1985 Pontiac Fiero Fuel Pump: The Complete Guide to Replacement, Diagnosis, and Upgrades
Understanding Your 1985 Pontiac Fiero Fuel Pump
The fuel pump in your 1985 Fiero is an electric, in-tank unit. When you turn the key to the "ON" position, the Engine Control Module (ECM) activates the fuel pump relay to build up pressure quickly. If the engine is not started, the ECM waits for cranking signals. There is also an oil pressure switch that acts as a backup: if the fuel pump relay fails, the oil pressure switch will keep the pump running once oil pressure exceeds about 7 psi.
The OEM part number for the 1985 Pontiac Fiero fuel pump is 25168719, which replaces several older numbers including 25166252, 6472232, 25166717, and 6472011. This pump fits all 1985-1988 Fiero models, both 2.5L and 2.8L.
Key specifications:
- Location: Inside the fuel tank, mounted to the sending unit assembly
- Type: Electric, non-repairable
- Voltage: 12V DC
- Flow rate (stock): Approximately 30-40 liters per hour (LPH) for the factory pump
- Function: Delivers fuel from the tank to the throttle body (2.5L) or fuel injection system (2.8L)
Top 5 Signs Your 1985 Fiero Fuel Pump Is Failing
-
Engine cranks but won't start. The most common symptom. If the pump is dead, the engine will crank but never fire because no fuel reaches the injectors or throttle body. You can test this by listening for a 2-3 second hum from the rear when you first turn the key to "ON".
-
Whining or loud humming noise from the fuel tank. A healthy fuel pump makes a faint, barely audible sound. If you hear a loud, high-pitched whine, especially when the tank is full, the pump is wearing out and will likely fail soon.
-
Car starts then dies after a few seconds. This happens when the pump can build initial pressure but cannot maintain it. The engine fires up using the fuel in the lines, then stalls as pressure drops.
-
Sputtering, hesitation, or loss of power under load. A weak pump cannot deliver enough fuel during acceleration or uphill driving. This can feel like a fuel starvation issue, especially when the gas tank is below half full.
-
Long cranking time before the engine starts. If the pump's check valve is failing, fuel pressure bleeds back to the tank overnight. This means you have to crank the engine longer to rebuild pressure the next morning.
How to Test Your 1985 Fiero Fuel Pump Before Replacing It
Before you drop the fuel tank, you should verify the pump is actually bad. Here is a simple diagnostic procedure:
Step 1: Listen for the pump. Turn the key to the "ON" position (do not start the engine). Listen near the rear of the car. You should hear a brief humming sound for about 2-3 seconds as the pump primes the system. If you hear nothing, the pump is not getting power or is dead.
Step 2: Check the fuel pump relay. The relay is located in the engine compartment fuse box. Swap it with a similar relay (like the horn relay) to see if the pump starts working. If it does, the relay is bad.
Step 3: Check the oil pressure switch bypass. If the relay is good but the pump still does not run, try cranking the engine for 10-15 seconds. If the oil pressure switch is working, it will power the pump once oil pressure builds. If the engine starts and runs only while you crank, the relay circuit is likely the problem.
Step 4: Use a fuel pressure gauge. Attach a fuel pressure gauge to the fuel rail valve (on the 2.8L V6) or the test port near the throttle body (on the 2.5L). With the key on, you should see 40-60 psi for the 2.8L and about 9-13 psi for the 2.5L throttle body injection system. If you see zero pressure, check the fuse and wiring first.
Step-by-Step: How to Replace the 1985 Pontiac Fiero Fuel Pump
Replacing the fuel pump requires dropping the fuel tank. This is a moderately difficult job that takes about 3-5 hours. Safety first: Work in a well-ventilated area, disconnect the battery, and keep a fire extinguisher nearby.
Tools you will need:
- Floor jack and two jack stands
- 13mm deep socket and ratchet
- 7mm socket for the heat shield screws
- Brass punch and hammer (never use steel tools near fuel)
- Flathead screwdriver
- Fuel pressure gauge (for bleeding pressure)
- Fuel-safe container for catching gas
- Safety glasses and gloves
Step 1: Relieve fuel system pressure. For the 2.5L four-cylinder, pull the fuel pump fuse from the fuse block, then start the engine and let it run until it stalls. For the 2.8L V6, locate the fuel rail valve, attach a fuel pressure gauge, wrap a rag around it, and bleed the pressure into a fuel-safe container.
Step 2: Disconnect the battery. Remove the negative battery cable. This prevents any sparks while you work near fuel.
Step 3: Raise the car and support it. Jack up the rear of the car and place it securely on jack stands. Lock the front wheels with chocks and engage the parking brake.
Step 4: Remove the heat shield. The heat shield is located between the engine and the fuel lines. It is held in place by four screws with 7mm heads. Remove them and set the shield aside.
Step 5: Disconnect the fuel pump wiring harness. The fuel pump wiring connector is located almost dead center along the firewall, above the heater hoses. Disconnect both plugs.
Step 6: Loosen the hose clamps. Loosen the clamps on the fuel feed hose, return hose, and the two filler neck hoses. Do not remove them yet—just loosen them so you can disconnect them after the tank is lowered.
Step 7: Support the fuel tank. Place a floor jack under the fuel tank with a 2x4 board laid across the jack to distribute the weight. Raise the jack just enough to take the weight off the tank straps.
Step 8: Remove the tank straps and support bracket. Use a 13mm deep socket to remove the two nuts on the long threaded bolts that hold the tank straps. Then remove the four 13mm bolts that hold the center support bracket.
