The Complete Guide to 1987 Jeep Comanche Fuel Pump Replacement and Troubleshooting

Replacing the fuel pump in your 1987 Jeep Comanche is a challenging but achievable task for many home mechanics. Located inside the fuel tank, this critical component requires dropping the tank for access. Understanding the process, common symptoms of failure, and the nuances of the Comanche's specific setup is essential for a successful repair. This guide covers everything you need to know, from diagnosis to installation, ensuring your MJ truck gets back on the road reliably.

Why the Fuel Pump is Crucial in Your 1987 Comanche

The fuel pump in your 1987 Jeep Comanche serves one vital function: it delivers gasoline from the fuel tank, located under the bed between the frame rails, to the engine's fuel injection system at precisely the correct pressure. Without consistent fuel pressure and volume, your Comanche's engine will not run properly, if at all. The 1987 model year used throttle body injection (TBI), meaning the pump needs to supply fuel at around 15-20 PSI to the injector unit mounted on top of the intake manifold. Unlike older vehicles with mechanical fuel pumps mounted on the engine, the Comanche uses an electric pump submerged in the fuel tank. This design helps cool the pump and reduce noise but makes replacement significantly more involved.

Common Symptoms of a Failing 1987 Jeep Comanche Fuel Pump

Recognizing the signs of a failing pump can help you diagnose the problem before you’re left stranded:

  • Engine Cranking But Not Starting: The most obvious symptom. If the engine spins over but doesn’t fire, and you don’t hear the brief fuel pump "whirring" sound when you first turn the ignition key to "ON" (before cranking), it strongly points to a pump issue or its related circuitry.
  • Engine Stalling Intermittently: A pump starting to fail might work occasionally, especially when cold, but cut out unpredictably, particularly under load or when warmed up. This can feel like the engine is losing power momentarily and then recovering, or it could stall completely and may restart after sitting for a few minutes.
  • Loss of Power Under Load: If the pump can’t deliver sufficient pressure or volume at higher engine speeds or throttle openings, the engine will hesitate, surge, or lack power during acceleration or when climbing hills.
  • Surging or Sputtering at High Speeds or Loads: Similar to power loss, this indicates the pump cannot maintain consistent fuel flow when demand is higher.
  • Loud Whining or Humming Noise from Fuel Tank Area: While the pump always makes some noise, a significant increase in volume, pitch, or a "grinding" sound indicates internal wear or impending failure. Listen near the tank or through the filler neck.
  • Difficulty Starting When Engine is Hot (Heat Soak): Heat can exacerbate failing pump motor windings or commutator problems. After driving and stopping, then trying to restart within 30-60 minutes, the engine struggles or refuses to start.

Accurately Diagnosing a Fuel System Problem

Do not assume the pump is bad just because the engine won't start! Jumping straight to replacing the pump is expensive and time-consuming. Follow a logical diagnostic path:

  1. Confirm Fuel Pump Activation: Turn the ignition key to "ON" (not START). You should hear a distinct "whirring" sound from the rear of the truck (tank area) lasting 1-2 seconds. If you hear nothing, the problem could be the pump, but is more likely its control circuit (relay, fuse, wiring) or the pump ground.
  2. Check Fuel Pump Relay and Fuse: Locate the Power Distribution Center (PDC), usually found on the driver's side firewall in the engine compartment. Identify the fuel pump relay (refer to the diagram on the PDC lid or a service manual). Swap it with a known good identical relay (like the horn relay - ensure specs match). Check the fuse related to the fuel pump circuit (usually a 20-amp).
  3. Verify Electrical Power at the Pump Connector: This requires accessing the connector near the top of the fuel tank. You might be able to see it without dropping the tank completely by removing the spare tire and any shields. Carefully unplug the connector. Have an assistant turn the ignition key to "ON" while you measure for voltage at the harness side connector using a multimeter. You should see approximately 12 volts across two of the pins for 1-2 seconds. If voltage is present, the problem is likely the pump itself or the ground connection near the pump mounting flange. If voltage is absent, trace the circuit back (relay, fuse, wiring) to the source of the failure. Crucially, check the dedicated ground wire connection near the fuel tank strap mounting bolt – clean its contact point thoroughly.
  4. Perform a Fuel Pressure Test: This is the definitive test. Rent or buy a fuel pressure test kit designed for TBI systems (usually 0-60 PSI range). Locate the Schrader valve test port on the throttle body fuel injection unit. Connect the gauge according to kit instructions. Turn the ignition key to "ON" to pressurize the system without starting the engine. Observe the reading. Good pressure for an '87 Comanche should be approximately 15-20 PSI and must hold steady for several minutes after the pump stops. Significantly lower pressure, pressure that bleeds down very quickly, or no pressure indicates a problem with the pump, the fuel pressure regulator on the TBI unit, or possibly a severe fuel leak (use caution!). Low pressure does not automatically condemn the pump – the regulator could be faulty. Verify regulator function if possible.

