Replacing The Fuel Pump In Your 1991 Mercedes 300Se: Step-By-Step Guide

The fuel pump in a 1991 Mercedes 300SE is located in the fuel tank. The fuel pump is responsible for delivering fuel from the gas tank to the engine. It is typically an electric pump that generates pressure to move the fuel through the fuel lines and into the engine. Depending on the model, the 300SE may have one or two fuel pumps.

The fuel pump relay, which controls the power to the fuel pump, is located in the MAS relay unit, which is situated between the two firewalls. The MAS relay unit is a large unit with a knob on top and can be found behind a cover behind the battery.

Fuel pump location

The fuel pump on a 1991 Mercedes 300SE is located in the engine bay, near the firewall on the right-hand side. It is situated behind the battery, between the two firewalls, in the centre.

To access the fuel pump, you will need to remove several components, including the radiator, shroud, fan, and drive belts. It is also recommended to safely lift and support the car for easier access.

• Begin by draining the coolant. Locate the plastic drain plug on the bottom of the lower front passenger side of the radiator. Place a drain bucket under the hole and use a large screwdriver to turn the plug.
• Remove the radiator cap to break the vacuum and allow the coolant to flow freely.
• Disconnect the overflow hose and upper hose from the radiator.
• Lift the charcoal canister on the left side of the radiator mount to provide more room for manoeuvring.
• Undo the clips holding the shroud to the radiator and radiator to the frame.
• Disconnect the transmission oil coolant lines from the lower part of the radiator.
• Lift the shroud from its locating taps on the bottom of the radiator and move it back over the fan.
• Remove the two lower water hoses from the radiator by wiggling the shroud to clear the outlet pipes.
• The radiator is now free and can be lifted out. It has two rubber mounts on the lower cross frame, so be careful when removing it.
• Next, remove the fan by holding it in place and unscrewing the four 10mm bolts at the back of the Viscofan.
• Remove the drive belts by loosening the tension on the respective pumps/alternators using Allen keys and wrenches of various sizes (6mm, 8mm, 13mm, 19mm).
• With the belts removed, you will have better access to the area near the firewall, where the fuel pump is located.

By following these steps, you should be able to locate and access the fuel pump on your 1991 Mercedes 300SE.

Fuel pump relay location

The fuel pump relay on a 1991 Mercedes 300SE is located behind a cover behind the battery. It will look like one of the two relays pictured below. The relay is part of the MAS relay unit, which is a large relay with a knob on top that needs to be unscrewed and released. The MAS relay unit is located between the two firewalls, about in the centre.

Fuel pump replacement

Step 1: Locate the Fuel Pump Relay

The fuel pump relay on a 1991 300SE is typically located behind a cover near the battery, between the two firewalls. It is a large relay with a knob on top that needs to be unscrewed to release it.

Step 2: Access the Fuel Pump

Before accessing the fuel pump, make sure the car is cool and place a drain bucket under the plastic drain plug on the bottom of the lower front passenger side of the radiator. Turn the plug with a large screwdriver to drain the coolant.

Next, move to the coolant reservoir and undo the cap to break the vacuum, allowing the coolant to flow freely. Then, undo and separate the overflow hose and upper hose from the radiator.

Give yourself more room by lifting the charcoal canister on the left side of the radiator mount and undo the clips holding the shroud to the radiator and radiator to the frame.

Disconnect the transmission oil coolant lines from the lower part of the radiator, as there will be some fluid spillage. Lift the shroud from its locating taps on the bottom of the radiator and move it back over the fan.

Step 3: Remove the Fuel Pump

Undo and remove the two lower water hoses from the radiator. You may need to wiggle the shroud to clear the outlet pipes. The radiator can then be lifted out.

Now, move on to removing the fan. There are four 10mm bolts holding the back of the fan to the front of the cooling pump. Hold the fan in place while unscrewing the bolts. Do NOT place anything between the fan blades to keep it from turning, as this can cause damage.

Step 4: Install the New Fuel Pump

Carefully clean the mounting surface on the engine of any old gasket or sealant. When installing the new fuel pump, use only a new gasket, as the two clean flat surfaces should provide an adequate seal.

Step 5: Reassemble

Reassemble all the parts in the reverse order of removal, making sure to tighten all bolts and clips securely.

Warning:

It is important to note that working with fuel and electrical systems can be dangerous. Always take the necessary safety precautions and refer to a professional mechanic if you are unsure about any part of the process.

Fuel pump problems

Common Fuel Pump Problems

• Whining or Howling Noises From The Gas Tank: A failing fuel pump can produce a loud, high-pitched whine, and during high-speed driving, you may hear sputtering sounds due to insufficient fuel pressure.
• Loss of Power: Inconsistent fuel volume delivery to the engine, especially during acceleration or elevation changes, can cause a loss of power. This can also lead to engine misfires, jerky acceleration, and stalling.
• Check Engine Light: When the check engine light illuminates, use a diagnostic scanner to read fault codes. This will help identify issues, including potential problems with the fuel pump or related components.
• Poor Fuel Economy: A faulty fuel pump may cause a significant decrease in fuel economy, and in severe cases, the car may not start at all due to a lack of fuel delivery to the engine.

Troubleshooting and Replacement

To diagnose fuel pump issues, you can check the fuel pressure using a fuel pressure gauge. Here are the steps:

• Open the hood and remove the engine cover.
• Locate the fuel pressure port on the fuel rail and remove the Schrader valve cap.
• Connect a compatible fuel pressure tester to the port.
• Turn on the vehicle without cranking the engine.
• Check for a normal fuel pressure range of 47-55 PSI on idle. If the vacuum hose is disconnected, acceptable readings may vary between 54-61 PSI.

If you determine that your fuel pump needs to be replaced, here's a general step-by-step guide:

• Put on safety goggles and drain the fuel pump to avoid spills and vapors.
• Disconnect the negative battery cable and remove the rear seat to access the integrated floor access cover.
• Disconnect fuel hoses, emissions hoses, and connections from the pump assembly.
• Remove the pump assembly's retainer ring and wipe the pump access hole clean.
• Remove the old fuel pump.
• Align and mount the new fuel pump, ensuring the seal ring is in place.
• Reconnect all fuel and emissions hoses and remount the floor access cover and rear seat.
• Reconnect the negative battery cable, refuel, and test-drive the vehicle to ensure proper functioning.

It's important to note that unlike fuel filters, fuel pumps do not require regular replacement. However, high mileage can increase the likelihood of fuel pump wear and tear. To maintain your fuel pump, it's recommended to fill up your fuel tank regularly and check for clogged fuel filters.

Fuel pump cost

The cost of a fuel pump for a 1991 Mercedes 300SE varies depending on the manufacturer and the retailer.

Adsit offers a Bosch fuel pump for the 1991 300SE, but the price is not listed. They also offer a fuel pump with no brand listed, which is the only fuel pump on their website with a listed price, at $279.97.

AutohausAZ offers a range of fuel pumps for the 1991 300SE, with prices ranging from $5.19 to $279.97. The cheapest option is a fuel tank screen seal O-ring, while the most expensive is a fuel pump kit.

Walbro offers an OEM in-line fuel pump for the 1991 300SE for an unspecified price.

It is important to note that the price of a fuel pump can vary depending on factors such as the quality of parts and maintenance. It is recommended to have the fuel pump inspected and serviced regularly to ensure optimal performance and longevity.

Frequently asked questions