The Complete Guide to Your 1985 Nissan 720 Fuel Pump: Symptoms, Replacement, and Troubleshooting

Conclusion Up Front: Dealing with fuel pump problems in your vintage 1985 Nissan 720 pickup? You're likely facing classic symptoms like hard starting, engine sputtering, or loss of power. Replacing a faulty 1985 Nissan 720 fuel pump is a manageable repair for most DIY enthusiasts. This guide provides everything you need to know about diagnosing, sourcing, and replacing the fuel pump on your 4x4 or 2WD model, restoring reliable performance to your classic Nissan truck.

The fuel pump is the heart of your 1985 Nissan 720's fuel delivery system. Mounted inside the fuel tank on most models (though very early 720s might have an inline pump), its sole job is to draw fuel from the tank and deliver it at the correct pressure to the carburetor. Over nearly four decades, these pumps inevitably wear out or succumb to issues like clogged filters, corrosion, or electrical problems. Recognizing the signs of failure, understanding how to diagnose it accurately, and knowing how to perform a safe replacement are crucial skills for keeping your classic Nissan running strong.

Recognizing Symptoms of a Failing 1985 Nissan 720 Fuel Pump

A failing fuel pump rarely stops working instantly. It usually gives ample warning signs as its internal components wear down or become restricted. Pay close attention to these common symptoms associated with a failing fuel pump in your 1985 Nissan 720:

  1. Engine Sputtering at High Speed or Under Load: This is one of the most common indicators. As the pump struggles to maintain adequate pressure and volume, the engine will lose power, stumble, or sputter significantly when you demand more fuel, such as climbing hills, accelerating hard, or cruising at sustained highway speeds. The engine may seem fine at idle or low speeds but falters when the fuel demand increases.
  2. Loss of Power: General sluggishness and a noticeable lack of the pickup truck's usual responsiveness, even under moderate acceleration, point towards inadequate fuel delivery. The engine feels strained and cannot produce its expected power output.
  3. Difficulty Starting or Extended Cranking: While hard starting can have many causes, a weak fuel pump makes starting difficult because insufficient fuel pressure builds up in the carburetor bowl before cranking. You might have to crank the engine excessively before it fires. This can also drain the battery significantly.
  4. Engine Stalling: Intermittent stalling, especially after the engine has warmed up or when coming to a stop, is a classic sign. As the pump weakens or overheats, it can intermittently fail to deliver enough fuel, causing the engine to die suddenly. It might restart after sitting for a few minutes, only to stall again later.
  5. Sudden Engine Shutdown: A more severe symptom is the engine stopping abruptly while driving. This typically happens when the pump fails completely. The engine will not restart until the pump is replaced.
  6. Whining or Humming Noise from the Fuel Tank Area: While some pump noise is normal (a low hum), a significantly louder than usual, high-pitched whining, grinding, or shrieking noise coming from the vicinity of the fuel tank strongly suggests the pump motor or internal bearings are failing. This noise often increases in intensity before a final failure.
  7. Poor Fuel Economy: A struggling pump might cause the engine to run lean under load, but paradoxically, inefficiencies or constant cranking attempts to start can sometimes lead to decreased miles per gallon. It's a less common but possible symptom related to overall compromised engine operation.

Essential Diagnostics Before Replacing Your 720's Fuel Pump

Never immediately assume the fuel pump is dead just because of the symptoms listed above. Other components in the fuel and ignition systems can cause similar problems. Always perform some basic diagnostic checks before replacing the pump:

  1. Verify Fuel Level: Sounds obvious, but always double-check you actually have fuel in the tank! A malfunctioning gauge is common on older trucks.
  2. Inspect Fuel Filter(s): A severely clogged fuel filter is a frequent cause of starvation symptoms mimicking a bad pump. The 1985 Nissan 720 has at least one primary fuel filter. Locate it (usually inline between the tank and pump, or between the pump and carburetor) and check for heavy sediment or blockage. Replace it regardless as cheap preventative maintenance.
  3. Check for Clogged Fuel Tank Vent: A blocked tank vent (often integrated into the gas cap) creates a vacuum lock, preventing fuel from flowing freely to the pump. Try loosening the gas cap. If symptoms immediately improve or you hear a strong sucking sound, the venting system is faulty.
  4. Listen for Pump Operation:
    • Turn the ignition key to the "ON" position (do not start the engine). Listen carefully near the fuel tank for a distinct whirring or humming sound lasting 1-2 seconds. This confirms the pump is getting power and its motor is activating briefly during prime. Complete silence suggests an electrical problem (fuse, relay, wiring) or a completely dead pump.
    • Have an assistant crank the engine while you listen near the tank. You should hear the pump run continuously.
  5. Inspect Electrical Connections:
    • Fuse: Locate the fuel pump fuse in the fusebox (refer to the owner's manual or a repair manual diagram). Visually inspect it. If blown, replace it with the correct amperage fuse. A blown fuse indicates an electrical short somewhere, which needs fixing.
    • Relay: The fuel pump relay controls power to the pump. Swapping it with a known good relay (like the horn relay – check schematics to ensure compatibility) is a quick way to test it. If the pump works with the swapped relay, replace the fuel pump relay.
    • Wiring: Visually inspect the wiring harness near the fuel tank pump access and along the frame rail for any obvious damage, chafing, corrosion, or disconnected plugs. Check both power and ground wires.
  6. Check Fuel Pressure (Most Critical Test): This is the definitive test. You must confirm low fuel pressure before replacing the pump.
    • Acquire a low-pressure fuel pressure gauge (carburetor-specific, range usually 0-15 PSI).
    • Locate a test port (if equipped) on the carburetor inlet line or disconnect the fuel line going to the carburetor carefully. Have plenty of rags ready, wear safety glasses, and work in a well-ventilated area away from sparks/flames.
    • Connect the gauge inline between the fuel pump outlet and the carburetor inlet.
    • Turn the ignition ON to prime the pump and note the pressure. It should jump to spec and hold.
    • Start the engine and let it idle. Observe the pressure reading.
    • The typical fuel pressure specification for a carbureted 1985 Nissan 720 is between 3 PSI and 5 PSI. Always verify the exact spec in a factory service manual if possible, but 3-5 PSI is the standard range for carbureted vehicles like the 720.
    • Results:
      • No Pressure: Pump not running, severe blockage, or failed pump diaphragm/valve.
      • Very Low Pressure (Below 2-3 PSI): Weak pump, clogged inlet strainer, or blockage downstream.
      • Pressure Fluctuates Erratically: Likely failing pump internals or severe vapor lock.
      • Pressure Holds Steady in Spec: Fuel pump pressure is likely not the primary problem causing your symptoms (look elsewhere, like ignition or carb issues).

Sourcing the Right Replacement Fuel Pump for Your '85 720

Finding the correct replacement part is critical. The 1985 Nissan 720 uses an in-tank electric fuel pump.

  1. Identify Your Engine: The most common engines are the Z24 (2.4L SOHC inline-4) and the SD25 (2.5L Diesel inline-4). These use completely different fuel pumps. This guide primarily addresses the gasoline (Z24) engine pump. Diesel pump replacement is significantly different and often involves additional priming procedures.
  2. Part Type: Confirm you need the in-tank fuel pump assembly. This typically includes the pump motor itself, an integrated strainer (sock filter), and the hanger assembly that suspends it in the tank, incorporating the fuel level sender unit and electrical connections. Buying a complete assembly is strongly recommended over trying to source just the bare pump motor.
  3. Brands & Quality:
    • OEM (Original Equipment Manufacturer): Genuine Nissan (often referred to as NIS) parts are the ideal choice for fit, performance, and longevity, but may be harder to find or significantly more expensive for a 39-year-old truck. Check Nissan dealership parts departments specializing in classic parts or authorized classic Nissan parts suppliers online.
    • Aftermarket Quality Brands: Several reputable manufacturers produce quality fuel pumps for the 1985 720 Z24. Look for brands like Airtex, Delphi, Carter, Bosch (verify specific fit), Denso (verify specific fit), and Spectra Premium. Generally, avoid obscure ultra-budget brands sold online without reviews.
    • Read Reviews: Search forums dedicated to classic Datsuns/Nissans and read reviews specifically mentioning the 720 fuel pump from the brand you're considering. Real-world experiences are invaluable.
  4. Includes Strainer/Sock Filter: Ensure the replacement assembly comes with a new inlet strainer (the sock filter). This fine mesh filter attached to the pump inlet prevents large contaminants from entering the pump. This should always be replaced when installing a new pump. If your assembly doesn't include one, purchase it separately.
  5. Includes Lock Ring: Many assemblies include the large locking ring and seal needed to secure the pump assembly to the tank. If yours doesn't, you MUST purchase the ring and seal separately – reusing the old ring is risky as it can be brittle and difficult to seal properly.
  6. Fuel Level Sender: Verify if the assembly includes the fuel level sending unit integrated into the hanger. Some assemblies might come without, requiring you to transfer your old sender, which can be delicate. It's often safer to get one with the sender included unless yours is known to be very accurate.
  7. Purchasing Sources: Reputable auto parts stores (NAPA, O'Reilly, AutoZone, etc. – check inventory online first), specialized online retailers (RockAuto, CarParts.com), and dedicated Datsun/Nissan classic parts suppliers (e.g., NissanPartsCC, DatsunParts.com, RockAuto - best selection often) are all viable options. Compare prices, shipping, and reviews.