Step 9: Lower the fuel tank. Slowly lower the jack, keeping the front of the tank tipped down (the outlets are at the rear). You may need to wiggle the tank and pry around the A/C brackets to get it past the frame. Disconnect the hoses as the tank comes down.
Step 10: Remove the fuel pump assembly. Once the tank is on the ground, clean the top of the tank. The fuel pump/sending unit is held in place by a lockring. Turn the inner ring clockwise (using a brass punch and hammer) until the locking cams are free of the retaining tangs. Carefully lift the assembly out of the tank, tilting it sideways to get the float out.
Step 11: Replace the fuel pump. The pump is attached to the sending unit by a rubber connector. Push the pump upward into the rubber connector, then slide it out from the bottom support. Remove the old strainer from the bottom of the pump. Install the new pump in reverse, and push a new strainer onto the bottom.
Step 12: Reinstall everything. Place the new pump/sending unit assembly back into the tank, making sure the rubber gasket is in good condition. Replace the lockring by turning it counterclockwise until the cams engage. Reinstall the tank, reconnect the hoses and wiring, and reinstall the heat shield.
Step 13: Test the system. Before starting the engine, turn the key to "ON" several times (about 5-6 times, waiting 10 seconds between each) to prime the fuel system. Check for leaks at all connections. Then start the engine and check for leaks again.
Replacement Options: OEM vs. Performance Fuel Pumps
When choosing a replacement fuel pump for your 1985 Fiero, you have several options:
1. OEM GM Fuel Pump (Part #25168719)
This is the direct factory replacement, built to GM specifications. It comes with a manufacturer's warranty and is guaranteed to fit your Fiero perfectly. Best for: Stock engines and daily drivers who want a reliable, no-hassle replacement.
2. Standard Replacement Pumps (255 LPH)
Pumps like the Quantum 255LPH or Edelbrock 17933 offer higher flow rates—255 liters per hour—compared to the stock pump. This is ideal for engines with mild performance upgrades like a cam, intake manifold, or exhaust. These pumps are nearly silent during operation and include installation kits.
3. High-Performance Pumps (340 LPH)
For modified Fieros with turbochargers, superchargers, or nitrous systems, consider a 340LPH pump like the Quantum 340LPH or Aeromotive 340 Stealth pump. These pumps support up to 600+ horsepower and feature higher bypass valve pressures (about 125 psi) to prevent fuel starvation under boost. Many are E85 compatible, which is important if you plan to run ethanol fuel.
4. KEMSO High-Performance Fuel Pumps
For a balance of affordability, reliability, and performance, KEMSO offers a 340LPH high-performance fuel pump that fits the 1985-1988 Pontiac Fiero. KEMSO pumps are designed as OEM replacement units with a lifetime warranty. Each pump is individually flow-tested, and the company follows strict quality control standards (ISO/TSO). The KEMSO pump is a direct fit for the Fiero, saving you the hassle of modifying wiring or hoses. It is ideal for both stock rebuilds and engines with mild to moderate performance upgrades. Visit KEMSO Racing at https://www.kemsoracing.com/ to find the right fuel pump for your Fiero and take advantage of their lifetime warranty.
Frequently Asked Questions
Q: Is the fuel pump the same for the 2.5L and 2.8L Fiero?
A: Yes. The 1985-1988 Pontiac Fiero uses the same in-tank fuel pump for both the 2.5L four-cylinder and the 2.8L V6.
Q: Can I replace the fuel pump without dropping the tank?
A: No. The fuel pump is located inside the fuel tank, and there is no access panel. You must remove the tank to access the pump.
Q: How much does it cost to replace the fuel pump on a 1985 Fiero?
A: A basic replacement pump costs between $60 and $80. High-performance pumps range from $70 to $120. If you have a mechanic do the work, expect labor costs of $200-$400, depending on your area.
Q: How long does a Fiero fuel pump last?
A: The average lifespan is 100,000 to 150,000 miles. Running the tank low on fuel frequently shortens the pump's life because the fuel cools and lubricates the pump.
Q: What happens if I install a higher-flow pump on a stock engine?
A: It is safe, as long as your fuel pressure regulator can handle the additional flow. The pump will only deliver what the engine demands, so a 255LPH pump on a stock engine will not cause problems.
Maintenance Tips to Extend Your Fuel Pump's Life
- Never run the tank below 1/4 full. The fuel pump relies on gasoline for cooling and lubrication. Running the tank dry causes the pump to overheat and fail prematurely.
- Replace the fuel filter every 30,000 miles. A clogged filter forces the pump to work harder, reducing its lifespan.
- Use quality fuel. Cheap gas can contain dirt and water that damage the pump over time.
- Listen for changes. If you hear a new whining noise from the rear, address it immediately. A noisy pump is a dying pump.
Conclusion
The 1985 Pontiac Fiero fuel pump is a critical component that, when it fails, leaves you stranded. Recognizing the symptoms early—hard starting, whining noises, or stalling—can save you from being stuck on the side of the road. Replacing the pump requires dropping the fuel tank, but with the right tools and this guide, it is a job you can handle in a weekend. Whether you choose an OEM replacement for reliability or a high-performance upgrade like the 340LPH KEMSO pump for more power, the key is to use a quality part that fits correctly. For a fuel pump that combines OEM quality, high performance, and a lifetime warranty, visit https://www.kemsoracing.com/ to find the perfect pump for your Fiero.