Introduction: Why Upgrade to a FlexFuel Kit?

The 2019 Mustang GT is already a formidable machine, but its true potential is unlocked when you give it access to higher-octane fuels like E85. The Lund Racing FlexFuel Kit is a purpose-built solution that allows the engine's ECU to automatically adjust timing and fuel delivery based on the ethanol content in the tank. This means you can run anything from pump 93 to E85 without swapping tunes or hardware—just fill up and go. E85 offers a cooling effect in the combustion chamber, higher effective octane, and reduced knock sensitivity, often translating to 25–40 wheel horsepower gains on the 5.0L Coyote engine, especially when combined with a proper Lund Performance tune.

This walkthrough covers a complete mechanical and electrical installation of the Lund Racing FlexFuel Kit on a 2019 Mustang GT. We'll go beyond the basic steps to include best practices, torque specifications, and troubleshooting insights gathered from the Mustang community. Whether you're a seasoned DIYer or a first-time modifier, following this guide will help you get the job done safely and correctly.

Tools and Components Required

Before popping the hood, assemble everything you need. Having the right tools prevents frustration and ensures the installation proceeds smoothly.

Kit Contents

  • Lund Racing FlexFuel Sensor (Continental / VDO style, calibrated for ethanol content)
  • Pre-assembled fuel line adapter harness (connects sensor to factory fuel line)
  • Wiring harness with pigtail connectors for sensor and OBD-II pass-through
  • Mounting bracket and hardware (stainless steel zip ties, screws, etc.)
  • Lund Racing NGauge or device-specific calibration file (purchased separately)

Tools & Consumables

  • 3/8” and 1/2” drive socket set with extensions (metric and SAE)
  • Combination wrenches: 10mm, 13mm, 15mm, 18mm
  • Flathead and Phillips screwdrivers
  • Wire strippers, crimpers, and heat shrink tubing
  • Electrical tape (high-temp rated) and zip ties (black UV-resistant)
  • Safety glasses and mechanic gloves (fuel system work demands protection)
  • Shop rags and a fire extinguisher rated for fuel fires
  • OBD-II scan tool (optional but recommended for post-install checks)
Important: Lund Racing requires you to have a custom tune loaded via their NGauge or SCT device that supports FlexFuel. The kit itself only provides the hardware—you must purchase the corresponding calibration from Lund. Do not start the engine without the correct tune installed.

Preparation and Safety

Safety is paramount when working with the fuel system. Gasoline is volatile, and even residual pressure can spray fuel when disconnecting lines.

Vehicle Prep

  • Park the Mustang GT on a level surface in a well-ventilated area. Avoid garages with pilot lights or open flames.
  • Disconnect the battery negative terminal (10mm nut) and wait 5 minutes for the PCM capacitors to discharge.
  • Relieve fuel system pressure: Locate the fuel pump fuse (F22 in the BCM fuse box) and run the engine until it stalls, then crank for 2 seconds more. Remove the fuse and verify no pressure by pressing the Schrader valve on the fuel rail (wrap a rag around it).
  • Ensure the engine is cool to the touch to avoid burns and to prevent fuel from vaporizing excessively.

Workspace Setup

  • Lay down cardboard or a drop cloth to catch any drips.
  • Have a container for old fuel lines or fittings.
  • Keep a phone or camera handy to document steps—taking pictures before disassembly helps with reassembly.

Removing Stock Components

The 2019 Mustang GT uses a returnless fuel system. The factory fuel line runs from the tank along the driver side frame rail to the fuel rail. Lund’s kit intercepts this line near the firewall.

Step 1: Access the Factory Fuel Line

  • Remove the air intake tube and air box (two 8mm bolts and a hose clamp) to make room on the passenger side; the sensor will mount near the firewall on the driver side, but the wiring route often crosses the engine bay.
  • Locate the factory fuel line quick-disconnect fitting on the driver side, just aft of the engine, near the brake booster.
  • Use a fuel line disconnect tool (3/8” and 5/16” sizes) to separate the line from the chassis-to-engine jumper. Be prepared for residual fuel—have rags ready.

Step 2: Remove the Factory FlexFuel Sensor (if equipped)

  • Some 2019 Mustangs built after March 2019 may already have a factory flex fuel sensor mounted on the subframe. If present, unplug and remove it (two 10mm bolts). The Lund kit replaces this with a more accurate sensor that communicates directly with the ECU.
  • If no factory sensor is present, skip this step.

