1987 S10 Fuel Pump: Complete Guide to Diagnosis, Replacement, and Troubleshooting

If your 1987 Chevy S10 cranks but won’t start, the fuel pump is the most likely culprit. The 1987 S10 uses an in-tank electric fuel pump that must prime the system before the engine can fire. When this pump fails, you get no fuel pressure, and the engine simply won’t start. The good news is that you can diagnose the problem yourself with basic tools, and replacement is a straightforward job. This guide covers everything you need to know: how the system works, where to find the relay and fuse, step-by-step testing procedures, and how to replace the pump when it fails.

How the 1987 S10 Fuel Pump System Works

The 1987 S10 fuel pump circuit is controlled by a relay, typically located on the firewall under the hood . When you turn the ignition key to the "On" position, the engine control module (ECM) triggers the fuel pump relay for about two seconds . This primes the fuel system before you start the engine. If you don’t hear a brief humming sound from the rear of the truck when you turn the key, the pump isn’t priming.

There’s a backup system built into these trucks. The fuel pump circuit also runs through the oil pressure sending unit . Once the engine starts and builds oil pressure, the oil pressure switch acts as a redundant power source for the fuel pump . This is why some 1987 S10s with a bad relay will still start after extended cranking—the engine builds enough oil pressure to trigger the backup circuit and power the pump . But if both the relay and the oil pressure switch fail, the pump won’t run at all.

Where to Find the Fuel Pump Relay and Fuse

Fuel Pump Relay Location: The fuel pump relay on a 1987 S10 is mounted on the passenger side of the firewall, near the top of the engine compartment . It’s usually part of a small relay bracket along with other relays. Look for a green connector that may be sticking out of the wiring harness—this is the fuel pump prime connector .

Fuel Pump Fuse Location: The fuel pump fuse is not under the dashboard. It’s located in the under-hood fuse box . Look for the "ECM B" fuse, which powers the fuel pump circuit . This fuse is often at the bottom left of the under-hood fuse box. A blown ECM B fuse is a common cause of no fuel pump operation.

Step-by-Step Diagnosis: How to Test the 1987 S10 Fuel Pump

Before you drop the fuel tank and replace the pump, run through these tests. Many times the problem is simpler than a dead pump.

1. Listen for the Prime Cycle
Turn the ignition key to the "On" position without starting the engine. Listen carefully near the rear of the truck. You should hear a brief humming sound lasting 2–3 seconds . If you hear it, the pump relay, fuse, and pump are likely working. If you don’t hear anything, move to the next step.

2. Check the Fuel Pump Fuse
Open the under-hood fuse box and locate the ECM B fuse . Use a test light or multimeter to check for continuity. If the fuse is blown, replace it with one of the same amperage. Never use a higher amperage fuse—this can cause a fire . If the new fuse blows immediately, there’s a short in the wiring that needs to be traced.

3. Test the Fuel Pump Relay
Locate the relay on the passenger side firewall . You can swap it with a known-good relay of the same type (like the horn relay) to see if the pump starts working. Or, use a multimeter to check for power at the relay socket with the ignition on. If there’s power at the relay but the pump doesn’t run, the relay itself may be bad.

4. Use the Prime Connector
Look for a green connector sticking out of the wiring harness near the master cylinder area . This is the fuel pump prime connector. With a fused jumper wire, apply 12 volts to this connector . If the fuel pump runs, you’ve confirmed the pump itself is good. The problem is then in the relay circuit, the ECM trigger signal, or the wiring between them.

5. Check for Power at the Fuel Pump Connector
If you have access to the fuel pump connector on top of the tank, use a multimeter to test for 12 volts at the connector with the ignition on . The 1987 S10 uses a Tan/White wire for power and a Black wire for ground . If you have voltage at the connector but the pump doesn’t run, the pump is bad. If you have no voltage, trace the wiring back toward the relay.

6. Test Fuel Pressure
Connect a fuel pressure gauge to the Schrader valve on the fuel rail . The 1987 S10 TBI system should show 9–13 psi of fuel pressure . If the pressure is low or zero, the pump is failing or dead.

Common Failure Points on the 1987 S10

Corroded Ground Connections: The 1987 S10 has three primary ground points that must be clean and tight: the negative battery cable to the engine block, the engine block to the firewall (braided strap), and the wiring harness ground to the radiator support . Corrosion at any of these points can cause intermittent electrical problems that are hard to diagnose.