Step-by-Step Guide: Replacing the Fuel Pump on a 1985 Nissan 720 (Z24 Gasoline)

Important Safety Precautions:

  • Work in a well-ventilated area. Gasoline fumes are highly explosive.
  • Disconnect the NEGATIVE battery terminal before starting.
  • Have a Class B fire extinguisher readily available.
  • Absolutely NO sparks, flames, or smoking anywhere near the work area. Remove power sources like phones from your pocket.
  • Wear safety glasses and gloves.
  • Wrap wrenches with a rag when undoing fuel lines to prevent sparks if they slip.

Tools & Supplies Needed:

  • Correct Replacement In-Tank Fuel Pump Assembly
  • Basic Hand Tools: Sockets & Ratchet (Metric sizes, usually 8mm, 10mm, 12mm, 14mm), Wrenches, Screwdrivers (Flathead & Phillips)
  • Needle Nose Pliers / Hose Clamp Pliers
  • Large Channel Lock Pliers or Strap Wrench (for lock ring)
  • Brass Punch or Drift Punch & Hammer (optional, for stubborn ring)
  • Flat Head Screwdriver (small, for lock ring tabs - be careful!)
  • Jack & Jack Stands (properly rated) OR Vehicle Ramps (ensure secure!)
  • New Inline Fuel Filter (highly recommended while you're there)
  • Drain Pan (for residual fuel)
  • Clean Shop Rags - LOTS of them
  • Safety Glasses & Nitrile Gloves
  • New Tank Locking Ring Seal (often included with pump assembly, but good to confirm)
  • Flashlight/Work Light
  • Penetrating Oil (like PB Blaster or Liquid Wrench - if bolts are rusty)

Procedure:

