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Mercedes Fuel Pressure Test

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how to check fuel pressure mercedesThe fuel pump plays a vital role in Mercedes-Benz cars. The fuel pump is located in the fuel tank. It pumps the gasoline from the fuel tank to the engine. While fuel pump failure isn’t very common in MB cars, we have seen several Mercedes cars such as ML320, C-Class, and E-Class models having a fuel pump go bad as early as 120k miles. In this tutorial, we will show you how to diagnose fuel-related problems and perform a fuel pressure test on Mercedes gasoline engines. If you need help diagnosing fuel problems on a diesel engine, check out this tutorial.

Mercedes Fuel Pressure Test Instructions


mercedes fuel pressure test kit gauageIf you suspect that your fuel pump is not working, you need to perform a fuel pressure test to verify. A fuel pressure check on your Mercedes-Benz will tell you if the fuel pressure is withing range or not. For this test, you will need a fuel pressure gauge. The Actron CP7818 fuel tester works on most Mercedes-Benz and has the required Schrader valve required for MB cars.

This test applies to Mercedes-Benz vehicles equipped with 4-cylinder, V6, V8 and V12 gasoline engines such as M112, M113, M119, M104 M272 and M273.

IMPORTANT NOTE
Be extremely careful during this test procedure. If fuel falls on hot engine parts, you may cause a fire. Keep a fire extinguisher nearby in case of emergency. Never carry this test if the engine is hot.

Mercedes Fuel Test Procedure

Tools Required:

Step 1

Open the hood of your Mercedes-Benz. Remove the engine cover. The engine cover can be pulled up as it is held in place with four tabs and no screws.

Step 2

Locate the fuel test port on the fuel rail. If you have a hard time finding the fuel pressure port (Schrader valve) on your Mercedes, start from one of the fuel injectors and follow the fuel rail.fuel pressure rail schrader port fuel test port

Step 3

Remove the Schrader valve cap. Mercedes fuel pressure test instructions Connect your fuel pressure tester.diy mercedes fuel pump test

Step 4

If you turn the ignition ON the fuel pressure should build up, before you even crank the engine. mercedes fuel pressure checkTry starting the motor and get a fuel pressure reading. As you can see from our video, the fuel pressure in our case was ranging between 54-56 PSI which is normal. At idle we were getting about 55 PSI with slight fluctuation (+/-2 PSI) when revving up the engine. Watch the video for more details. You will notice that the fuel pressure didn’t change much when RPMs were kept constant at 4000, 2000 and idle, which is a good sign.

Recommended fuel pressure range

  • Idle:  47 – 55 PSI
  • With vacuum hose disconnected: 54 – 61 PSI

*Note that under normal operation the fuel pressure should build up as soon as you turn on the ignition before you even try to crank the engine. 

Troubleshooting Mercedes Fuel Pump Problems


A bad fuel pump can cause: engine no start condition or poor engine performance.

Typically problems related to the fuel pump and fuel pressure include:

  • loss of power
  • acceleration problems
  • engine sputtering at high speed
  • engine not starting
  • engine stalling
  • problems starting the car

Under normal operation conditions, the fuel pump provides a constant flow of fuel.  A failing fuel pump typically delivers too little fuel from the gas tank or in certain cases too much. Mercedes-Benz cars use an electrical fuel pump to pump the fuel to the fuel injectors.

Let’s look at the top fuel problems and how you can troubleshoot some of them.

Test 1: Fuel Pump Hearing Test

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A simple way to check if the fuel pump is working is to turn ignition to position II (all dash lights will turn on). Do not start the car. As soon as you turn the ignition on, listen carefully by 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.

Test 2: Check Fuses

If your Mercedes cranks but won’t start it is possible that the fuel pump has failed. The first thing that you should do is check the fuel pump fuse. If the fuse is good, you will need to test the fuel pressure.

Test 3: Check Voltage at Fuel Pump

If you performed a fuel pressure test and find out that the fuel pump is not generating fuel pressure, don’t rush into the conclusion that the fuel pump is bad. Perform the following test before you replace the fuel pump.

  1. Check fuel pump relay
    Check the fuel pump relay. You can either test the relay if you know how to or you can swap it with the same relay from a different system.
  2. Check voltage at the fuel pump
    Let’s say you check the fuel pump fuse and relay and they are both good. Next, we want to make sure that power is going to the fuel pump. It is very common that for various reasons power is not going to the fuel pump. This can be a damaged electrical wire issue or the Engine Control Unit not powering the fuel pump for various reasons.
    How you to check if the fuel pump is getting power.

