Checking Fuel Pressure In A 2005 Mercedes E500

If you want to check the fuel pressure on your 2005 Mercedes E500, you'll need to test it with a fuel pressure gauge. This is attached to the Schrader valve on the fuel rail, which is located on the right side of the engine. You can buy a fuel pressure gauge from most auto parts stores for around $30-75. Once you have the gauge, follow these steps:

1. Remove the brass cap from the Schrader valve using a 15mm box- or open-end wrench.

2. Screw the fuel pressure gauge onto the Schrader valve and tighten it as much as you can by hand.

3. Start the engine or turn the key to the ACC position.

4. The gauge should immediately jump to the 3.2-3.6 bar (46.4-52.2 PSI) mark, indicating that the system is fully pressurised.

5. With the engine running, carefully disconnect the rubber vacuum fitting from the nipple extending from the side of the FPR.

6. The needle on the gauge should now jump to 3.7-4.2 bar (53.6-61 PSI).

7. Reconnect the vacuum fitting – the needle should fall back to the previous reading of 3.2-3.6 bar.

8. If the pressure is less than these factory specs, further investigation is needed, starting with the FPR and moving towards the fuel pumps.

Check the fuel pump relay

To check the fuel pump relay of your Mercedes E500 2005, you will need a digital volt-ohm meter (DVOM) and a wiring diagram. The fuel pump relay is located in the fuse and relay box in the engine compartment.

First, you need to confirm that the fuel pump is receiving the correct voltage when commanded on by the fuel pump relay. To do this, use the DVOM to check for voltage at the fuel pump. If the correct voltage is present but the pump does not activate, it is likely faulty.

Next, use the DVOM in conjunction with the wiring diagram to determine the correct terminals for testing. You can access these terminals under the rear seat cushion, below the access panel.

Now, locate the fuel pump relay in the fuse and relay box. Remove the relay and use a fused jumper wire to connect the terminals indicated by red arrows in Figure 15. This will activate your fuel pump. Check for voltage at the fuel pump as before. If battery voltage is present, the relay is functioning correctly.

If you do not have voltage at the fuel pump, there may be an issue with the relay circuit. You can check this using the DVOM. Connect the black lead of the DVOM to the battery negative and use the other lead to backprobe the terminals. Refer to Figure 15 and the list below to cross-reference your voltage readings:

• Terminal 30: Constant battery positive
• Terminal 86: Switched battery positive, present when the key is in the ON position
• Terminal 85: DME relay control, close to zero volts when DME turns relay on
• Terminal 87: Feed to fuel pump, battery positive when relay is activated

If you do not have voltage at the correct terminals, the relay may be faulty. You can also try load testing the relay using a test light and a DVOM. Connect the DVOM across the fuel pump electrical connector terminals and take a reading. It should read battery volts when the key is turned on. Then, connect an incandescent bulb-style test light to the battery ground and touch the test light probe tip to the positive wire you are backprobing with the DVOM. Your reading should hold steady, with a maximum voltage drop of 0.5 volts.

If the relay is not functioning correctly, you may need to replace it. You can find fuel pump relays for the Mercedes E500 starting at $42.17 from suppliers such as Advance Auto Parts.

Test the fuel pump

To test the fuel pump of a 2005 Mercedes E500, you can start by checking the fuse and relay. If those are fine, the next step is to check the voltage at the fuel pump.

Checking the Voltage at the Fuel Pump

• Get a hold of a digital multimeter.
• Locate the fuel pump, which is usually under the rear seat.
• Remove the electrical connector from the fuel pump. You may need to remove a metal cover to access the top of the fuel pump.
• Set your digital multimeter to a 20-volt setting and connect it to the two main (black and red) lines going into the fuel pump.
• Turn the key to position II and look at the reading. You should get around an 11-13 volt reading. Note that when you turn on the ignition, the fuel pump gets voltage for about 5 seconds to prime the system. Cycle the ignition on and off a couple of times and look at the reading.
• If you are not getting around 12 volts at the two lines that feed the fuel pump, that means that you most likely either have a bad fuse or relay, or the ECU (Engine Control Unit) is detecting a problem (often security-related) and is not feeding the fuel pump.

Other Fuel Pump Tests

Before concluding that the fuel pump is faulty, you can perform other tests, such as:

• A fuel pump hearing test: Turn the ignition to position II (all dash lights will turn on). Do not start the car. Listen carefully to the rear seat (near the fuel tank). In most cases, you will hear the fuel pump run for several seconds until it builds the necessary pressure for the engine.
• A fuel pressure test: This test will tell you whether the fuel pressure is within range. For this test, you will need a fuel pressure gauge, such as the Actron CP7818 fuel tester.

Check the Schrader valve

Checking the Schrader valve is a crucial step in diagnosing fuel-related issues in your Mercedes-Benz E500 2005. Here is a detailed guide on how to do it:

Step 1: Understand the Purpose

The Schrader valve, also known as the fuel pressure valve, is used to check the fuel pressure in your Mercedes-Benz E500. This valve is located on the fuel rail, which is the tube that connects the car's fuel injectors. By measuring the fuel pressure, you can gain insights into the health of various components, such as the fuel pressure regulator, check valves, and fuel pumps.

Step 2: Park Your Car and Prepare

Before you begin, make sure your car is parked in a well-ventilated area and the engine is turned off. Working with fuel can be dangerous, so ensure you have a fire extinguisher nearby in case of any emergencies.

