Mercedes Fuel Pressure Test

The 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 have a fuel pump go bad as early as 120k miles. This tutorial 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

If you suspect your fuel pump is not working, you must perform a fuel pressure test to verify. A fuel pressure check on your Mercedes-Benz will tell you whether the fuel pressure is within range. 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:

Fuel Pressure Tester (eBay link)

Step 1

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

Step 2

Locate the fuel test port on the fuel rail. If you have difficulty finding your Mercedes’s fuel pressure port (Schrader valve), start from one of the fuel injectors and follow the fuel rail.

Step 3

Remove the Schrader valve cap. Connect your fuel pressure tester.

Step 4

If you turn the ignition ON, the fuel pressure should build up before you even crank the engine. Try starting the motor and get a fuel pressure reading. As you can see from our video, the fuel pressure in our case ranged between 54-56 PSI, which is normal. At idle, we got about 55 PSI with slight fluctuation (+/-2 PSI) when revving 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 crank the engine.

Troubleshooting Mercedes Fuel Pump Problems

A bad fuel pump can cause the engine no start condition or poor 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 fuel flow. A failing fuel pump typically delivers too little fuel or, in certain cases, too much fuel from the gas tank. Mercedes-Benz cars use an electric 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

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

Test 2: Check Fuses

The fuel pump may have failed if your Mercedes cranks but won’t start. The first thing that you should do is check the fuel pump fuse. If the fuse is good, you must test the fuel pressure.

Test 3: Check the Voltage at the Fuel Pump

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

Check the fuel pump relay.
Check the fuel pump relay. You can either test the relay if you know how to or swap it with the same relay from a different system.
Check the voltage at the fuel pump.
Let’s say you check the fuel pump fuse and relay, which are both good. Next, we want to ensure power goes 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 do you 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 must remove the metal cover to access the top of the fuel pump.
4. 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.
5. 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.

Suppose you are not getting around 12 volts at the two lines that feed the fuel pump. In that case, 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, you may 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.

Note that a defective fuel injector (eBay link) can also be the root of many problems. You may experience problems such as engine shaking, misfire codes, or even the inability 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 a lack of acceleration when you press the gas pedal or a vehicle trying to stall. The fuel pump does not provide the required fuel pressure to meet 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. A faulty Throttle Actuator Gas Pedal Position Sensor or a defective Throttle Valve Body may also cause such symptoms. 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. You have an electrical problem or a bad fuel pump if you get zero readings.

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 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 the Star Diagnostic scanner to perform a complete system scan.

Mercedes fuel pump replacement cost

Replace the fuel pump on a Mercedes-Benz 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 in car repair, you can replace the Mercedes fuel pump for under $250 (parts) or less. Replacement fuel pumps for Mercedes can be purchased online to save money.

Where to buy a Mercedes fuel pump?

This is a list of Mercedes fuel pumps for different MB models. Links to where you can buy a 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 cause similar symptoms, such as the engine running rough or lacking power.

What is the Best Scanner to Use for Diagnosing my Mercedes-Benz?

If you’re experiencing problems with your Mercedes-Benz, the YOUCANIC full system diagnostic scanner can provide invaluable assistance in troubleshooting the issues. By connecting to your car’s onboard computer, the scanner can read fault codes and provide detailed information about any problems with your engine, transmission, brakes, and other systems. Additionally, the scanner can perform diagnostic tests on various systems, such as the ABS, airbag, and steering systems, to help identify the underlying cause of any issues.

In addition to providing diagnostic information, the YOUCANIC scanner can save you time and money on repairs by enabling you to perform simple fixes yourself. For example, if you notice the check engine light on, the scanner can assist you in identifying the problem and determining if it’s a simple issue you can address at home or if you need to take it to a mechanic. This can help you avoid unnecessary trips to the repair shop, saving you both time and money.

Overall, the YOUCANIC full system diagnostic scanner is a powerful tool to help you maintain and troubleshoot your Mercedes-Benz. It’s an excellent investment for both DIY mechanics and professional technicians, as it can assist in diagnosing and resolving even the most complex automotive problems easily, saving you time and money on costly repairs.

Conclusion

Troubleshooting a bad fuel pump problem on a Mercedes-Benz is easy. This job requires you to be extremely careful because you will work near fuel, and there is always a fire risk. It is recommended that you empty the tank or have it 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 reason why the fuel pump isn’t working is that it is not getting power due to electrical problems. Always perform tests to confirm a defective part 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. Changing the fuel pump on a Mercedes-Benz is quite easy, especially on E-Class, C-Class, S-Class, and ML models.

Before replacing, ensure you have the right tool to remove the fuel pump. Next, you must find out how to remove the rear seat.
Always disconnect the negative battery terminal before working on your fuel system.

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