Diagnosing Fuel Pump Relay Circuits on 94-95 C/K Trucks with 6.5L Diesel (OBD2)

If you’re troubleshooting a no-start or driveability issue on your 1994 or 1995 Chevrolet or GMC C/K truck equipped with a 6.5L diesel engine and OBD2 system, a crucial step is to diagnose the fuel pump relay circuit. While these model years share similarities, it’s important to note a specific difference for ’94 models: the Diagnostic Link Connector (DLC) “Fuel Pump Test Terminal” is designated as “G” on the ’94, which is important for specific diagnostic procedures. This guide will walk you through diagnosing the fuel pump relay circuit and performing essential fuel supply system checks.

Understanding the fuel supply system is paramount when addressing engine performance issues. A properly functioning fuel system ensures consistent fuel delivery, critical for the operation of your 6.5L diesel. If the system isn’t delivering enough fuel, or worse, if air is being introduced, it can lead to significant driveability problems, including a frustrating “cranks but will not run” scenario. If your initial diagnostics point towards fuel delivery issues, or if you suspect the fuel supply system, the following checks are essential.

Important Fuel Supply System Checks – Preliminary Steps

Before diving into detailed tests, consider these critical points:

• Air Leaks and Restrictions: Air leaks or blockages on the suction side of the fuel pump can severely impact its performance. Always address these first.
• Fuel Level: Ensure you have sufficient fuel in the tank. It seems obvious, but it’s often overlooked!
• Leak Inspection: Thoroughly inspect all fuel connections from the fuel tank to the injection pump for any signs of leaks.
• Connection Tightness: Tighten any fuel line connections that appear loose.
• Hose and Line Condition: With the engine running, carefully check all fuel hoses and lines for flattening or kinks that could restrict fuel flow.

Lift Pump Flow Check

This test measures the basic fuel flow rate from your lift pump.

1. Disable Injection Pump Solenoid: Disconnect the electrical connector for the engine shutoff solenoid at the injection pump. This prevents the engine from starting during the test.
2. Disconnect Outlet Pipe: Locate and disconnect the fuel pipe at the lift pump outlet fitting.
3. Prepare Fuel Collection: Attach a hose to the lift pump outlet fitting and direct the other end into a 1 liter/quart container to collect fuel.
4. Crank Engine and Measure: Crank the engine for 15 seconds and measure the amount of fuel collected in the container.
   • Sufficient Flow: If you collect more than 0.24 liters (1/2 pint) in 15 seconds, proceed to the “Lift Pump Pressure Check” section.
   • Insufficient Flow: If you collect less than 0.24 liters (1/2 pint) in 15 seconds, proceed to the “Lift Pump Suction Line Check” section.

Lift Pump Suction Line Check

This check helps isolate issues within the fuel suction line from the tank.

1. Remove Fuel Tank Cap and Repeat Flow Check: Remove the fuel tank cap to eliminate potential vacuum issues and repeat the “Lift Pump Flow Check.”
   • Improved Flow: If flow improves to more than 0.24 liters (1/2 pint) in 15 seconds, suspect a defective fuel cap causing a vacuum lock. Replace the fuel cap and proceed to the “Lift Pump Pressure Check.”
   • Still Insufficient Flow: If flow remains below 0.24 liters (1/2 pint) in 15 seconds, continue to the next step.
2. Separate Suction Line: Disconnect the lift pump suction line from the fuel sender unit at the fuel tank.
3. Bypass Fuel Tank: Connect the suction line to a source of clean fuel (like a can of diesel) using an additional length of hose. This bypasses the fuel tank and sender.
4. Repeat Flow Check Again: Repeat the “Lift Pump Flow Check” with the bypassed fuel source.
   • Flow Restored: If flow is now more than 0.24 liters (1/2 pint) in 15 seconds, the restriction is likely within the fuel sender unit in the tank. Remove the fuel sender and inspect it for blockages or restrictions.
   • Flow Still Low: If flow is still less than 0.24 liters (1/2 pint) in 15 seconds, proceed to step 5.
5. Inspect Suction Line for Restriction: Examine the lift pump suction line itself for kinks, clogs, or damage.
   • Restriction Found: If a restriction is found, repair or replace the suction line and re-perform the lift pump flow check.
   • No Restriction Found: If no restriction is apparent in the line, the lift pump itself is likely faulty. Replace the lift pump and recheck the fuel flow.
6. Reconnect Suction Line: Once the issue is resolved or the pump is replaced, reattach the lift pump suction line to the fuel sender unit.

Lift Pump Pressure Test

This test verifies the pressure output of the lift pump.

1. Install Tee Adapter: Install a tee adapter at the fuel inlet connection of the injection pump.
2. Connect Pressure Gauge: Connect a pressure gauge capable of reading 0 to 15 psi (0 to 103 kPa) to the tee adapter.
3. Measure Fuel Pressure: Start and run the engine and observe the fuel pressure reading on the gauge.
   • Adequate Pressure: If the pressure is at least 3 psi (27 kPa), proceed to step 4.
   • Low Pressure: If the pressure is less than 3 psi (27 kPa), refer back to Chart A-5 in SECTION 3 (Fuel Pump Relay Circuit Diagnosis) (as seen in the diagram) before replacing the lift pump. This chart likely details electrical circuit checks for the fuel pump relay.
4. Remove Test Equipment: Remove the pressure gauge and tee adapter.
5. Reconnect Outlet Pipe: Reconnect the outlet pipe at the lift pump outlet fitting.
6. Clean Up: Clean any spilled fuel.
7. Check for Leaks: Run the engine again and inspect for any fuel leaks at the connections you worked on.

