel and stalls.

Disconnect the fuel lines. Push the line, press the tabs on each side with your fingers, and remove it. Caution: the fuel line in the centre (feeding line) is still pressurised from the fuel filter, which stores almost half a quart of remaining fuel. Collect the fuel into a cup immediately after disconnecting the line. Fuel will come from the line, not the pump. There will be some fuel spillage, so place towels around to minimise inhalation of the fuel vapour.

Turn the locking ring of the pump counter-clockwise and remove it. Lift the fuel pump. It won't come straight up because of the fuel sensor floating arm and secondary intake line (tail-like extension tube). Tilting and turning will be enough. Caution: there is some fuel inside the pump reservoir (the bottom pan). Tilting the pump more than 45 degrees may cause fuel spillage. Be careful not to drop anything inside the tank at this point.

To install the new pump, carefully insert it, checking the alignment of the fuel lines and making sure it is the same as the original. Caution: wrong alignment of the pump may prevent the fuel sensor floating arm from lifting completely when the tank is full, meaning the fuel gauge will never show a full tank.

Make sure that the rubber gasket has a tight fit between the pump and the tank. Otherwise, it may cause a leak when the tank is full. Put the locking ring over the pump and draw a line with a pen (mark the initial position). Turn the locking ring clockwise for at least 360 degrees (one full turn). If you have the specific MB tool to remove and install the locking ring, the specification is 65Nm. If you don't have the specific tool, make sure it is tight enough or do one full turn. Otherwise, it may cause a leak when the tank is full.

Snap the fuel lines in place. Refer to the picture to confirm the feed line, which plugs into the centre. Plug in the power cable. Start the engine and use body sealant to seal the cap. Finally, install everything in reverse order. Do not forget to connect the SRS cable. Use Loctite for the seat nuts and bolts (40 Nm).

Frequently asked questions