Checking Fuel Pressure Relay: A Step-By-Step Guide

If your car is experiencing issues such as ignition problems, a continuously running fuel pump, or an illuminated check engine light, it may be suffering from a faulty fuel pump relay. A fuel pump relay is an electromagnetic switch that uses low current to control a higher current circuit. Testing a fuel pump relay can be done by swapping it with another relay from the power distribution box or by using a multimeter to check the resistance. To test fuel pressure levels, a fuel pressure test kit can be used by attaching the gauge to the fuel pump test point and activating the engine.

Using a multimeter to check the relay's resistance

To check the resistance of a fuel pump relay using a multimeter, you'll first need to locate the fuel pump relay in your vehicle. This is usually located in the fuse box under the hood or in the passenger compartment. You can consult your owner's manual or a wiring diagram to find the exact location. Once you've found the fuel pump relay, follow these steps:

• Read the circuit diagram: Look on the main body of the relay for a printed diagram of the circuit it controls. Identify the two points on the diagram that have a box between them, as these will be the power prongs. The two points with a break in the line connecting them are the prongs you'll need to test. Note the numbers printed next to the prongs.
• Set your multimeter to the ohm setting: Turn the dial on your multimeter to the ohm symbol (Ω) to test the relay. If your multimeter has multiple ranges within the ohm setting, choose the lowest range for the most accurate reading.
• Attach jumper wires to the battery terminals: Connect one jumper wire to the positive terminal of your vehicle's battery and the other to the negative terminal. Jumper wires are small cables with alligator clips on both ends, allowing for easy current transfer.
• Connect the jumper wires to the relay terminals: Flip the relay so the prongs point up, and clip the wire from the negative terminal to the terminal labelled 85. Then, attach the wire from the positive terminal to the pin labelled 86.
• Listen for a clicking sound: When you attach the jumper wires to the relay, you should hear a switch close inside the circuit, making an audible clicking noise. If you don't hear this, unclip and reattach the wire, touching the end of the wire to the prong repeatedly to listen for the click.
• Hold the multimeter probes against the remaining prongs: Plug the multimeter probes into the ports and hold them against the remaining prongs on the relay, labelled 30 and 87. The multimeter will now measure the resistance of the internal circuit between these prongs.
• Check the multimeter reading: If the circuit inside the relay has power, the prongs should show no resistance between them, resulting in a reading of 0 on the multimeter. A reading greater than 0 indicates a faulty circuit. Depending on the ohm range, the reading may be a small decimal, such as 0.001 or 0.005.

If your relay functions correctly but you still experience issues, there may be a different problem with your fuel pump or electrical system.

Locating the fuel pump relay in the fuse box

The fuel pump relay is an important component of your vehicle's fuel system. It is responsible for supplying power to the fuel pump, which creates pressure in the fuel lines to push gasoline to the combustion engine. When the ignition is turned on, the fuel pump relay supplies a consistent voltage to the fuel pump, allowing the engine to run.

The fuel pump relay is usually located in the fuse box, which is often a long black box found in the engine bay or under the dashboard. Depending on the year, make, and model of your vehicle, the fuse box may also be located under the hood, in the trunk, near the steering column, or on the firewall.

To locate the fuel pump relay, first find the fuse box. Then, look for a diagram on the fuse box cover that shows the location of the relays. This diagram should indicate the position of the fuel pump relay relative to the other relays in the box. If there is no diagram, you may need to check your owner's manual or search online for a diagram specific to your vehicle model.

Once you have located the fuel pump relay, you can inspect it for any signs of corrosion or overheating. You can also use a multimeter to test the continuity of the relay's contacts. If the fuel pump relay is not working properly, it may need to be cleaned, resoldered, or replaced.

Identifying bad fuel pump relay symptoms

A fuel pump relay is an electromagnetic switch that uses low current to control a higher current circuit. It is an electronic component found on virtually all vehicles equipped with an internal combustion engine. The fuel pump relay is usually controlled by the ignition or Powertrain Control Module (PCM). When switched on, it provides current to the fuel pump so that it may function.

