1999 Mustang GT Fuel Pump Relay Location: A Complete Guide to Finding and Fixing Your Fuel System

The fuel pump relay on a 1999 Ford Mustang GT is located in the engine compartment, inside the relay box mounted on the driver's side fender near the battery. This small black box contains several relays, and the fuel pump relay is typically labeled "Fuel Pump" or "FP." However, because the 1999 Mustang GT uses a returnless fuel system, it also has a Fuel Pump Driver Module (FPDM) located underneath the luggage compartment trim panel in the trunk. Understanding both components is essential for diagnosing fuel delivery issues.

Understanding the Fuel System in Your 1999 Mustang GT

The 1999 Ford Mustang GT came with a returnless fuel system. This is different from older Mustangs. Instead of a traditional fuel pump relay that simply turns the pump on and off, the 1999 model uses a Fuel Pump Driver Module to control the pump's speed and power. This system is designed to be more efficient and reduce evaporative emissions.

The fuel pump itself is located inside the fuel tank. It is an electric pump that sits submerged in gasoline. This design keeps the pump cool and helps it last longer. The fuel pump relay is the component that sends power to the pump, while the FPDM modulates that power based on engine demand.

Step-by-Step Location of the Fuel Pump Relay

1. Open the Hood

Start by parking your Mustang on a level surface and engaging the parking brake. Open the hood and locate the battery. The relay box is usually mounted on the driver's side fender, very close to the battery.

2. Find the Relay Box

Look for a black plastic box with a removable cover. This is the power distribution box or relay box. On the 1999 Mustang GT, it is typically mounted on the driver's side fender, near the battery. The box is rectangular and has a lid that snaps or clips on.

3. Remove the Cover

Carefully remove the cover of the relay box. You may need to press on clips or use a flathead screwdriver to gently pry it open. Inside, you will see several relays and fuses arranged in a grid pattern.

4. Locate the Fuel Pump Relay

Look for a relay labeled "Fuel Pump" or "FP." The fuel pump relay is usually a small black or gray box with four terminals. It will be one of the larger relays in the box. On some models, it may be labeled with a diagram on the inside of the cover. If you are unsure, consult your owner's manual.

5. Remove and Inspect the Relay

Once you have located the correct relay, gently pull it straight up to remove it from the socket. Inspect the relay for any signs of damage, corrosion, or burnt terminals. The relay should be clean and the pins should be straight.

Where is the Fuel Pump Driver Module?

Because the 1999 Mustang GT uses a returnless fuel system, it has a Fuel Pump Driver Module in addition to the relay. The FPDM is located underneath the luggage compartment trim panel in the trunk. To access it:

  1. Open the trunk.
  2. Remove the carpet or trim panel on the driver's side of the trunk.
  3. You will see a small black module attached to the body of the car.
  4. The FPDM has a wiring harness connected to it.
  5. This module controls the power to the fuel pump and can fail, causing the pump not to run even if the relay is good.

How to Test the Fuel Pump Relay

Before you replace the fuel pump, you should test the relay. Here is a simple way to do it:

Tools you will need:

  • A multimeter or test light
  • A helper to turn the key

Steps:

  1. Locate the fuel pump relay in the engine compartment relay box.
  2. Remove the relay.
  3. Turn the ignition key to the "RUN" position (do not start the engine).
  4. Use a test light or multimeter to check for power at the relay socket terminals.
  5. You should see 12 volts on one of the terminals.
  6. Have a helper turn the key to the "START" position. You should hear the relay click and see power on the output terminal.
  7. If you do not get power, check the fuse first. The fuel pump fuse is a 20 amp mini fuse located in the power distribution box under the hood. It is usually labeled as position 14.

Common Symptoms of a Bad Fuel Pump Relay

If the fuel pump relay is failing, you may notice these symptoms:

1. Engine Cranks But Does Not Start: This is the most common symptom. You can hear the starter motor turning the engine, but it never fires up. This often means the fuel pump is not getting power.

2. No Fuel Pump Prime Noise: When you turn the key to the "ON" position, you should hear a brief whirring sound from the fuel tank. This is the fuel pump priming. If you hear nothing, the relay or the pump itself may be bad.

3. Intermittent Starting Issues: The car may start sometimes and not others. This can indicate a failing relay that works when cold but fails when hot.

4. Engine Stalls While Driving: If the relay fails while the car is running, the engine will lose fuel pressure and stall. This can be dangerous, especially on the highway.

How to Test the Fuel Pump Itself

If the relay tests good, the problem may be the fuel pump itself. The fuel pump is located inside the gas tank. You can test it without removing the tank.