Fuel System Air Leak Test

Air in the fuel system can cause significant performance problems. This test helps identify air leaks.

1. Install Transparent Hose (Inlet): Place a transparent hose between the fuel manager/filter outlet and the injection pump fuel inlet. This allows you to visually inspect the fuel for air bubbles.
2. Observe for Air Bubbles (Inlet): Start and idle the engine and carefully observe the fuel flowing through the transparent hose for the presence of air bubbles.
   • No Bubbles (Inlet): If no air bubbles are visible, stop the engine and proceed to step 6 (checking the return line).
   • Bubbles Present (Inlet): If air bubbles are present in the inlet line, stop the engine and continue to step 3 to investigate suction line leaks.
3. Check Suction Line for Air Leakage:
   • Plug Fuel Sender Line: Disconnect the fuel pipe from the fuel sender and plug the sender line to isolate the tank.
   • Connect Vacuum Pump: Disconnect the fuel pipe from the lift pump and connect a handheld vacuum pump with a gauge to the fuel pipe leading to the tank.
   • Apply Vacuum and Observe: Apply vacuum to the fuel pipe and watch the gauge reading.
     • Vacuum Holds: If the vacuum reading remains steady and doesn’t drop, the suction line is likely sealed. Reconnect the fuel pipe and proceed to step 4 to check the fuel sender.
     • Vacuum Drops: If the vacuum drops, there’s an air leak in the suction line. Locate and repair the air leak in the suction line, then reconnect the suction line pipe and hose and retest from step 5.
4. Check Fuel Sender for Air Leakage:
   • Remove Fuel Tank and Sender: For thorough access, remove the fuel tank from the vehicle. Then, remove the fuel sender unit from the fuel tank. Remove the fuel strainer from the sender and plug the bottom of the pickup tube.
   • Apply Vacuum to Sender: Apply vacuum to the upper end of the pickup tube on the fuel sender and observe the gauge reading.
     • Vacuum Holds (Sender): If the vacuum holds steady, the fuel sender is likely not leaking. Reinstall the fuel sender and fuel tank.
     • Vacuum Drops (Sender): If the vacuum drops, the fuel sender is leaking air. Replace the fuel sender, reinstall the fuel tank, and reconnect the fuel pipe. Proceed to step 5.
5. Re-check for Bubbles (Inlet after Suction/Sender Check): Start and run the engine again and observe the fuel in the transparent hose (inlet line) for air bubbles.
   • Bubbles Still Present: If air bubbles are still present, stop the engine and re-examine steps 3 and 4, ensuring you’ve thoroughly addressed potential leaks in the suction line and fuel sender.
   • No Bubbles: If no air bubbles are present in the inlet line, stop the engine and continue to step 6 to check the return line.
6. Remove Transparent Hose (Inlet) and Reconnect: Remove the transparent hose from the fuel manager/filter outlet and injection pump inlet and reconnect the original fuel hose.
7. Disconnect Return Hose: Disconnect the fuel return hose from the injection pump.
8. Install Transparent Hose (Return): Install a transparent hose between the injection pump return port and the disconnected fuel return hose.
9. Observe for Air Bubbles (Return): Start and run the engine and observe the fuel in the transparent hose (return line) for air bubbles.

Important Notice Regarding Air Bubbles in Return Line: It is normal to see a small stream of air bubbles during snap acceleration in the return line. However, continuous or excessive bubbles are problematic.

• No Excessive Bubbles (Return): If no excessive air bubbles are present in the return line, proceed to step 10.
• Excessive Bubbles (Return): If excessive air bubbles are present in the return line, the injection pump itself may be drawing in air or malfunctioning. Consider injection pump replacement.

10. Stop Engine.
11. Remove Transparent Hose (Return) and Reconnect: Remove the transparent hose from the return line and reconnect the original fuel return hose to the injection pump.
12. Clean Up. Clean any spilled fuel.
13. Check for Leaks. Run the engine and check for fuel leaks at all connections.

Clearing Diagnostic Trouble Codes (DTCs)

As a standard procedure after any fuel system test, routine maintenance like fuel filter changes, or repairs, and once normal engine operation is restored, it is crucial to check for, note down, and importantly, CLEAR any Diagnostic Trouble Codes (DTCs). This ensures that any stored codes from the troubleshooting process do not interfere with future diagnostics.

This comprehensive guide should assist you in diagnosing fuel pump relay circuit and fuel supply system issues on your ’94 or ’95 C/K truck with a 6.5L diesel engine and OBD2. Remember to always prioritize safety when working with fuel systems and consult a qualified mechanic if you are unsure about any of these procedures.