A bad or failing fuel pump relay will usually produce a few symptoms that can alert the driver of the issues. Here are some common symptoms of a faulty fuel pump relay:

• Engine Stalling or Not Starting: If the fuel pump relay has an issue while the vehicle is operating, it will cut off power to the fuel pump, causing the engine to stall. A faulty relay may allow the vehicle to be restarted a short while later, while a completely failed relay will not.
• No Noise from the Fuel Pump: Most fuel pumps produce a low-volume hum or whine that can be heard from inside the vehicle or near the fuel tank. If the fuel pump relay fails, it will cut off power to the fuel pump, rendering it inoperable and silent when the ignition is on.
• Fuel Pump Runs Continuously: In rare cases, the fuel pump relay can stick in a closed position, causing the fuel pump to run continuously. This usually happens in circuits that supply constant power to the fuel pump fuse.
• Ignition Issues: A failing fuel pump relay can cause ignition issues, such as multiple attempts to start, especially in the morning when the vehicle is cold. Over time, the problem worsens, and the vehicle won't start at all.
• Unexplained Battery Drainage: A fuel pump relay stuck in the closed/on position can cause unexplained battery drainage. The fuel pump may continue running even after the vehicle is turned off, and this issue may persist even after installing a new battery or fuel pump.
• Illuminated Check Engine Light: In some vehicles, the PCM monitors the fuel pump relay and its circuit. If the PCM detects a problem, it turns on the check engine light and stores a diagnostic trouble code (DTC) in its memory.

It is important to note that these symptoms may also be caused by other issues. Therefore, it is recommended to have the vehicle properly diagnosed by a professional technician.

Testing fuel pressure with a fuel pressure tester kit

Turn the ignition to "on" and check the psi reading. A drop in psi over time indicates a leak in the system. If the reading remains the same after 5-10 minutes, the system is holding pressure well. If the fuel pressure drops over this period, there may be a leak in the fuel system. Check for drips underneath the vehicle to help locate the source of the leak. However, be aware that the leak could be internal, originating from a faulty fuel injector.

Next, start the engine and let it idle. The fuel pressure should remain steady, within a few psi of the recommended pressure for your vehicle. Once the engine has warmed up, slowly rev the engine and ensure the pressure rises with the RPMs. If the fuel pressure holds steady and rises with engine speed, your engine problem is likely not fuel-related.

Zero fuel pressure indicates that the fuel pump is dead or not receiving power. In this case, check the fuel pump fuse and verify power to the pump with a multimeter. If the pump is receiving power, it may need to be replaced.

Low fuel pressure can be caused by a clogged fuel filter or a failing fuel pump. If the fuel filter is serviceable, it may be beneficial to replace it. Low fuel pressure can also be caused by improper tank venting due to a loose or damaged gas cap. Ensure the gas cap gasket is intact and tighten it securely.

High fuel pressure can be the result of a clogged or kinked fuel return line, a faulty fuel pump driver module, or a faulty powertrain control module. These issues may trigger a "check engine" light and store a code. High fuel pressure can also be caused by a faulty fuel pressure regulator.

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

Interpreting fuel pressure readings

Fuel pressure readings are an essential part of fuel injection system troubleshooting. The correct fuel pressure will depend on the engine. Older throttle-body injected systems may need as little as 10 psi, while multi-port injection can see as high as 60. The same vehicle can also vary depending on the model year. For example, the LT1 V8 in the 1996 Corvette wants 40-42 psi, while the next year's LS1 V8 should see 58 psi. Therefore, it is important to consult a repair manual or the manufacturer's specifications to determine the correct fuel pressure for your specific vehicle.

Zero fuel pressure means the engine will not run. A complete lack of pressure indicates that the pump is dead or not getting power. First, check the fuel pump fuse. Then, verify power to the pump with a multimeter. If power is present, the fuel pump likely needs to be replaced.

Low fuel pressure can sometimes allow the engine to operate, but with issues. Signs of low fuel pressure may include a slow start-up, low performance, misfires, and stalling. Low fuel pressure can be caused by a clogged fuel filter or a failing fuel pump. If it is a serviceable type filter, try changing the fuel filter. It could also be caused by improper tank venting or an emissions issue caused by a loose gas cap. Check that the cap gasket is not damaged and tighten it until it clicks.

High fuel pressure will cause the engine to run rich. This can lead to excessive fuel consumption, black smoke from unburned gas, an overheating catalytic converter, and rough idle. High fuel pressure can be caused by a clogged or kinked fuel return line, a bad fuel pump driver module, or a faulty fuel pressure regulator. These issues may trigger a "check engine" light and store a code.

Fluctuating fuel pressure can also indicate a problem with the fuel system. If the reading is fluctuating by 20-30% or more, there may be an issue. However, it is important to note that electric fuel pumps and regulators rarely lose pressure and usually fail altogether.

Frequently asked questions