  1. Depressurize the System (Minimal Pressure): While carbureted systems hold much less pressure than fuel injection, it’s still good practice. With the engine OFF and the ignition key OFF, locate the primary inline fuel filter under the hood (if equipped). Place rags underneath it. Carefully loosen the clamp and disconnect the outlet fuel line from the filter (the side going towards the carb). Allow any residual fuel to drain into the rag/pan. Re-tighten the line or plug it temporarily. Alternatively, you can just be very cautious with the lines at the tank.
  2. Disconnect Battery: Remove the NEGATIVE (-) cable from the battery terminal. Secure it away.
  3. Access the Fuel Pump: The fuel pump is accessed through an access panel inside the truck bed, behind the driver's seat. Move the driver's seat fully forward.
    • Remove the bolts securing the metal access cover plate. There might be sealant or a gasket underneath; carefully peel it back. Place the cover aside.
    • Note: You will now see the top of the fuel pump/sender assembly secured by a large lock ring. Be prepared for gasoline smell.
  4. Disconnect Electrical Connectors:
    • Identify the multi-pin electrical connector for the fuel pump (usually 2-3 wires) and the connector for the fuel level sender (usually 2 wires). Take pictures or note wire colors/locations carefully.
    • Depress the locking tabs on each connector and disconnect them. Keep them clear of the area.
  5. Disconnect Fuel Lines:
    • Locate the fuel supply line (coming from the tank to the pump) and the outlet line (going from the pump to the front of the truck). Your pump assembly has nipples these lines attach to.
    • Identify any hose clamps. Use hose clamp pliers or screwdriver (depending on clamp type) to carefully loosen them. Be prepared for more residual fuel – have rags ready and plug the lines quickly if possible.
    • Gently twist and pull the hoses off the pump assembly nipples.
  6. Remove the Locking Ring: This is often the trickiest part.
    • The large metal ring around the assembly flange threads onto the tank neck. It has lugs or notches.
    • Method 1: Use large Channel Lock pliers or a specialty strap wrench. Engage the notches and turn the ring counter-clockwise (lefty-loosey). It might be very tight or corroded.
    • Method 2 (Careful!): Use a brass punch or drift punch and a small hammer. Place the punch tip against one of the ring lugs and tap firmly counter-clockwise. Work your way around the ring, tapping each lug progressively until it loosens. Brass reduces spark risk. DO NOT HIT THE PLASTIC FLANGE.
  7. Remove Pump Assembly: Once the locking ring is loose and unscrewed by hand, lift it off.
    • Carefully lift the entire pump assembly straight up and out of the tank. It has the float arm for the fuel sender attached – angle it gently to clear the tank opening without bending the arm. Do this slowly to minimize splashing fuel residue.
    • Place the old assembly into a drain pan or onto a large rag.
  8. Transfer Fuel Level Sender (If Necessary): SKIP if new assembly includes sender. If your new assembly does not include the fuel level sending unit: VERY carefully detach the float arm assembly from the old pump hanger. Take pictures and note exactly how the retaining clips or screws work. Attach it securely to the new pump assembly in the identical orientation. Bending the float arm can ruin calibration.
  9. Prepare New Pump Assembly:
    • Replace Strainer/Sock: If not pre-installed, press the new inlet strainer/filter sock firmly and evenly onto the pump inlet tube until it clicks or seats securely. Ensure the plastic collar (if present) locks it on.
    • Compare the old assembly's height and orientation with the new assembly. Ensure the float arm shape and position match exactly. This is critical for accurate fuel gauge readings.
    • Install the new lock ring seal onto the tank neck groove. Ensure it fits snugly and evenly. Lubricate it lightly with petroleum jelly (Vaseline) or fresh gasoline to aid installation and sealing. Never use oil-based grease which can degrade it.
  10. Install New Pump Assembly:
    • Carefully lower the new pump assembly down into the tank, gently guiding the float arm. Make sure it seats fully and evenly onto the seal on the tank neck. The orientation usually aligns with notches or marks – ensure it matches how the old one came out.
  11. Install Locking Ring:
    • Place the locking ring over the assembly flange and onto the tank neck threads. Hand-start it clockwise (righty-tighty). Ensure it's seated correctly on the threads.
    • Tighten the ring firmly. You can use Channel Locks or the brass punch/hammer method (tapping clockwise on the lugs) until it's very snug. Avoid overtightening, which can crack the flange or tank neck. It needs to be tight enough to compress the seal fully and prevent leaks. Listen for the lugs clicking past the thread stops as a guide; it should feel substantially tight. Ensure the pump flange is pulled down evenly and flush with the ring.
  12. Reconnect Fuel Lines: Push the fuel supply and outlet hoses firmly onto their respective nipples on the new pump assembly until they seat fully. Secure each one with a new hose clamp positioned properly over the barb. Tighten them securely. Double-check these connections are leak-proof later!
  13. Reconnect Electrical Connectors: Reconnect the electrical connectors for the fuel pump and fuel level sender, pushing them together firmly until the locking tabs click into place. Double-check they are seated correctly and locked.
  14. Install Access Cover: Clean the mating surfaces on the tank flange and the underside of the metal access cover plate if necessary. Place the cover plate back over the hole and secure it with its bolts. Tighten evenly.
  15. Replace Fuel Filter: This is an excellent time to replace the main inline fuel filter under the hood. Disconnect lines (have rags ready), remove the old filter, note the direction of flow (arrow on filter casing), install new filter in the correct orientation, and reconnect lines with clamps.
  16. Reconnect Battery: Reconnect the NEGATIVE (-) battery terminal.
  17. Prime the System and Check for Leaks:
    • Turn the ignition key to the "ON" position (do not start) for 2-3 seconds. Listen for the new pump to activate briefly (you should hear it). Turn key OFF. Repeat 2-3 times. This allows the pump to fill the carburetor bowl.
    • Carefully inspect EVERY fuel line connection you touched – at the pump assembly under the access plate, and the new filter under the hood – for any signs of dripping fuel. Be meticulous. Small leaks can become dangerous fires.
    • If you see ANY leak, shut off the ignition, disconnect the battery, and fix it immediately before proceeding.
  18. Test Drive: Start the engine. It might take a few cranks as air purges fully through the lines. Once running, let it idle for a minute, checking for smooth operation. Check again for leaks at idle.
    • Take the truck for a careful test drive. Check starting, low-speed operation, acceleration, and cruising at sustained speed. Ensure the symptoms you experienced are resolved.
    • Monitor the fuel gauge over the next few days to ensure it's reading accurately (if you replaced the sender).

