1994 C/K Truck 6.5 Diesel Fuel Pump Relay Circuit Diagnosis for OBD2 Systems

Diagnosing fuel supply issues in your 1994 or 1995 C/K truck, especially with the 6.5L diesel engine, can be crucial for maintaining optimal performance. While this guide primarily references ’95 C/K Trucks, it’s highly applicable to ’94 models as well, particularly when considering the nuances of the OBD2 system and the fuel pump setup on the 6.5 diesel. It’s important to note that for ’94 models, the DLC “Fuel Pump Test Terminal” is designated as “G”.

Fuel Supply System Checks: Ensuring Proper Fuel Delivery for Your 6.5 Diesel

A malfunctioning fuel supply system can significantly impact your vehicle’s driveability, potentially leading to a frustrating “Cranks But Will Not Run” situation. This is often exacerbated by issues within the fuel pump relay circuit, a common concern in 1994 C/K trucks with the 6.5 diesel engine. If your diagnostics point towards fuel delivery problems, or if you suspect inadequate fuel supply or air intrusion, the following checks are essential.

Important Considerations Before You Begin:

• Air Leaks and Restrictions: Remember that any air leaks or restrictions on the suction side of the fuel pump will drastically reduce pump efficiency.
• Fuel Level: Always ensure there’s sufficient fuel in the tank before starting any diagnostic procedures.
• Leak Inspection: Thoroughly inspect all fuel connections from the fuel tank to the injection pump for any signs of leakage.
• Connection Integrity: Tighten any loose fuel line connections you find.
• Hose and Line Check: With the engine running briefly, carefully examine all fuel hoses and lines for flattening or kinks that could impede fuel flow.

Lift Pump Flow Check: Assessing Fuel Pump Delivery Rate

This test measures the volume of fuel delivered by the lift pump, a critical component in the 6.5 diesel fuel system of your ’94 C/K truck.

1. Disable Shutoff Solenoid: Disconnect the electrical connector for the engine shutoff solenoid located at the injection pump.
2. Disconnect Outlet Pipe: Detach the pipe at the lift pump outlet fitting.
3. Prepare Fuel Collection: Attach a hose to the lift pump outlet fitting and position the other end into a container capable of holding at least one liter or quart of fluid.
4. Crank and Measure: Crank the engine for 15 seconds and measure the amount of fuel collected in the container.
   • Adequate Flow: If you collect more than 0.24 liters (approximately 1/2 pint) in 15 seconds, proceed to the “Lift Pump Pressure Check” section.
   • Insufficient Flow: If you collect less than 0.24 liters (approximately 1/2 pint) in 15 seconds, refer to the “Lift Pump Suction Line Check” section to investigate potential blockages or leaks in the fuel supply line.

Lift Pump Suction Line Check: Investigating Fuel Supply Obstructions

If the Lift Pump Flow Check indicates insufficient fuel delivery, the suction line is the next area to investigate for potential restrictions or air leaks, common culprits in 6.5 diesel fuel system issues in ’94 C/K trucks.

1. Vent Fuel Tank: Remove the fuel tank cap to eliminate potential vacuum lock and repeat the “Lift Pump Flow Check.”
   • Improved Flow: If flow improves to more than 0.24 liters (1/2 pint) in 15 seconds, replace the fuel cap as it may be defective, and then proceed to the “Lift Pump Pressure Check.”
   • Continued Low Flow: If flow remains below 0.24 liters (1/2 pint) in 15 seconds, continue to the next step.
2. Isolate Suction Line: Disconnect the lift pump suction line from the fuel sender unit.
3. External Fuel Source: Connect the suction line to a source of clean fuel, using an additional length of hose. This bypasses the in-tank fuel system to isolate potential issues.
4. Repeat Flow Check: Perform the “Lift Pump Flow Check” again with the external fuel source.
   • Improved Flow with External Source: If flow now exceeds 0.24 liters (1/2 pint) in 15 seconds, remove the fuel sender unit from the tank and inspect it for any restrictions within the fuel pickup or lines.
   • Still Low Flow: If flow remains less than 0.24 liters (1/2 pint) in 15 seconds, proceed to step 5 to further investigate the suction line and pump itself.
5. Suction Line Restriction Check: Inspect the lift pump suction line itself for any kinks, collapses, or external damage that could be causing a restriction.
   • Restriction Found: If a restriction is identified, repair or replace the suction line and re-perform the lift pump flow check.
   • No Restriction: If no visible restriction is found in the suction line, the lift pump itself may be faulty. Replace the lift pump and recheck the lift pump flow.
6. Reattach Suction Line: Once the suction line or pump issues are addressed, reattach the lift pump suction line to the fuel sender.

Lift Pump Pressure Test: Verifying Minimum Fuel Pressure

This test confirms that the lift pump is generating adequate pressure, essential for proper fuel delivery to the injection pump in your 6.5 diesel ’94 C/K truck.

