Fuel Pressure: Testing and Inspection
Fuel System DiagnosisIf the fuel supply system is not delivering enough fuel driveability could be greatly affected. If air is being drawn into the fuel injection system a Cranks But Will Not Run, or a Hard Start symptom could exist. If another diagnosis indicates, or if the fuel supply system is suspected of not delivering enough fuel or drawing air, the following systems should be checked:
- Any air leaks or restrictions between the fuel injection pump and the fuel tank will seriously affect pump output.
- Insufficient fuel in the fuel tank.
- Any loose fuel component connections.
- Any hoses and lines that are flattening when the engine is running, or have kinks that would restrict the flow of fuel.
Fuel System Air Leak Check
1. Install the J 44638 Vacuum Gage to the fuel system service port on the right front side of the engine.
2. Remove the air duct from the air cleaner assembly to the turbo inlet.
3. Remove the generator for visibility of the fuel supply lines.
4. Prime the fuel system until 10 psi is indicated on the J 44638.
5. Check for external fuel leaks between the fuel filter assembly and the fuel injection pump.
- If fuel is leaking externally, replace or repair the leaking component.
- If no external fuel leaks are found, go to step 6.
6. Reprime the fuel system until 10 psi is indicated on the J 44638.
- If the fuel pressure drops from 10 psi to 2 psi in less than 1 minute, go to step 24.
- If the fuel pressure drops from 10 psi to 2 psi in more than 1 minute, go to step 7.
7. Disconnect the fuel supply line (2) at the engine.
8. Cap off the fuel supply pipe (2) on the engine with a 1/2 inch rubber plug (3) and clamp.
9. Disconnect the fuel return line (1) at the engine.
10. Install a J 23738-A Mityvac on the fuel return pipe on the engine.
11. Check the fuel system components on top of the engine for vacuum leaks.
11.1. Apply at least 25 inches Hg of vacuum to the fuel return pipe with the J 23738-A and wait more than 1 minute for the system to stabilize.
11.2. Repeat step 11.A 6 times.
11.3. Observe the J 23738-A.
- If the vacuum does not drop below 8 inches within 10 minutes, go to step 12.
- If the vacuum drops below 8 inches within 10 minutes, go to step 22.
IMPORTANT: There is an orifice in the fuel injection pump between the fuel supply and fuel return systems. This will delay the vacuum being pulled from the fuel supply lines.
12. Reconnect the fuel supply line at the engine.
13. Disconnect the chassis fuel line from the short fuel line between the fuel sender and the chassis fuel line in front of the fuel tank (1).
14. Plug the chassis fuel line (1) using a 1/2 inch rubber plug, and clamp the plug.
15. Check the fuel system components on top of the engine for vacuum leaks.
15.1. Apply at least 25 inches Hg of vacuum to the fuel return pipe with the J 23738-A and wait more than 1 minute for the system to stabilize.
15.2. Repeat step 15.A 10 times.
15.3. Observe the J 23738-A.
- If the vacuum does not drop below 8 inches within 10 minutes, go to step 16.
- If the vacuum does drop below 8 inches within 10 minutes, inspect the O-ring sealing surfaces of the engine fuel supply line for damage and correct as necessary. If the sealing surfaces are not damaged, replace the chassis fuel lines and go to step 29.
IMPORTANT: There is an orifice in the fuel injection pump between the fuel supply and fuel return systems. This will delay the vacuum being pulled from the fuel supply lines.
16. Reconnect the fuel line at the fuel tank and the fuel return line at the engine.
17. Install a transparent hose between the fuel injection control module (FICM) outlet and the fuel filter inlet, with a vertical loop to observe the incoming fuel for air bubbles.
18. Install the generator.
19. Prime the fuel system 30 times to remove the air in the fuel system.
20. Start and idle the engine.
IMPORTANT: The engine must be run for at least 10 minutes to allow the fuel system to stabilize and remove all air from the fuel system.
21. Observe the fuel in the transparent hose for air bubbles.
- If there are no air bubbles of any kind entering the transparent hose with the incoming fuel, the fuel suction system is OK. Go to step 29.
- If any air bubbles are entering the transparent hose with the incoming fuel, stop the engine and go to step 23.
22. Check the fuel injection pump and the on-engine fuel suction system for air leakage.
22.1. Disconnect the fuel outlet hose (2) from the outlet pipe (1) of the fuel filter assembly.
22.2. Open the drain on the fuel filter assembly and drain the fuel into a suitable container.
22.3. Close the drain.
22.4. Install a J 23738-A on the outlet port (1) of the fuel filter assembly.
22.5. Apply 15 inches Hg of vacuum to the fuel pipe and observe the J 23738-A.
- If the vacuum does not drop, connect the fuel supply line at the engine and go to step 22.F.
- If the vacuum drops, repair the air leak between the fuel supply line fitting and the fuel filter assembly.
22.6. Install a J 23738-A to the fuel outlet hose (2) from the fuel filter assembly.
22.7. Remove the Air Intake Pipe.
22.8. Remove the supply hose (3) from the fuel injection pump (4) and plug the hose.
22.9. Apply 15 inches Hg of vacuum and observe the reading.
- If the vacuum drops, repair the leak between the fuel filter assembly outlet and the fuel injection pump inlet.
- If the vacuum does not drop, go to step 22.J.
22.10. Disconnect the fuel injection pump return hose (1).
22.11. Cap off the fuel return pipe on the fuel injection pump with a 1/2 inch rubber plug, and clamp the plug.
22.12. Install a J 23738-A to the fuel inlet pipe of the fuel injection pump.
22.13. Apply 20 inches of vacuum and observe the J 23738-A.
- If the vacuum drops, replace the fuel injection pump.
- If the vacuum does not drop, replace the fuel injection pump inlet supply hose (3). Then go to step 29.
23. Check the fuel sender for air leakage.
23.1. Remove the fuel tank.
23.2. Remove the fuel sender from the fuel tank.
23.3. Remove the strainer from the fuel sender and plug the bottom end of the pickup tube.
23.4. Apply 15 inches Hg of vacuum to the upper end of the pickup tube and observe the J 23738-A.
- If the vacuum holds, inspect the O-ring sealing surfaces of the fuel sending unit and the chassis fuel line for damage and correct as necessary. If the sealing surfaces are not damaged, replace the fuel line between the chassis line and the fuel sending unit. Then go to Step 29.
- If the vacuum does not hold replace the fuel sender, reinstall the fuel tank, and go to step 29.
24. Prime the fuel system 30 times or until fuel pressure is 10 psi to remove the air from the fuel system. Air will be forced out of the system within 2 minutes.
25. Remove the hose from the fuel filter assembly outlet fitting and plug the hose.
26. Verify that the J 44638 is installed.
27. Remove the ignition 1 relay.
28. Crank the engine 2-3 times in 15 second intervals and observe the J 44638.
- If the indicates 5 inches Hg of vacuum or more, go to step 6.
- If the indicates less than 5 inches Hg of vacuum, replace the fuel injection pump.
29. Remove the transparent hose if necessary and attach any disconnected components or fuel lines.
30. Prime the fuel system 30 times to remove the air in the fuel hoses
31. Clean any fuel spillage.
32. Start the engine. If the engine starts and stalls, repeat step 30.
33. Run the engine to check for fuel leakage.
34. Verify that the customer concern is corrected. If the customer concern is not corrected, go to Symptoms - Computers and Control Systems. Symptom Related Diagnostic Procedures