Fuel System Diagnosis (Dual Tanks)

If the fuel supply system is not delivering enough fuel, or air is being drawn into the fuel injection system, driveability could be greatly affected or a Cranks But Will Not Run 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.
^ Air leaks or restrictions on the suction side of the fuel pump will seriously affect pump output.
^ Make certain that there is sufficient fuel in the tank.
^ Check for leaks at all of the fuel connections from the fuel tank to the injection pump.
^ Tighten any loose connections.
^ With the engine running, check all of the hoses and the lines for flattening or 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. Prime the fuel manager until 10 psi is indicated on the J44638.
3. Check for external fuel leaks between the fuel filter/heater 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 4.
4. 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 15.
^ If the fuel pressure drops from 10 psi to 2 psi in more than 1 minute, go to step 5.
5. Install a transparent hose between the fuel injection control module (FICM) outlet and the fuel filter inlet.
6. Prime the fuel manager 30 times to remove air in the hose.

IMPORTANT: If the engine does not start in the next step, go to step 13.

7. Attempt to start and idle the engine.
8. Observe the fuel for air bubbles.
^ If air bubbles are not present, stop the engine and go to step 14.
^ If air bubbles are present, stop the engine and go to step 9.
9. Check the fuel injection pump suction line for air leakage.




9.1. Disconnect the fuel outlet hose (2) from the outlet pipe (1) of the fuel filter/heater element housing.
9.2. Open the drain on the fuel filter/heater element housing and drain fuel into a suitable container.
9.3. Close the drain.
9.4. Install a J23738-A Hand Held Vacuum Pump (3) on the outlet port (1) of the fuel filter/heater element housing.




9.5. Disconnect the supply line (2) at the engine. Refer to Quick Connect Fitting(s) Service(Metal Collar).
9.6. Cap off the fuel supply line (2) with a 1/2 inch rubber plug (3) and clamp.
9.7. With a J 23738-A, apply 15 inches of vacuum and observe the reading.
^ If the vacuum drops, repair the leak between the supply line fitting and the fuel filter/heater element housing.
^ If the vacuum does not drop, go to step 10.
10. Check the fuel sender for air leakage.
10.1. Remove the front fuel tank. Refer to Fuel Tank Replacement (Pickup) or Fuel Tank Replacement (Cab/Chassis - Primary) or Fuel Tank Replacement (Cab/Chassis)
10.2. Remove the fuel sender from the front fuel tank.
10.3. Remove the strainer and plug the bottom end of the pickup tube.
10.4. Apply a vacuum to the upper end of the pickup tube.
10.5. Observe the vacuum gage.
^ If the vacuum holds replace the fuel line between the front fuel tank and the engine, reinstall the fuel sender and the fuel tank, and go to step 11.
^ If the vacuum does not hold, replace the fuel sender, reinstall the front fuel tank, connect the fuel pipe, and go to step 11.
11. Prime the fuel system 30 times to remove the air from the fuel hoses.
12. Start and run the engine.
13. Observe the fuel for air bubbles.
^ If air bubbles are present, stop the engine and recheck steps 9 and 10.
^ If air bubbles are not present, stop the engine and go to step 14.
14. Remove the transparent hose and reconnect the hose between the filter inlet and the FICM outlet fitting.
15. 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.
16. Remove the hose from the fuel filter/heater assembly outlet fitting and plug the hose.
17. Verify that the J 44638 is installed.
18. Remove the ignition 1 relay.
19. Crank the engine for 15 seconds and observe the J44638.
^ If the J 44638 indicates 5 inches Hg of vacuum or more, go to step 20.
^ If the J 44638 indicates less than 5 inches Hg of vacuum, replace the fuel injection pump. Refer to Fuel Injection Pump Replacement.
20. Stop the engine.
21. Remove the transparent hose and attach the fuel return hose at the injection pump.
22. 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.
23. Clean any fuel spillage.
24. Run the engine to check for fuel leakage.