Diagnostic Aids

Enter this test only when all steps under QUICK TEST have been successfully completed and engine still does not start or if directed here from another test or chart. This test is only intended to diagnose:

• Fuel rail pressure circuit.
• Fuel tank pressure (FTP) sensor.
• Signal return (SIG RTN), FTP and FTP power (FTP PWR) circuits.
• Compuvalve.

CAUTION: High FRPREAB value indicates high pressure in system. Use caution when working with components in system.
1. 1) Check FRPREAB PID
   Turn ignition switch to OFF position. Connect New Generation Star (NGS) scan tool. Turn ignition switch to ON position (engine off). Using scan tool, access FRPREAB PID. If FRPREAB PID reading is less than one psi, replace compuvalve. If FRPREAB PID reading is more than 150 psi, go to next step. For all other readings, system is operating correctly at this time. See INTERMITTENT TESTS .
CAUTION: High FRPREAB value indicates high pressure in system. Use caution when working with components in system.
2. 2) Check FRPREAB PID Change
   Turn ignition switch to OFF position. Release ALT fuel system pressure. See FUEL SYSTEM VENTING in REMOVAL, OVERHAUL & INSTALLATION - CNG/LPG (BI-FUEL) article. With scan tool connected, turn ignition switch to ON position (engine off). Access FRPREAB PID. If FRPREAB PID reading is more than 150 psi, replace compuvalve. If FRPREAB PID reading is 150 psi or less, go to next step.
3. 3) Check Fuel Tank Pressure (FTP) Sensor For Opens
   
   Turn ignition switch to OFF position. Disconnect in-line connector C4003. Measure and record resistances between in-line connector C4003 terminals No. 6 and 4, terminals No. 6 and 5, and terminals No. 5 and 4. See Figure. Total the 3 resistance measurements. If resistance total is 10 k/ohms, go to next step. If resistance total is not 10 k/ohms, go to step  8).
4. 4) Check Compuvalve Main Harness
   
   Ensure ignition switch is turned to OFF position. Disconnect bi-fuel relay connector C4026, compuvalve connector C4025 and in-line connector C4030. See Figure, Figure and Figure . Ensure in-line connector C4003 is disconnected. Use COMPUVALVE HARNESS CONTINUITY CHECK  table and check for continuity between specified harness terminals. If continuity is present at specified terminals, go to next step. If continuity is not present at specified terminals, repair open in affected circuit.
   COMPUVALVE HARNESS CONTINUITY CHECK
   
   

   Connector/Terminal No.	Connector C4025 Terminal No.
   C4003/4	4
   C4003/5	6
   C4003/6	8
   C4030/1	43
   C4030/2	42

5. 5) Check Fuel Pump Circuits
   
   Ensure ignition switch is turned to OFF position. Disconnect Inertia Fuel Shutoff (IFS) switch connector and fuel pump assembly connector. Ensure in-line connector C4030 is disconnected. Using FUEL PUMP CIRCUIT CONTINUITY CHECK  table, check continuity between specified terminals. See Figure, Figure and Figure . If continuity exists, reconnect all connectors and go to next step. If continuity does not exist, repair open in affected circuit.
   FUEL PUMP CIRCUIT CONTINUITY CHECK
   
   

   Connector/Terminal No.		Connector/Terminal No.
   Contour
   C4030/3		C4420/3
   C4030/2		C732/5
   F150
   C4019/2		C415/1
   C4030/4		C4302/16
   F350
   C4302/16		C4019/1
   C1045/16		C4019/2

6. 6) Check Power To FTP Sensor
   
   Disconnect in-line harness connector C4003. Turn ignition switch to ON position (engine off). Measure voltage between in-line harness connector C4003 (compuvalve side) terminal No. 4 (White wire) and ground. If voltage is about 5 volts, go to next step. If voltage is not about 5 volts, repair open or short to ground in affected circuit.
7. 7) Check Compuvalve Main Harness
   
   Turn ignition switch to OFF position. Disconnect compuvalve harness connector C4025. Ensure in-line harness connector C4003 is disconnected. Using COMPUVALVE HARNESS CONTINUITY CHECK  table, check continuity between the specified connector terminals. If continuity exists, replace compuvalve. If continuity does not exist, repair open in affected circuit.
   COMPUVALVE HARNESS CONTINUITY CHECK
   
   

   Connector/Terminal No.	Connector/Terminal No.
   C4003/4	C4025/4
   C4003/5	C4025/6
   C4003/6	C4025/8

8. 8) Check FTP Sensor Harness
   
   Ensure in-line harness connector C4003 is disconnected. On F350 with dual tanks, remove LPG sender relay No. 1. On Contour and F150 models, disconnect FTP sensor connector. On F350 models, disconnect LPG sender connector(s). On all models, disconnect compuvalve harness connector C4025. Using FUEL TANK PRESSURE SENSOR HARNESS CONTINUITY CHECK  table, check continuity between FTP sensor (Contour and F150) or LPG sender (F350) and specified connector terminals. If continuity exists, replace FTP sensor (Contour or F150 models) or LPG sender(s) (F350 models). If continuity does not exist, repair open in affected circuit.
   FUEL TANK PRESSURE SENSOR HARNESS CONTINUITY CHECK
   
   

   Connector/Terminal No.				Connector/Terminal No.
   Contour
   C4003/4				C4050/2
   C4003/5				C4050/3
   C4003/6				C4050/1
   F150 (1)
   CNG
   C4003/4				C4025/4
   C4003/5				C4025/6
   C4003/6				C4025/8
   LPG
   C4303/4				C4025/4
   C4303/5				C4025/6
   C4303/6				C4025/8
   F350
   Single Tank
   C4303/4				C4025/4
   C4303/5				C4025/6
   C4303/6				C4025/8
   Dual Tank
   Front Tank
   C4903/4				C4025/4
   C4903/5 (2)				C4025/6
   C4903/6				C4025/8
   Rear Tank
   C4904/4				C4025/4
   C4904/5 (3)				C4025/6
   C4904/6				C4025/8
   (1) Diagnostics can be performed by removing plug from in-line harness connector C4003. (2) Connect jumper wire between LPG sender relay No. 1 connector terminals No. 30 and 87. See Figure. (3) Connect jumper wire between LPG sender relay No. 1 connector terminals No. 30 and 87A. See Figure.