Step 3: Locate the Wiring Pass-Through

  • Prepare the grommet on the driver side firewall (near the brake master cylinder). You'll need to run the sensor’s harness through here to reach the OBD-II port and the power source.
  • If necessary, drill a small pilot hole and carefully enlarge it with a step bit. Seal any new hole with RTV silicone to prevent water intrusion.

Installing the Lund Racing FlexFuel Sensor

Now we place the sensor and route the fuel lines. The sensor must be positioned vertically or slightly tilted so that any air bubbles can rise through it—mounting it horizontally can cause erratic readings.

Step 1: Mount the Sensor Bracket

  • Lund provides a bracket that attaches to the driver side strut tower bolts or to the inner fender. Choose a location where the sensor will be at least 6 inches away from exhaust heat sources (header, manifold, catalytic converter).
  • Secure the bracket using existing nuts or self-tapping screws with the provided hardware. Do not mount to plastic panels—they are not stable at high temperature.

Step 2: Connect Fuel Lines

  • Install the included fuel line adapter between the factory hard line and the flex fuel sensor. Use new O-rings supplied in the kit (lubricate with a small amount of clean engine oil).
  • From the sensor, run the provided hose to the engine fuel rail inlet. Use the supplied clamps—do not overtighten.
  • Ensure no kinks or sharp bends are present. Route the line away from moving parts (belt, alternator, steering shaft).

Step 3: Torque and Verify

  • Tighten all banjo bolts or threaded fittings to manufacturer specs: typically 18–22 lb-ft for the sensor to bracket, 8–10 lb-ft for small AN fittings. Do not guess—use a torque wrench.
  • Double-check that the flow direction arrow on the sensor points toward the engine (away from the tank).
Tip from the forums: Many owners choose to wrap the sensor in a reflective heat shield (e.g., DEI Cool Tape) if it sits near the exhaust. The sensor can overheat and give false ethanol content readings if subjected to prolonged radiant heat.

Wiring the FlexFuel Kit

Accurate wiring is critical. The Lund kit includes a harness that connects to the OBD-II port for power and communication. Improper connections can cause error codes or even damage the PCM.

Step 1: Run the Sensor Harness

  • Connect the 4-pin pigtail to the sensor. The wires are: Red (+12V ignition-switched), Black (ground), Green (CAN High), White (CAN Low).
  • Route the harness through your prepared firewall grommet. Use a coat hanger or wire puller to guide it.
  • Inside the cabin, route the harness to the OBD-II port location (driver side kick panel). Secure the harness along existing wire looms with zip ties—do not leave it loose.

Step 2: Tap into Power and Ground

  • For +12V switched power, the cleanest method is to use an Add-A-Circuit fuse tap in the interior fuse box (Fuse #24 is a common choice: “Not Used (Spare)” with ignition-switched power). Insert a 5-amp fuse for the sensor.
  • Ground the black wire to a chassis bolt near the OBD-II port (sand down any paint).
  • Alternatively, you can connect to the OBD-II harness itself using Posi-Taps, but the fuse tap is more reliable and reversible.

Step 3: CAN Bus Connection

  • The Green and White wires (CAN+ and CAN-) must be spliced into the OBD-II port’s pins 6 and 14 respectively. Use solder or crimp connectors with heat shrink—twist and tape is not acceptable for CAN bus.
  • Test continuity with a multimeter before sealing everything up.
  • Wrap all connections with high-quality electrical tape or use adhesive-lined heat shrink to prevent moisture ingress.

Step 4: Reconnect Battery

  • Only after all wiring is fully secured and no bare wires are exposed, reconnect the battery negative terminal.
  • Do not start the engine yet—proceed to testing.

Configuration, Testing, and Tune

Hardware installation is only half the job. The vehicle must be calibrated to recognize the FlexFuel sensor and adapt fueling accordingly.

Step 1: Load the Lund Racing FlexFuel Tune

  • Connect your Lund NGauge or SCT device to the OBD-II port.
  • If you haven’t already, purchase and download the FlexFuel calibration from Lund Racing’s website (provide your ECU strategy and hardware version).
  • Follow the device instructions to flash the tune. This typically takes 5–8 minutes. Do not interrupt the flash—ensure the battery is fully charged or use a battery maintainer.

Step 2: Initial Engine Start

  • After the flash, turn the key to the ON position without starting; wait for the fuel pump to prime (listen for the hum).
  • Check for fuel leaks visually—use a flashlight around all connections. If you see any wetness, tighten fittings.
  • Start the engine. It may idle roughly for a few seconds as the ECU learns the ethanol content. Let it idle for 2–3 minutes.
  • Monitor ethanol content on your NGauge (it should be displayed as a percentage). With pump E85, expect values between 60–85% depending on location and season.