Troubleshooting After Replacement

Most of the time, a correctly installed new pump solves the problem. If issues persist:

  1. Double-Check ALL Connections: Revisit every electrical connector and fuel line connection. Ensure electrical plugs are fully seated and locked. Ensure fuel lines are pushed on fully and clamps are tight in the right positions. Inspect again for leaks.
  2. Fuel Pressure Test: Perform the fuel pressure test again. Ensure you are getting 3-5 PSI at the carburetor inlet. If pressure is still low, the problem could be a restriction (clog in the line, stuck tank pickup) or a problem with the pump relay/wiring limiting voltage even though the pump runs.
  3. Check Voltage at Pump Connector: With the pump running (ignition ON or cranking), use a multimeter to check voltage at the pump's electrical connector (behind the connector pins carefully). You should see near battery voltage (approx. 12V). Significantly less (like 9-10V or less) indicates high resistance in the circuit: corroded terminals, bad relay contacts, failing wire, or poor ground.
  4. Inspect Ground: Find where the pump's ground wire connects to the chassis (usually near the tank area under the truck bed). Clean the connection point (wire terminal and chassis) thoroughly to bare metal and ensure it's tight. A poor ground prevents the pump from getting enough voltage.
  5. Review Diagnostics: Rethink the original diagnosis. Did you replace the fuel filter? Could the carburetor itself be blocked or malfunctioning? Check ignition components (spark plugs, wires, distributor cap, rotor, points/ignition module) again. Don't rule out vacuum leaks affecting engine operation.

Long-Term Maintenance for Your 720's Fuel System

To maximize the life of your new fuel pump and ensure reliable operation:

  1. Keep the Tank Clean: Avoid running the tank consistently below 1/4 full. Sediment settles at the bottom. When replacing a pump, if you saw significant debris in the tank, consider professional cleaning or replacement. Modern ethanol-blended fuels can stir up old rust and deposits.
  2. Replace Fuel Filters Regularly: The inline fuel filter is your main line of defense after the pump strainer. Replace it according to your Nissan's maintenance schedule, or more frequently if you suspect contamination (e.g., 12-15k miles, or sooner if symptoms arise). The strainer is not a replaceable service item and should only be replaced when the pump is.
  3. Use Quality Fuel: While modern fuels aren't always ideal for classics, buying gasoline from reputable stations reduces the risk of water or severe contamination.
  4. Address Rust Concerns: If your truck sits a lot and is in a humid climate, fuel tank rust is a real concern. A rusty tank will quickly destroy a new pump. Consider fuel tank restoration or replacement if internal rust is found.
  5. Ensure Venting Works: Keep the gas cap and any tank vent lines clean and unobstructed. Symptoms like vapor lock or difficulty filling the tank indicate vent issues.

Conclusion

A failing fuel pump can sideline your dependable 1985 Nissan 720, but fortunately, diagnosing and replacing it is a well-defined repair achievable by most home mechanics. By understanding the symptoms, performing careful diagnostics (especially confirming low pressure), sourcing a quality replacement assembly, and methodically following the replacement steps while prioritizing safety, you can restore reliable fuel delivery to your classic truck. Regular fuel filter changes and good maintenance habits will help ensure your newly installed 1985 Nissan 720 fuel pump provides years of faithful service, keeping your vintage Nissan on the road where it belongs. Don't let a worn-out pump keep you from enjoying your classic 720 – tackle the repair confidently using the information in this guide.

Back to blog