Diagnosing Low Fuel Pressure: Simple Checks To Try

Low fuel pressure can cause a car's engine to have little to no response. This can be caused by a bad fuel pump, a clogged fuel filter, a defective fuel pressure regulator, a stuck fuel injector, a fuel pressure sensor, or a smashed fuel pressure line. To check for low fuel pressure, you can use a fuel pressure tester or gauge, or an OBD-II diagnostic code reader. If you don't have access to these tools, you can listen out for a buzzing sound when you turn your key to the 'ON' position, indicating that your fuel pump is working correctly.

Listen for a buzzing sound when turning the key in the ignition

If you hear a buzzing sound when turning the key in the ignition, it could be due to a weak electrical current. This is usually caused by a weak battery that doesn't have enough energy to crank the engine. However, the battery may have enough power to close the relay field. When the field closes, the relay closes the contacts for the starter, which then attempts to crank the starter, drawing down the battery. This cycle repeats itself very quickly, causing the buzzing noise.

To address this issue, ensure that the battery cables, terminals, and other connections are not corroded. Corrosion can occur in the connection between the battery and the starter motor, causing high resistance and a voltage drop when the solenoid engages. This can lead to a loud, annoying buzz as the voltage rises and falls rapidly.

If the buzzing sound persists, it may be due to a faulty switch connected to the relay or a problem with the low-voltage relay itself. Replacing the defective switch or the relay may be necessary to resolve the issue.

It is important to note that a buzzing sound when turning the key in the ignition could also be indicative of low fuel pressure. However, other common symptoms of low fuel pressure include difficulty starting the car, an unresponsive throttle, a stalling engine, a check engine light on the dashboard, misfires, and low performance.

Check the battery voltage

Checking your battery voltage is an important step in diagnosing low fuel pressure. Before you begin, ensure that you have a digital multimeter (DMM) to hand.

First, turn off your ignition and any accessories in your vehicle, such as the lights or radio. Set your multimeter to 20V DC if it is not auto-ranging. If your multimeter is auto-ranging, you can set it to 20 volts D-C scale or D-C.

Next, place the red lead on the positive battery terminal, which is indicated by a red cover or a + symbol. Then, place the black lead on the negative terminal. If you see a negative number, don't worry—this just means that you've attached the leads backwards, but the number is still accurate.

Read the measurement. If the reading is below 12.4 volts, your battery may need charging. If the battery voltage drops below 9.6 volts during cranking, this is an indicator of a weak battery.

Low voltage can cause a pump to underperform, so it's important to check this before replacing your fuel pump.

Check engine codes

If you're experiencing low fuel pressure, you may be able to diagnose the issue by checking your engine codes. Modern vehicles are equipped with an On-Board Diagnostics (OBD) system that can provide valuable information about any problems your car may be experiencing.

When your car's onboard computer detects an issue, it will log a fault code, which can be retrieved using an OBD-II scanner. These codes can help identify specific issues with your vehicle and suggest potential causes and solutions.

One common code related to low fuel pressure is the P0087 code, which indicates that the fuel rail pressure is below the desired level. This code is usually triggered by a malfunctioning fuel pump, a clogged fuel filter, or a faulty fuel pressure regulator.

Other possible codes related to low fuel pressure may include:

• P0190: Fuel Rail Pressure Sensor Circuit Malfunction
• P0191: Fuel Rail Pressure Sensor Circuit Range/Performance
• EID 398: Low Fuel Rail Pressure

Once you've retrieved the error codes from your vehicle's onboard computer, you can use them to identify and address the specific issue causing the low fuel pressure. It's important to address these issues promptly to avoid further damage to your vehicle's engine and fuel system.

Test the fuel pressure with a fuel pressure gauge

To test the fuel pressure with a fuel pressure gauge, follow these steps:

Firstly, ensure you have a fuel pressure gauge that is suitable for your vehicle. Most fuel pressure gauges will work with domestic fuel-injected vehicles, and many will also work with imported cars. If your car doesn't have the right valve, you may need an adapter.

Now, follow these steps to test your fuel pressure:

• With the engine entirely cold, open the hood.
• Find the Schrader valve fitting on the fuel rail. This may be hidden under a fuel rail cover or another plastic engine cover.
• Remove the Schrader valve cap.
• Attach the appropriate fuel pressure tester fitting. Ensure that it threads on properly for a leak-proof fit.
• Turn the ignition to "on", not start.
• Check the psi reading and wait. Watch for a drop in psi, which indicates a leak in the system. If the reading stays the same after 5-10 minutes, the system is holding pressure well.
• If the fuel pressure drops over this time, there is a leak in the fuel system. Look for drips underneath to help locate the source of the leak. It could also be leaking internally from a bad fuel injector.
• Start your engine and let it idle. You should see a steady fuel pressure, within a few psi of the recommended pressure.
• Once the engine is warmed up, slowly rev the engine, making sure the pressure rises with the RPMs.

If your fuel pressure holds steady, rises with engine speed, and is at the recommended pressure, your engine problem is likely not fuel-related.

It is important to note that fuel vapors are extremely flammable, so ensure these steps are performed in a well-ventilated area with a fire extinguisher nearby.

Additionally, different engines require different fuel pressures. Older throttle-body injected systems may need as little as 10 psi, while multi-port injection can see as high as 60 psi. Always refer to the specifications for your particular engine.

Check the fuel pressure with the vehicle running

Checking the fuel pressure with the vehicle running is the final step in diagnosing low fuel pressure. Before this, you should have already checked the battery voltage, engine codes, and fuel pressure with the vehicle off.

To check the fuel pressure with the vehicle running, you will need to install a fuel pressure gauge. First, park your vehicle and apply the parking brake. Then, turn off the engine and allow it to cool down. Locate the fuel pressure test port and place a rag underneath to catch any fuel that may be released during the test. Install the pressure tester to the port, then start the engine and record the pressure reading.

Observe the fuel pressure gauge needle for excessive movement. If the needle fluctuates and bounces, this indicates an issue with the fuel system. If the fuel pump doesn't meet the required pressure specifications, the fuel pump may be failing to pump fuel to the engine.

It is important to note that releasing fuel under pressure can be dangerous, so safety precautions such as wearing safety glasses and gloves and working in a well-ventilated area are crucial.

Frequently asked questions