Understanding the 1987 Comanche Fuel Pump Assembly

The fuel pump is not purchased or replaced as a standalone component on the Comanche. It is part of an integrated assembly called the fuel pump and sender module:

  • Module Housing: A metal or plastic assembly that mounts through a hole on top of the fuel tank, sealed with a large O-ring.
  • Fuel Pump: A submerged electric motor-driven turbine or roller vane pump mounted within the housing. It pulls fuel through the strainer sock.
  • Fuel Sending Unit: This consists of a float arm attached to a variable resistor that sends the signal to your dashboard fuel gauge. A bad sending unit can cause erratic fuel gauge readings even if the pump itself is functional.
  • Fuel Strainer Sock: A filter sock attached to the inlet of the pump inside the tank. Its purpose is to trap large debris before it reaches the pump.
  • Locking Ring: A large threaded or snap-ring that holds the entire module assembly securely into the tank.
  • Electrical Connector: The plug connecting the pump motor and sender unit to the vehicle wiring harness.
  • Fuel Lines: Short sections of rigid fuel line connecting the module to the longer supply and return lines running to the engine.

Why Replacement Typically Means the Whole Assembly

While it's technically possible to replace just the pump motor itself on many vehicles, it is strongly recommended to replace the entire assembly on a 1987 Comanche:

  • Cost: Buying just the pump motor is usually only marginally cheaper than the whole assembly from quality aftermarket suppliers.
  • Labor: The labor to drop the tank is significant. Once it's out, replacing the whole assembly ensures all critical internal components (pump, sender, strainer sock, seals) are new and won't require dropping the tank again soon because the 35+ year old sending unit failed.
  • Seal Integrity: New assemblies come with a new seal and updated lock ring, preventing leaks.
  • Sending Unit Reliability: Old sender units are prone to failure, leading to inaccurate fuel readings. A new assembly fixes this.
  • Quality Assurance: Reputable assembly suppliers use components designed to work together optimally.

Choosing the Right Replacement Assembly

Opting for quality is paramount. Avoid the cheapest parts store options:

  • Reputable Brands: Spectra Premium (SPI), Bosch, Carter, Delphi, and ACDelco (Professional line) are generally well-regarded. Spectra Premium (often sold under the Master brand too) is a popular and cost-effective choice for the Comanche.
  • Ensure Correct Application: Verify the replacement assembly is specifically listed for 1987 Jeep Comanche. 1987 is a specific year for emissions and TBI configuration. While 1986-1988 Cherokees (XJ) with the 2.5L or 4.0L share the same assembly, confirm fitment before purchase. Crucially, double-check engine size compatibility (2.5L 4-cylinder or 4.0L inline-6).
  • Avoid Sub-Standard Units: Cheap, no-name assemblies often use inferior pumps and sender units that fail prematurely, sometimes within months.

Tools and Supplies Needed for the Job

Be prepared before you start:

  • Required: Floor jack & robust jack stands (essential for safe lifting), wheel chocks, basic hand tools (wrenches, sockets in metric sizes, ratchets, extensions), screwdrivers, pliers.
  • Essential: Torch (Penetrating fluid is useless on tank straps!), Wire brush / sandpaper / wire wheel for cleaning bolts and ground points.
  • Highly Recommended: Fuel line disconnect tools (specifically for the 5/16" & 3/8" quick-connect fittings on the pump module lines).
  • Supplies: Replacement fuel pump and sender assembly, New fuel filter (located near the tank), Replacement fuel tank lock ring seal/gasket (often comes with pump assembly, but get one if not), Replacement O-rings for fuel line fittings (optional but good practice), Safety glasses, Nitrile gloves.
  • Safety: Fire extinguisher rated for fuel fires (ABC or BC), Ample ventilation (preferably outdoors). Have no sparks, flames, or sources of ignition nearby!

Step-by-Step Guide to Dropping the Tank and Replacing the Pump

Warning: Gasoline is extremely flammable. Exercise utmost caution. Disconnect the negative battery terminal first! Work in a well-ventilated area. Have a fire extinguisher within reach.

