2006-2014 Mustang Fuel Pump: Why It Stalls With Half a Tank of Gas

The Saddle Tank Problem: Stalling with a Full Tank

The most common and confusing issue with the 2006-2014 Mustang fuel system is when the engine stalls as if it's out of gas, but the fuel gauge reads between 1/4 and 1/2 full. This happens because the Mustang has two fuel level sensors—one on each side of its saddle-shaped tank. The fuel gauge displays an average of these two readings.

The main fuel pump is on the driver's side. The passenger side contains a transfer pump assembly (this part) that siphons fuel from the right side of the tank and sends it to the driver's side. When this passenger-side transfer pump fails, it can no longer move fuel across the tank's central hump. The main pump on the driver's side runs its section of the tank dry, starving the engine of fuel and causing a stall. Meanwhile, the passenger side remains full of unusable fuel, tricking the gauge into showing a higher-than-actual fuel level.

Symptoms of a Failing Right-Side Fuel Pump

Diagnosing this part is straightforward if you know what to look for. The symptoms are very specific to its function.

• Engine stalls with 1/4 to 1/2 tank on the gauge: This is the most definitive symptom. The car will behave exactly as if it has run out of gas, but the gauge will indicate otherwise.
• Fuel gauge drops suddenly to empty: The vehicle's computer may detect a large difference between the two fuel level sensors and, as a safety measure, drop the gauge to 'E' and trigger the low fuel light to warn the driver of a potential issue.
• Car only accepts 7-10 gallons when refueling: After stalling, if you go to refuel, you'll find the tank only takes about half of its 15-16 gallon capacity, confirming that one side of the tank was still full.
• Whining noise from the fuel tank: Like any electric pump, a failing unit may produce a noticeable whining or humming sound from the rear seat area.
• Hesitation or loss of power at low fuel levels: Before it fails completely, the pump may struggle to transfer fuel efficiently, causing the engine to sputter or lose power when the gauge approaches the 1/4 mark, especially during cornering.

Known Issues, TSBs, and Recalls

While there are no specific NHTSA recalls for this fuel pump issue on 2006-2014 Mustangs, Ford did issue a Technical Service Bulletin (TSB) for a nearly identical problem.

TSB 10-21-3: Fuel Gauge Drops to Empty

This TSB was released for 2011 Mustangs exhibiting the classic symptoms: the fuel gauge dropping from 1/2 tank to empty and the car only taking a partial fill-up. Ford identified the cause as a potential internal restriction in the main Fuel Pump Module (FPM) on the driver's side, which prevented it from properly siphoning fuel from the passenger side. The TSB recommends replacing the FPM. Although this TSB targets the main pump, the symptoms are identical to a failure of the passenger-side transfer pump, and the issue is widely reported by owners of all model years in this generation.

Buying a Used 2006-2014 Mustang Fuel Pump: What to Check

A used OEM fuel pump assembly can be a reliable and cost-effective alternative to a new part, especially when compared to cheap aftermarket units which may have lower quality standards. Here’s what to inspect:

• Verify the Part Number: Check the part number on the unit against the list of known OEM numbers to ensure compatibility.
• Inspect the Plastic Housing: Look for any cracks, deep scratches, or signs of heat damage on the white plastic assembly. The housing should be intact and free from warping.
• Check the Electrical Connector: The pins in the electrical connector should be straight and free of corrosion. Any green or white crust indicates moisture intrusion, and the part should be rejected.
• Examine the Fuel Level Sender: The float arm should move freely without binding. The float itself should be intact and not saturated with fuel.
• Look at the Strainer: While the small filter sock at the bottom is often discolored, it should not be clogged with heavy debris or sludge.
• Ask About the Donor Vehicle: If possible, find out the mileage of the vehicle the part came from. A pump from a lower-mileage vehicle is always preferable.

Cost Comparison: New vs. Used

Prices for a replacement passenger-side fuel pump assembly can vary significantly. Here is an estimated breakdown:

Replacement Overview

Replacing the passenger-side fuel pump is a manageable DIY job for those with basic mechanical skills. Unlike older Mustangs that required dropping the fuel tank, this generation provides access through a cover under the rear seat cushion.

1. Relieve Fuel Pressure: Remove the fuel pump fuse from the fuse box and run the engine until it stalls.
2. Disconnect the Battery: Disconnect the negative battery terminal as a safety precaution.
3. Remove the Rear Seat: The rear seat bottom cushion typically pops out by pulling up firmly on the front edge.
4. Access the Pump: Under the seat, you will find a metal access cover on the passenger side. Remove the screws to expose the top of the fuel pump assembly.
5. Disconnect and Remove: Carefully disconnect the electrical connector and fuel lines. Use a specialized tool or a brass punch and hammer to carefully rotate and remove the lock ring holding the pump in place.
6. Install the New Pump: Gently lift the old unit out and install the new one, ensuring the new tank seal is seated correctly. Reinstall the lock ring, connect the lines and electrical connector, and reassemble.

OEM Part Numbers

This part has been updated several times by Ford. The following part numbers are known to be associated with the right-side fuel pump assembly for this generation. Always verify fitment with the vehicle's VIN.

• 6R3Z-9H307-AA (superseded by 7R3Z-9H307-C)
• 7R3Z-9H307-A
• 7R3Z-9H307-C
• BR3Z-9H307-B (referenced in TSB 10-21-3)
• CR3Z-9H307-A
• CR3Z-9H307-B
• PFS952 (Motorcraft)

Frequently Asked Questions (FAQ)