Checking Fuel Pressure In Your Bmw E46: A Step-By-Step Guide

The fuel supply system in a BMW E46 consists of a fuel filler, evaporative emissions control valve, pressure pump, fuel pump assembly, fuel pressure regulator, fuel rail, and electric fuel injectors. A faulty fuel pump can cause poor engine performance and hard starting problems. To check the fuel pressure, you can use a fuel pressure gauge to measure the pressure at the fuel rail. The fuel pressure should be within the specified range when the engine is idling and also after turning off the engine for a certain period. If the fuel pressure is low or drops too quickly, it could indicate a problem with the fuel pump, fuel filter, fuel injectors, or a leak in the fuel system. It is recommended to refer to the specific fuel pressure specifications for your BMW E46 model and perform thorough diagnostics to identify the root cause of any fuel pressure issues.

Check the fuel pressure gauge

Checking the fuel pressure gauge is a vital part of maintaining your BMW E46. Here's a detailed, step-by-step guide on how to do it:

Step 1: Prepare the Car

Before starting, ensure the car is parked on a level surface and the engine is cool. Locate the fuel test port, usually found near the fuel injectors. If you have difficulty finding it, refer to your car's service manual or seek assistance from a BMW technician.

Step 2: Attach the Fuel Pressure Gauge

Remove the fuel test port cap and install the fuel pressure gauge. Ensure all test hoses are securely connected and routed properly to avoid fuel leaks.

Step 3: Start the Engine and Check Pressure

Start the engine or attempt to start it if it is not running. The fuel pressure should be 3.5 bar +/- 0.2 bar (approximately 50 psi +/- 3 psi).

Step 4: Recharge the Fuel System

After taking the initial pressure reading, recharge the fuel system by cycling the key or running the fuel pump. Note the fuel pressure again.

Step 5: Allow the System to Sit and Re-check Pressure

Let the fuel system sit under pressure for about 15 minutes. A properly functioning fuel system should not lose more than 0.5 bar of pressure during this time. If the pressure drops significantly, it could indicate a faulty fuel pump check valve or a leak in the fuel system.

Step 6: Further Diagnostics

If you suspect a problem, further diagnostics may be required. This could involve checking for fuel leaks, testing the fuel pump relay, or even replacing the fuel filter and fuel pump if necessary. Always refer to your car's service manual or consult a qualified BMW technician for more complex diagnostics and repairs.

Tips for Checking Fuel Pressure:

• When working on your vehicle, always wear protective gear, including eye protection, gloves, and clothing that covers your body.
• Ensure you have the correct tools and a basic understanding of automotive systems before attempting any repairs.
• If you are unsure about any aspect of the process, don't hesitate to seek professional assistance.
• Fuel systems can be dangerous due to high pressures and flammable liquids. Always exercise caution and refer to manufacturer guidelines.

Check the fuel pump

To check the fuel pump, you will need a digital volt ohm-meter (DVOM). You can confirm the fuel pump has received the correct voltage when commanded ON by the fuel pump relay using the DVOM. If the correct voltage is present, but the pump does not activate, it is likely faulty.

• Use the DVOM in conjunction with a wiring diagram to determine the correct terminals for testing.
• Access the terminals under the rear seat cushion, below the access panel.
• Check if the correct voltage is present at the fuel pump terminals when the key is turned ON. The DVOM should read battery volts (around 12 volts). If not, check the relay and fuel pump fuse.
• If the correct voltage is present but the pump is not activating, use a test light and a DVOM for load testing. 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.
• Connect an incandescent bulb-style test light to battery ground and touch the test light probe tip to the positive wire you are back-probing with the DVOM. Your reading should hold steady, with a maximum drop in voltage of 0.5 volts.
• If the voltage drops more than 0.5 volts, there is a problem.
• Alternatively, you can jump out your fuel pump relay (to bypass it) for testing. Open the glove compartment and remove all items from within.
• Remove the suspension strut rivet and detach the strut from the glove compartment.
• Remove the suspension strap rivet and detach the strap from the glove compartment.
• Remove the five Phillips-head screws from the glove compartment.
• Lower the glove compartment, disconnect the flashlight electrical connector, and detach the wiring harness from the back side. Then, remove the glove compartment from the dashboard.
• Lower the plastic trim panel and disconnect the footwell light electrical connector. Then, remove the trim panel from the vehicle.
• Working under the passenger side of the dashboard, remove the relay 10mm panel support fastener. Then, pull the relay panel out of the top clips and pull it down.
• Remove the fuel pump relay.
• Connect a fused jumper wire between terminals 30 (battery positive) and 87 (fuel pump feed). This will activate your fuel pump. Check for voltage at the fuel pump as you did earlier. Battery volts should be present.