    1. Get a hold of digital multimeter.
    2. Locate the fuel pump. In most cases under the rear seat.
    3. Remove the electrical connector from the fuel pump. You will have to remove the metal cover to access the top of the fuel pump.
    4. Set you digital multimeter to 20-volt setting and connect it to the two main (black and red) lines going into the fuel pump.
    5. Turn the key to position II and look at the reading. You should get around 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.

check windshield wiper motor not workingIf 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 it is possible that the ECU (Engine Control Unit) is detecting a problem (often security related) and is not feeding the fuel pump.

If you change the fuel pump and it fails again soon a few months later, it is possible that you have a partially clogged fuel filter or a restriction in the fuel lines.  Under such conditions, the fuel pump may draw excessive current and blow the fuel pump fuse.

fuel injectorNote that a defective fuel injector can also be the root of many problems. You may experience problems such as engine shaking, misfire codes or even not being able to start the engine because a leaking fuel injector won’t allow the fuel pump to build enough fuel pressure to start the engine.

Bad Mercedes Fuel Pump Symptoms


Mercedes loses power when accelerating

Symptoms include lack of acceleration when you press the gas pedal or vehicle trying to stall. The fuel pump is not providing the required fuel pressure to keep up with the engine demands.

Mercedes may jerk during acceleration from a stop. Happen when you hit the gas pedal to the floor. The vehicle operates normally when you press the gas pedal slowly. Note that such symptoms may also be caused by a faulty a faulty Throttle Actuator Gas Pedal Position Sensor or a defective Throttle Valve Body. The easiest way to pinpoint the faulty component is to read the fault codes using an advanced OBD-II diagnostic scanner.

Mercedes Cranks but won’t start

If your Mercedes doesn’t start, check the fuel pressure. If you are getting zero reading, you either have an electrical problem or a bad fuel pump.

Check Engine Light

If the check engine light is on, consider yourself “lucky” as it can make your diagnosis easier. Note that not all faults will trigger the check engine light. Scan the fault codes using a multi-system diagnostic scanner that is capable of reading fault codes from multiple control units. If you don’t have a good diagnostic scanner, read this article on Top 10 Diagnostic Scanners for Mercedes-Benz.

Mercedes in limp mode

Don’t be surprised if your Mercedes goes into limp mode for fuel related problems. Use Star Diagnostic scanner to perform a complete system scan.

Mercedes fuel pump replacement cost

To replace the fuel pump on a Mercedes-Benz, it costs between $450 and $800 depending on the MB model. You will pay about $300-$450 for parts (fuel pump) at the dealer. If you have a couple of hours and some basic skills on car repair, you can replace Mercedes fuel pump yourself for under $250 (parts) or less. Replacement fuel pumps for Mercedes can be purchased online if you are looking to save money.

Where to buy Mercedes fuel pump?

This is a list of Mercedes fuel pumps for different MB models. Links to where you can buy Mercedes fuel pump are provided below.

FAQ

Can a bad fuel pump or fuel regulator trigger the check engine light?

Yes. You may get codes such as:

  • P2017 Self-adaptation of mixture formation to limit value of engine control module
  • P0170 at idle speed or P2086 Self-adaptation from the mixture forming (left engine) (P0173)
  • P0087 Fuel Rail System Pressure Too Low
  • P0230 Fuel Pump Primary Circuit Malfunction

My fuel pump is not the problem, what else could it be?

Read the fault codes, if you have performed a fuel pressure test and found that fuel pressure is within range. You may have problems with the MAF (mass air flow) sensor which can also cause similar symptoms such as the engine running rough or lack of power.

Conclusion


Troubleshooting a bad fuel pump problem on a Mercedes-Benz is easy.  This is a job where you need to be extremely careful because you will work near fuel and there is always the risk of fire. It is recommended that you empty the tank or have the fuel tank near empty when you replace the fuel pump. If your Mercedes-Benz doesn’t start, always test that you have enough fuel pressure. Before you replace the fuel pump make sure the pump is getting power. Often the reasons why the fuel pump isn’t working is because it is not getting power due to electrical problems. Always perform tests to confirm that a part is defective instead of just replacing parts. Seek the help of an auto mechanic when necessary.

Resources


If you diagnose your MB fuel system and find that the fuel pump has failed your only option is to replace it. The change the fuel pump on a Mercedes-Benz is quite easy especially on models such as E-Class, C-Class, S-Class and ML models.

Before replacing make sure that you have the right tool to remove the fuel pump. Next, you will need to find out how to remove the rear seat.
Always disconnect the negative battery terminal before you start working on your fuel system.

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