1. Install Tee Adaptor: Install a tee adaptor at the fuel inlet connection of the injection pump.
2. Connect Pressure Gauge: Connect a pressure gauge, capable of reading from 0 to 15 psi (0 to 103kPa), to the tee adaptor.
3. Measure Pressure: Start and run the engine, then observe the fuel pressure reading on the gauge.
   • Sufficient Pressure: If the pressure is at least 3 psi (27 kPa) or higher, proceed to step 4.
   • Insufficient Pressure: If the pressure is below 3 psi (27kPa), consult Chart A-5 in SECTION 3 (Fuel Pump Relay Circuit Diagnosis) (see below) before considering lift pump replacement. This chart provides crucial diagnostic steps for the fuel pump relay circuit, directly relevant to the “94 Pump On Obd2 6.5” keyword focus.
4. Remove Gauge and Adaptor: Carefully remove the pressure gauge and tee adaptor.
5. Reconnect Outlet Pipe: Reconnect the outlet pipe at the lift pump outlet fitting.
6. Clean Spillage: Clean up any fuel spillage that occurred during the test.
7. Leak Check: Run the engine again and check for any fuel leaks at the connections you just worked on.

Fuel System Air Leak Test: Identifying Air Intrusion Points

Air leaks in the fuel system can cause significant driveability problems and are a common issue in older vehicles like the ’94 C/K trucks. This test helps pinpoint air intrusion points.

1. Install Transparent Hose (Inlet): Install a transparent hose between the fuel manager/filter outlet and the injection pump fuel inlet.
2. Observe for Air Bubbles (Inlet): Start and idle the engine, carefully observing the fuel flow 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.
   • Bubbles Present (Inlet): If air bubbles are observed, stop the engine and proceed to step 3 to investigate the lift pump suction line.
3. Suction Line Air Leak Check:
   • Plug Fuel Sender Line: Disconnect the fuel pipe from the fuel sender and plug the sender line to isolate the tank.
   • Vacuum Pump Connection: Disconnect the fuel pipe from the lift pump and connect a hand-held vacuum pump with a gauge to the fuel pipe leading to the tank.
   • Apply Vacuum and Observe: Apply vacuum to the fuel pipe and monitor the gauge reading.
     • Vacuum Holds: If the vacuum reading remains stable and does not drop, reconnect the fuel pipe and proceed to step 4 to check the fuel sender.
     • Vacuum Drops: If the vacuum reading drops, it indicates an air leak in the suction line. Repair the air leak, reinstall the suction line pipe and hose, and retest for air bubbles (step 5).
4. Fuel Sender Air Leak Check:
   • Remove Fuel Tank & Sender: For thorough access, it’s often easiest to remove the fuel tank. Remove the fuel sender from the fuel tank, remove the strainer, and plug the bottom of the pickup tube.
   • Apply Vacuum to Sender: Apply a vacuum to the upper end of the pickup tube on the fuel sender and observe the gauge reading.
     • Vacuum Holds (Sender): If the vacuum reading remains stable, reinstall the fuel sender and fuel tank.
     • Vacuum Drops (Sender): If the vacuum reading drops, the fuel sender itself is leaking air and needs to be replaced. Install a new fuel sender, reinstall the fuel tank, reconnect the fuel pipe, and proceed to step 5.
5. Re-check for Bubbles (Inlet): Start and run the engine again, observing the fuel flow through the transparent hose (inlet) for air bubbles.
   • Bubbles Still Present (Inlet): If air bubbles are still present, stop the engine and re-examine steps 3 and 4 to ensure no leaks were missed.
   • No Bubbles (Inlet): If no air bubbles are now present in the inlet line, stop the engine and proceed to step 6 to check the return line.
6. Reconnect Inlet Hose: Remove the transparent hose from the fuel manager/filter outlet and injection pump inlet, and reconnect the original 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 and the disconnected return line hose.
9. Observe for Air Bubbles (Return): Start and run the engine, observing the fuel flow in the transparent return hose for air bubbles.

Important Notice: A small, intermittent stream of air bubbles during snap acceleration is considered normal.

• No Bubbles (Return): If no excessive air bubbles are present in the return line, proceed to step 10.
• Bubbles Present (Return): If excessive air bubbles are present in the return line, it may indicate an issue within the injection pump itself. Consider injection pump replacement.

10. Stop Engine.
11. Reconnect Return Hose: Remove the transparent hose and reattach the fuel return hose to the injection pump.
12. Clean Spillage.
13. Final Leak Check: Run the engine one last time and check for any fuel leaks at all connections.

Clearing Diagnostic Trouble Codes (DTCs)

As standard practice after completing any fuel system test, fuel filter change, or fuel system repair on your ’94 C/K truck 6.5 diesel (or any vehicle for that matter), and after confirming normal engine operation is restored, it is essential to check for, note down, and most importantly CLEAR any Diagnostic Trouble Codes (DTCs) that may have been triggered during the diagnostic process. This ensures accurate future diagnostics and prevents misinterpretations of old codes.

This guide should provide a solid foundation for diagnosing fuel pump and fuel system issues in your 1994 C/K truck with a 6.5 diesel engine and OBD2 system, specifically addressing concerns around the “94 pump on obd2 6.5”. Remember to always prioritize safety and consult a qualified mechanic if you are unsure about any of these procedures.