Note: Terminals may vary, so double-check your model against a wiring diagram.

Check the fuel filter

To check the fuel filter on your BMW E46, you will need to locate the fuel filter, which is on the left side of the vehicle, approximately midway back, and is covered with a metal plate. Remove the bolts securing the fuel filter cover to the chassis of the vehicle. Note that the last two bolts may be difficult to access, so a deep 8mm socket on a spinner handle can be used for this.

Once the cover is removed, you will see the fuel filter and fuel lines. To remove the fuel filter, you will need to:

• Disconnect the fuel lines from the filter, being careful to catch any spilled fuel.
• Remove the vacuum line from the front of the fuel filter.
• Detach the fuel filter mounting bracket from the chassis.
• Remove the fuel filter from the vehicle and drain any excess fuel.

Before installing a new fuel filter, it is important to prepare it by:

• Removing the filter outlet fuel hose from the old filter and installing it on the new filter with new hose clamps.
• Transferring the metal mounting bracket from the old filter to the new one, ensuring that the arrow on the fuel filter points towards the direction of fuel flow.

To install the new fuel filter:

• Connect the rear fuel inlet(s) first and secure with new hose clamps.
• Connect the vacuum line and front metal fuel line, again using new hose clamps.
• Re-attach the mounting bracket to the chassis and replace the mounting bracket bolt and fuel filter cover.

Check the fuel injectors

To check the fuel injectors on a BMW E46, you will need a set of sockets (10mm, 13mm), a flathead screwdriver, and a digital volt-ohm meter (DVOM).

Step 1: Relieve the fuel system pressure

Remove the fuel pump fuse and run the vehicle until the engine stalls. This will minimise the amount of fuel spilled when you disconnect the fuel lines.

Step 2: Disconnect the battery

Disconnect the negative (-) battery cable.

Step 3: Remove the engine cover

Remove the engine cover over the fuel injectors.

Step 4: Disconnect the oxygen sensor connectors

Locate the oxygen sensor connectors at the top of the intake manifold and slide them out of the mounting bracket.

Step 5: Disconnect the intake air temperature sensor electrical connector

Disconnect the electrical connector by releasing the tab and pulling it off.

Step 6: Disconnect the intake VANOS solenoid electrical connector

Disconnect the electrical connector by releasing the tab and pulling it off.

Step 7: Disconnect the fuel injector harness

Use a flathead screwdriver to gently pry up each fuel injector to release the spring clips that hold the electrical harness in place. Work your way down the injectors while disconnecting the injector harness.

Step 8: Disconnect the fuel line

Reach behind the intake manifold, near the firewall, to locate the fuel line. Press the release tab and slide the fuel line off the connection. Have a rag handy to catch any dripping fuel.

Step 9: Remove the fuel rail fasteners

Remove the four 10mm fuel rail fasteners.

Step 10: Remove the fuel rail

Pull the fuel rail straight up and out of the intake manifold. Be careful not to damage the fuel injectors. If there are any O-rings stuck, use a flathead screwdriver to gently pry the fuel rail up.

Step 11: Remove the fuel injectors

To remove the individual fuel injectors, slide the retaining clip off the fuel rail using a flathead screwdriver. Then, pull the fuel injector straight out of the fuel rail.

When reinstalling the fuel injectors, replace the sealing O-rings and ensure that any retaining clips are properly seated. Press the fuel rail with its injectors into the intake manifold until they are fully seated. Install the fuel rail fasteners and torque them down to specs. Reassemble the remaining items and check for fuel leaks.

Check the fuel pressure regulator

To check the fuel pressure regulator, you will need to perform a fuel pressure test.

First, relieve the pressure in the fuel system, add the gauge, and cycle the pump. You should get a reading of 3.5 bar +/- 0.2 (50 psi +/- 3). Once you test the pressure, recharge the fuel system by cycling the key or running the fuel pump, noting the fuel pressure. Then, allow the fuel system to sit under pressure for 15 minutes. The pressure should not drop more than 0.5 bar in that time period. If it does, you could have a faulty fuel pump check valve or a leak in the fuel system.

If you suspect a faulty fuel pressure regulator, you can perform the following test. With the engine idling, check the fuel pressure. Then, rev the engine. The pressure should increase a few psi, then return to normal. Next, remove the vacuum line from the FPR and check the fuel pressure again. It should go up about 10 psi and stay there. If there is no change to the fuel pressure, suspect a stuck fuel pressure regulator.

If you do not have a Schrader valve on the fuel rail, you will need to tee into the fuel line at the filter with some hose and adapters. This job can be dangerous, as fuel is under pressure and could cause severe injury. Be sure to wear safety glasses and depressurize the fuel system to prevent fuel from spraying.

Frequently asked questions