Bad Tank Ground: On these trucks, the fuel pump ground is made through the tank straps and the tank itself . If the steel tank gets rusty, the ground connection fails. You can test this by touching a test light probe to the gas tank while the pump should be running. If the light turns on, the tank has a bad ground .

Failed Oil Pressure Sending Unit: The oil pressure switch is the backup power source for the fuel pump once the engine is running . If this switch fails, the engine may start but then stall, or it may require extended cranking to start .

Faulty ECM Trigger Signal: The ECM sends a signal to the fuel pump relay through a Dark Green/White wire . If the ECM itself fails, or if this wire is broken or shorted, the relay won’t trigger.

How to Replace the Fuel Pump on a 1987 S10

If testing confirms the pump is bad, replacement requires removing the fuel tank. The fuel pump is located inside the tank on the driver’s side of the truck .

What You’ll Need:

  • Jack and jack stands
  • Socket set and wrenches
  • Flathead screwdriver
  • Brass punch and hammer
  • Fuel line disconnect tool
  • New fuel pump assembly (OEM replacement recommended)
  • New O-ring for the tank seal
  • Safety glasses and gloves
  • Fire extinguisher (have it nearby)

Step-by-Step Procedure:

  1. Relieve fuel pressure: Remove the negative battery cable. Locate the Schrader valve on the fuel rail and push the valve core in with a small brass punch to release pressure . Remove the fuel fill cap.

  2. Drain the tank: Use a suction hose to remove as much fuel as possible . This makes the tank lighter and safer to work with.

  3. Raise the vehicle: Jack up the rear of the truck and place jack stands under the rear axle .

  4. Remove the rock shield: If your truck has a rock shield under the tank, remove the bolts and take it off .

  5. Disconnect hoses and lines: Remove the hose clamps from the filler hose and vent hose. Disconnect the evap hose at the canister. Remove the fuel line hose and clamps at the fuel tank .

  6. Support and lower the tank: Place a piece of plywood on top of a jack, raise it under the tank, and remove the strap bolts. Lower the tank just enough to unplug the electrical connector . Then slide the tank out from under the vehicle.

  7. Remove the fuel pump assembly: Mark the orientation of the pump assembly in the tank. Use channel locks or a brass punch and hammer to turn the retainer ring counterclockwise and remove it . Lift the pump assembly out of the tank.

  8. Install the new pump: Clean the O-ring groove in the tank. Install the new O-ring. Place the new pump assembly in the tank, aligning the tab or mark, and tighten the retainer ring .

  9. Reinstall the tank: Raise the tank back into position, reconnect the electrical plug, install the filler and vent hoses, and tighten the strap bolts. Reinstall the rock shield if equipped.

  10. Reconnect the battery and test: Lower the vehicle, reconnect the battery, and turn the key to the "On" position. Listen for the pump to prime. Check for fuel leaks before starting the engine.

Why You Should Consider KEMSO Fuel Pump for Your 1987 S10

When it’s time to replace your fuel pump, you want a product that delivers reliable performance and long-term durability. KEMSO offers high-performance fuel pumps designed as direct OEM replacements for the 1987 Chevy S10 and many other vehicles. Their pumps support up to 600 BHP in naturally aspirated, turbocharged, and supercharged applications, making them suitable for both stock and modified engines. Every KEMSO fuel pump is backed by a lifetime warranty, giving you confidence that your investment is protected. Built with quality materials and tested for function and performance, KEMSO pumps are engineered to meet or exceed OEM specifications. Whether you’re doing a simple replacement or upgrading your fuel system for more power, KEMSO has the right pump for your 1987 S10. Visit their website at https://www.kemsoracing.com/ to find the correct fuel pump for your truck and take advantage of their lifetime warranty and high-performance designs.

Final Thoughts

The 1987 S10 fuel pump system is well-engineered but has common failure points that are easy to diagnose. Start with the simple checks: listen for the prime, inspect the fuse, and test the relay. Most of the time, the problem is a blown fuse, corroded ground, or failed relay—not the pump itself. If you do need to replace the pump, the job is manageable with basic mechanical skills and the right tools. Always work safely around fuel, keep a fire extinguisher nearby, and never bypass fuses with higher amperage wire. With a systematic approach, you can get your 1987 S10 back on the road reliably.

For a high-quality replacement that will last, consider KEMSO fuel pumps. Visit https://www.kemsoracing.com/ for more information and to order the correct pump for your 1987 Chevy S10.


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