  1. Prepare the Vehicle: Park on a level, solid surface. Chock the front wheels. Disconnect the negative battery cable. Relieve fuel system pressure: Either pull the fuel pump relay/fuse and crank the engine until it stalls (if it was starting) OR cautiously remove the gas cap slowly.
  2. Drain the Fuel Tank: This is the safest method. Run the vehicle until it’s near empty if possible. If not, you need a transfer pump or siphoning equipment designed for gasoline. Never siphon by mouth! Drain the tank into approved gasoline containers. Removing a near-empty tank is vastly safer and lighter. Even a few gallons add significant weight.
  3. Access the Tank: Remove the spare tire if present. Carefully disconnect the filler neck hose clamp located inside the driver's side rear wheel well area. You will likely need to remove the plastic inner splash shield screws (torx or phillips) partially to access the clamp. Protect the hose from damage.
  4. Disconnect Electrical and Fuel Lines: Locate the electrical connector near the top of the tank (usually near one of the straps). Disconnect it. Locate the fuel supply and return lines (often along the frame rail). Identify and mark the lines (supply/return) if they differ. Use the appropriate sized fuel line disconnect tool to carefully release the quick-connect fittings where the lines attach to the hard lines coming from the top of the pump module. Expect residual fuel spillage; have rags ready. Cover the open line ends to prevent debris ingress.
  5. Support and Remove Tank Straps: Place a sturdy jack (a transmission jack with a flat plate adapter works best) securely under the fuel tank. The tank is heavy, especially if not drained. Locate the long bolts securing the front and rear tank straps to the vehicle frame. These bolts are notoriously seized. Apply significant penetrating fluid (like PB Blaster) and allow time to soak. Use heat (torch) carefully if necessary. Be prepared for bolt breakage – have extraction tools ready. Once the bolts are removed, carefully lower the tank a few inches using the jack.
  6. Final Disconnections and Removal: With the tank lowered enough to access the top, carefully disconnect the vapor recovery line (smaller diameter hose) connector if present. Ensure nothing else is connected. Slowly and carefully lower the tank fully to the ground. Maneuver it out from under the vehicle, taking care not to damage the lines or sender unit.
  7. Clean Tank Top: Thoroughly clean the area around the pump module flange on top of the tank before disassembly to prevent debris falling into the tank. Remove any dirt and grime.
  8. Remove Lock Ring: The module is held by a large locking ring. This may be threaded (rotate counter-clockwise with spanner wrench/brass punch & hammer) or a snap-ring type requiring pliers to compress and rotate. Penetrating fluid helps. Strap wrenches can also be useful. Be careful to not damage the tank flange.
  9. Remove Old Module: Once the lock ring is off, carefully lift the entire module assembly straight out of the tank. Note the orientation! Inspect the old strainer sock for debris which can indicate tank contamination.
  10. Install New Module: Thoroughly clean the tank flange surface and mating surface of the new module seal. Inspect the inside of the tank for excessive rust or debris – cleaning may be necessary if found. Insert the new module assembly into the tank in the exact correct orientation (aligning float arm position). Ensure the new seal is properly seated.
  11. Install New Lock Ring: Place the new lock ring over the module flange and rotate it firmly clockwise (or compress/rotate if snap-ring style) until it is fully seated and secure.
  12. Reinstall Tank: Carefully lift the tank back into position using the jack. Ensure no lines or hoses are pinched. Realign the tank straps. Insert the strap bolts and tighten securely (apply anti-seize compound to threads on new bolts). Reconnect the vapor line. Reconnect the fuel lines using the quick-connect fittings – ensure a positive "click". Reconnect the electrical connector. Reattach the filler neck hose and clamp securely inside the wheel well. Reinstall any splash shields.
  13. Final Steps: Install the new fuel filter if applicable (recommended). Reinstall the spare tire. Reconnect the negative battery cable. Before starting, turn the ignition key to "ON" several times (pause for a few seconds between each) to prime the fuel system and allow the pump to build pressure. Listen for leaks at the module top, fittings, and filler neck. If leaks are present, shut off immediately and address. Once confirmed leak-free, start the engine. Check the fuel gauge operation immediately. Monitor for leaks carefully during the first few minutes of operation and after driving.

Preventative Maintenance and Long-Term Reliability

Maximize the lifespan of your new fuel pump:

  • Keep Fuel Level Reasonable: Running the tank chronically very low increases the risk of the pump sucking in sediment from the bottom and can cause the pump to overheat as it relies on fuel submersion for cooling. Refill before you hit 1/4 tank.
  • Change the Fuel Filter: Replace the in-line fuel filter according to the manufacturer’s recommendation or more frequently if driving in dusty conditions or using questionable fuel sources. A clogged filter strains the pump.
  • Use Quality Fuel: Consistently using lower-tier fuels can contain more contaminants or lack effective detergents. Occasionally filling up at a high-volume station from a major brand can be beneficial.
  • Avoid E85: Ethanol blends higher than E10 (standard gasoline) can be corrosive and damaging to older fuel system components and seals not designed for it. Stick to regular unleaded.

Conclusion: Restoring Your 1987 Comanche's Vital Lifeline

Replacing the internal fuel pump in your 1987 Jeep Comanche is a significant repair, demanding safety awareness, patience, and preparedness – especially dealing with the fuel tank and potentially stubborn hardware. However, by understanding the diagnostic steps, choosing a quality replacement assembly, gathering the right tools, and methodically following the tank removal and module replacement process, it is a task well within reach for a dedicated DIY mechanic. Addressing this critical component properly ensures reliable fuel delivery for countless more miles, keeping your iconic Comanche pickup truck running strong. Remember, the initial investment in a quality part and careful installation pays dividends in long-term performance and peace of mind.

Back to blog