Step 3: Leak Check Under Pressure

  • Drive the car gently for a few miles, then park and re-check all fuel connections. The fuel system operates at 50–60 psi; any drip is dangerous.
  • If no leaks are found, proceed to test drive with moderate throttle. Listen for any abnormal fueling noises (knocking, pinging).
Note: If the ethanol reading stays at 0% or shows “ERR” on the display, recheck the CAN wiring connections (swap CAN high/low?) and ensure the sensor is powered. Also, confirm that your NGauge firmware is up to date from Lund.

Troubleshooting Common Issues

Even with careful installation, issues can arise. Here are the most common problems and their fixes based on community experience.

Sensor Not Recognized (No Ethanol Data)

  • Check the yellow LED on the sensor (with key on) – it should blink slowly. If not, verify +12V and ground.
  • Confirm CAN wiring: pin 6 (CAN+) to green wire, pin 14 (CAN-) to white wire. Swapping these will stop communication.
  • Update the tune file from Lund – some early 2019 models require a specific calibration revision.

Engine Misfire or Rough Idle on E85

  • E85 can cause misfires if the fuel system is not fully adapted. Drive 30–40 miles for the ECU to learn.
  • Inspect spark plugs – E85’s cooling effect may require a colder plug if you are running forced induction or high compression. Stock plugs on a naturally aspirated car are fine.
  • Ensure the fuel filter is not clogged; E85 acts as a solvent and can loosen debris in the tank.

Fuel Leaks at Fittings

  • Lubricate O-rings with clean fuel or Vaseline before assembly. Dry O-rings can tear.
  • Use two wrenches to tighten AN fittings – one on the fitting, one on the adapter to prevent twisting the hard line.
  • Replace any PTFE tape or thread sealant with manufacturer-recommended paste (most AN fittings are flare-type and do not use tape).

Performance Gains and Real‑World Tuning

Once the kit is installed and the tune is dialed in, you can expect substantial gains. On a 2019 Mustang GT with the Performance Pack, a switch from 93 octane to E85 typically yields an additional 30–35 wheel horsepower and 25 lb-ft of torque on a Lund Racing tune, without any other modifications. The ethanol content also reduces exhaust gas temperatures and knock sensor activity, allowing the ECU to advance timing aggressively.

For those who want to push further, Lund Racing offers custom tuning for vehicles with headers, cold air intakes, and larger throttle bodies. The FlexFuel kit seamlessly integrates with these mods because it reports real-time ethanol content, enabling the tuner to create a single calibration that works across all fuel blends—from 93 octane to E85. This eliminates the need for separate race gas tunes and reduces the risk of detonation when filling up at an unknown station.

External references for further study:

Maintenance and Long-Term Considerations

FlexFuel kits require minimal maintenance, but E85 is more corrosive than gasoline. Use E85-compatible motor oil (high TBN) and change it more frequently (every 3,000–4,000 miles) if you run E85 consistently. Also, flush the system with a tank of 93 octane every third fill-up to prevent moisture accumulation and varnish buildup in the sensor.

The sensor itself is a wear item. Most owners report 60,000–80,000 miles before ethanol readings become erratic. Replacement is straightforward—simply unplug and swap the sensor without removing the fuel lines. Keep a spare in your track toolbox.

Finally, be aware that not all gas stations label E85 accurately. The Lund kit’s real-time ethanol readout is invaluable—if the number drops below 50%, the ECU will automatically pull timing and add fuel to protect the engine. Always monitor the gauge during hard pulls if you’re near E10 fuel.

Conclusion

Installing the Lund Racing FlexFuel Kit on your 2019 Mustang GT is one of the highest-return upgrades you can perform. The process is well within the reach of an intermediate mechanic who can follow detailed instructions and exercise caution with the fuel system. By carefully mounting the sensor, connecting the wiring to the CAN bus and a switched power source, and flashing the accompanying Lund tune, you gain the ability to run any ethanol blend from pump gas to E85. The result is a noticeable increase in horsepower, improved engine cooling, and a more responsive throttle that transforms the driving character of the Coyote 5.0L.

This walkthrough covered the entire process from prep to post-install testing, including common pitfalls and community-tested solutions. With the right tools, patience, and the links provided, you can complete the installation in an afternoon and start enjoying the benefits on your next drive. Always remember: if you encounter anything beyond your skill, consult a professional tuner or mechanic—there’s no shame in asking for help when dealing with high-pressure fuel systems.