Step 3: Locate the Schrader Valve

Open the hood of your Mercedes-Benz E500 and locate the fuel rail. The Schrader valve is usually found on the fuel rail, and you can start by looking for it near the fuel injectors. If you have an M119 engine, the valve will be towards the front of the engine, in the base of the "U"-shaped fuel rail. For an M104 engine, the valve will be about two-thirds back along the straight fuel rail, next to the upper wiring harness cover.

Step 4: Prepare the Fuel Pressure Gauge

You will need a fuel pressure gauge to check the fuel pressure. These gauges are readily available at most auto parts stores and typically cost around $30 to $75. The gauge should have a rubber hose with a female Schrader valve fitting at the end. Before attaching the gauge, ensure that the car's engine is cool, and there is no risk of fire.

Step 5: Remove the Schrader Valve Cap

Once you have located the Schrader valve on the fuel rail, use a 15mm box-end or open-end wrench to remove the brass cap from the valve. Set the cap aside in a safe place.

Step 6: Attach the Fuel Pressure Gauge

After removing the cap, screw the fuel pressure gauge onto the Schrader valve. Tighten it as much as you comfortably can by hand. In most cases, you won't need to use pliers or any other tools for additional tightening.

Step 7: Start the Engine and Take Readings

Now, you can turn the ignition to the 'ACC' position or start the engine. With the engine running, carefully observe the gauge. The needle should immediately jump to the specified pressure range, indicating that the system is fully pressurized. At idle, the fuel pressure should be within the acceptable range, with slight fluctuations when revving the engine.

Step 8: Perform the Vacuum Test

For the next test, carefully disconnect the rubber vacuum fitting from the nipple extending from the side of the fuel pressure regulator. This nipple is usually found under the airbox for the M119 engine and on the front of the engine for the M104 engine. When you disconnect the vacuum fitting, the needle on the gauge should jump to a higher pressure range. Reconnect the vacuum fitting, and the needle should return to the previous pressure range.

Step 9: Interpret the Results

If the indicated pressure is lower than the factory specifications, further investigation is required. This could suggest issues with the fuel pressure regulator, check valves, or fuel pumps.

Step 10: Perform the Leakdown Test

The leakdown test will show how well the fuel injection system holds residual pressure over time. After turning off the engine, the fuel pressure should gradually decrease, but it should maintain a pressure of at least 2.5 bar (36.2 PSI) for 30 minutes after shutdown, according to Mercedes-Benz specifications.

Test the fuel pressure regulator

To test the fuel pressure regulator, you must first check for any signs of malfunction. These can include poor mileage, gas leaking, black smoke from the exhaust, or a rough-running engine. If you notice any of these issues, you should then follow these steps:

• Scan the computer system in the car for trouble codes.
• Inspect the fuel pressure regulator for leakage and proper operation.
• Inspect for any broken vacuum lines.
• Remove and replace the fuel pressure regulator if it is faulty.
• Change the engine oil and filter if the oil is contaminated due to fuel leakage.
• Clear any diagnostic trouble codes.
• Test drive the vehicle to ensure proper functioning.
• Open the hood of your Mercedes-Benz and remove the engine cover by pulling it up from the four tabs.
• Locate the fuel test port (Schrader valve) on the fuel rail. This is usually found near the fuel injectors.
• Remove the Schrader valve cap.
• Connect your fuel pressure tester. Ensure you are using a suitable tester, such as the Actron CP7818 fuel tester, which works on most Mercedes-Benz models.
• Turn the ignition on and check the fuel pressure reading. It should build up before you crank the engine.
• Start the engine and observe the fuel pressure. The pressure should range between 54-56 PSI at idle, with slight fluctuations when revving the engine.
• Compare your readings to the recommended fuel pressure range: with the vacuum hose disconnected, the pressure should be between 54-61 PSI.
• If your readings are lower than the recommended range, further investigation is needed, starting with the fuel pressure regulator.

Check the fuel injectors

To check the fuel injectors on a 2005 Mercedes E500, you will need to purchase a fuel pressure gauge. These can be bought at most auto parts stores for around $30-75. The fuel pressure gauge is generally a round dial gauge, attached to a rubber hose with a female Schrader valve fitting at the end.

First, remove the brass cap from the Schrader valve on the fuel rail using a 15mm box or open-end wrench. Then, screw the fuel pressure gauge onto the Schrader valve and tighten as much as possible by hand. You do not need to use pliers or any other tools to tighten it further.

Next, get into the car and turn the key to the ACC position, or start the engine. The gauge should immediately jump from the "0" mark to the 3.2-3.6 bar (46.4-52.2 PSI) mark, meaning the system is fully pressurised. With the engine running, carefully disconnect the rubber vacuum fitting from the nipple extending from the side of the FPR (fuel pressure regulator). The needle of the gauge should immediately jump to 3.7-4.2 bar (53.6-61 PSI) with the vacuum disconnected; and should fall back to 3.2-3.6 bar (46.4-52.2 PSI) when the vacuum fitting is re-connected. If the indicated pressure is less than these factory specs, then further investigation is warranted, starting with the FPR and going rearward toward the fuel pumps.

You can also perform a "leakdown" test to show how the fuel injection system (the fuel rail) holds residual pressure. This is done by turning off the engine and observing how the fuel pressure bleeds off over time. According to Mercedes-Benz specifications, the system should maintain greater than 2.5 bar (36.2 PSI) for a full half-hour after the engine is shut down, and should gradually bleed off after that.

If you are experiencing issues with your fuel injectors, there are a number of replacement options available online, including on eBay and specialty Mercedes-Benz parts websites.

Frequently asked questions