Fuel pressure test:

  1. Relieve the fuel system pressure by removing the fuel pump relay and starting the engine. Let it run until it stalls.
  2. Connect a fuel pressure tester to the test port on the fuel rail. The test port is located on the passenger side of the fuel rail near the front of the engine.
  3. Reinstall the fuel pump relay.
  4. Have a helper cycle the ignition key on and off three times to prime the system.
  5. Start the engine and read the pressure on the gauge.
  6. The fuel pressure should be at least 45 psi. If it is below 45 psi, the fuel pump is likely failing and needs to be replaced.

Electrical test:

  1. Locate the fuel pump electrical connector under the car, in front of the fuel tank, near the fuel lines.
  2. Use a circuit tester to check for power at the connector.
  3. Have a helper turn the key to the "START" position.
  4. If you see power at the connector, the pump is receiving power and should be running. If the pump does not run, it is likely bad.
  5. If you do not see power, the problem is in the wiring, relay, or FPDM.

The Fuel Pump Shut-Off Switch

The 1999 Mustang GT also has a fuel pump shut-off switch, also known as an inertia switch. This switch is designed to stop the fuel pump in the event of a collision. If the engine cranks but does not start after a jolt or accident, this switch may have been tripped.

Location of the inertia switch:
The fuel pump shut-off switch is located on the left side of the trunk, behind the trunk liner, near the driver's side taillight. There is an access hole in the trunk liner. To reset it, simply push the button on the top of the switch until it clicks.

Replacing the Fuel Pump

If you have determined that the fuel pump itself is bad, you will need to replace it. The fuel pump is located inside the fuel tank. To access it, you have two options:

Option 1: Access panel under the rear seat
Some 1999 Mustangs have an access panel under the rear seat cushion. To check:

  1. Remove the rear seat cushion by pulling up on the front edge.
  2. Look for a round or rectangular metal plate in the floor.
  3. If you see one, remove the screws to access the fuel pump assembly.

Option 2: Drop the fuel tank
If there is no access panel, you will need to drop the fuel tank. This is a more involved job but is common on many Mustangs.

  1. Relieve fuel pressure and drain the tank.
  2. Disconnect the fuel lines and electrical connector.
  3. Support the tank with a jack and remove the straps.
  4. Lower the tank carefully.
  5. Remove the fuel pump module assembly from the top of the tank.

Important: Always replace the O-ring seal when installing a new fuel pump. This prevents fuel leaks.

Recommended Fuel Pump: KEMSO

When it comes time to replace your fuel pump, you want a product that is reliable, durable, and performs like the original. That is why we recommend KEMSO fuel pumps.

Why choose KEMSO?

  • High Performance: KEMSO fuel pumps are designed to deliver up to 340 LPH (liters per hour), supporting up to 600 BHP. This makes them suitable for both stock and modified engines.
  • OEM Replacement: These pumps are direct OEM replacements. They match the original equipment specifications for voltage, amperage, pressure, and flow rate. This ensures your engine receives the correct fuel volume at all times.
  • Lifetime Warranty: Every KEMSO fuel pump comes with a lifetime warranty. If your pump ever fails due to a manufacturing defect, KEMSO replaces it free of charge. No questions, no hassle.
  • Versatile Fuel Compatibility: KEMSO pumps are designed for both gasoline and E85 applications. This gives you flexibility if you decide to run alternative fuels.
  • USA-Based Support: Backed by expert USA technicians, KEMSO provides peace of mind and reliable customer service.

How to install the KEMSO pump:
The KEMSO pump is designed to fit directly into your 1999 Mustang GT's fuel tank. It comes with the necessary hardware and instructions for installation. The wiring connector matches the original, so you do not need to cut or splice wires. Simply remove the old pump assembly, install the new KEMSO pump, and reconnect the wiring.

Where to buy:
To get your KEMSO fuel pump, visit the official website: https://www.kemsoracing.com/. Their team can help you confirm compatibility and answer any questions about installation.

Final Thoughts

Finding the fuel pump relay on your 1999 Ford Mustang GT is straightforward once you know where to look. The relay is in the engine compartment relay box on the driver's side fender. But remember, the 1999 model also uses a Fuel Pump Driver Module in the trunk. If you are having fuel delivery issues, always check the relay first, then the fuse, then the inertia switch, and finally the pump itself.

Fuel system problems can be frustrating, but with the right knowledge and tools, you can diagnose and fix them yourself. Always work safely: disconnect the battery, relieve fuel pressure, and avoid open flames. If you need to replace your fuel pump, choose a reliable brand like KEMSO. Their products are built to last and come with a lifetime warranty.

For more information and to purchase a high-quality fuel pump for your 1999 Mustang GT, visit https://www.kemsoracing.com/.


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