P0174-FUEL SYSTEM 2/1 LEAN





For a complete wiring diagram Refer to Section 8W.

Theory of Operation


The fuel feedback system will maintain a stoichiometric fuel/air mixture, 14.7:1, by modifying the injector pulse width according to the oxygen content of the exhaust gas. The PCM makes short term and long term fuel corrections to maintain stoichiometric fuel/air ratio for best catalytic converter efficiency. Short term fuel correction is based on upstream O2 sensor output and is designed for quick engine response. The long term fuel correction compensated for variations in the engine specifications, sensor tolerances and component aging and is designed to correct rich and lean conditions over a longer period of time.

  • When Monitored:

    With the engine running in closed loop mode, the ambient/battery temperature above -6.7°C (20°F) and altitude below 2590.8 m (8500 ft).

  • Set Condition:

    If the PCM multiplies short term compensation by long term adaptive and a certain percentage is exceeded for two trips, a freeze frame is stored, the MIL illuminates and a trouble code is stored. Two Trip Fault. Three good trips to turn off the MIL.


Possible Causes
  
RESTRICTED FUEL SUPPLY LINE
  
FUEL PUMP INLET STRAINER PLUGGED
  
FUEL PUMP MODULE
  
O2 SIGNAL CIRCUIT
  
O2 RETURN CIRCUIT
  
O2 SENSOR HEATER OPERATION
  
O2 SENSOR
  
MAP SENSOR OPERATION
  
ECT SENSOR OPERATION
  
ENGINE MECHANICAL
  
FUEL FILTER/PRESSURE REGULATOR (HIGH)
  
PCM
  

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING).



Diagnostic Test

1.ACTIVE DTC

NOTE: Check for contaminants that may have damaged an O2 Sensor: contaminated fuel, unapproved silicone, oil and coolant.






NOTE: It may be necessary to drive the vehicle to meet the conditions to set this DTC, try to repeat the conditions in which the fault originally set by reviewing the Freeze Frame data.



Is the DTC Active or Pending at this time?

Yes
  • Go To 2


No

2.CHECKING FUEL PRESSURE

WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released. Failure to follow these instructions can result in personal injury or death.





NOTE: Fuel pressure specification is 407 kPa +/- 34 kPa (59 psi +/- 5 psi).




Choose a conclusion that best matches your fuel pressure reading.

Within Specification
  • Go To 3


Above Specification


Below Specification
  • Go To 13

    CAUTION: Stop All Actuations.


3.CHECKING THE EXHAUST SYSTEM FOR LEAKS



WARNING: The normal operating temperature of the exhaust system is very high. Never work around or attempt to service any part of the exhaust system until it has cooled. Special care should be taken when working near the catalytic converter. The temperature of the converter rises to a high level after a short period of engine operating time



CAUTION: The air pressure must not exceed 27.6 kPa (4 psi), otherwise engine damage can occur.















Type 1 Leak

Type 2 Leak


Leak Location
  		Repair required if results at 27.6 kPa (4 psi) reveal bubble size:
  
Welded joints
  		Type 1, 1 mm (0.04 of an inch) or greater
  
O2 Sensor seal points
  		Type 1, 1 mm (0.04 of an inch) or greater
  
O2 Sensor boss welds
  		Type 1, 1 mm (0.04 of an inch) or greater
  
Flange / joint connections
  		Type 2, 8 mm (0.3 of an inch) or greater
  
Exhaust Manifold to cylinder head connections
  		Type 2, 8 mm (0.3 of an inch) or greater
  
EGR gasket and tube seal points
  		Type 2, 8 mm (0.3 of an inch) or greater
  





Were any exhaust leaks found?

Yes


No
  • Go To 4

4.O2 SENSOR OPERATION



NOTE: If one of the O2 Sensor's Signal or Return circuit is shorted to ground the scan tool will display all O2 Sensor voltage readings low. The O2 Sensor that is shorted to ground will display a voltage reading near or at 0 volts.


NOTE: If one of the O2 Sensor Signal or Return circuits are shorted to voltage, the scan tool will display all O2 Sensor voltage readings high.


NOTE: After the repairs have been made, verify proper O2 Sensor operation. If all the O2 Sensor voltage readings have not returned to normal, follow the diagnostic procedure for the remaining O2 Sensors.




Is the voltage switching between 2.5 and 3.4 volts?

Yes
  • Go To 5


No
  • Go To 9

5.2/1 O2 SENSOR HEATER OPERATION


NOTE: Wait a minimum of 10 minutes to allow the O2 Sensor to cool down before continuing the test. Allow the O2 Sensor voltage to stabilize at 5.0 volts.


NOTE: Perform the following test on all O2 Sensors.






Does the voltage stay above 4.5 volts?

Yes


No
  • Go To 6

6.MAP SENSOR OPERATION



NOTE: The Barometric Pressure should be approximately equal to the actual barometric pressure. If necessary, compare the value to the value of a known good vehicle of a similar make and model.






NOTE: If engine will not idle, maintain a constant RPM above idle.




Is the scan tool reading for MAP Vacuum within 1 inch of the Vacuum Gauge reading and is the Barometric Pressure reading of the vehicle the approximately the same as actual Barometric Pressure?

Yes
  • Go To 7


No

7.ECT SENSOR OPERATION

NOTE: For this test to be valid, the thermostat must be operating correctly.

NOTE: This test works best if performed on a cold engine (cold soak).




NOTE: If engine coolant temperature is above 82° C (180° F), allow the engine to cool until 65° C (150° F) is reached.





Did the ECT value increase smoothly and did it reach at least 82° C (180° F)?

Yes
  • Go To 8


No

8.ENGINE MECHANICAL














Are there any engine mechanical problems?

Yes


No

9.O2 SENSOR








NOTE: The voltage should drop from 5.0 volts to 2.5 volts with the jumper wire in place.



Did the O2 Sensor voltage change from 5.0 volts to 2.5 volts when the jumper wire was installed?

Yes


No
  • Go To 10

    NOTE: Remove the jumper wire before continuing.


10.O2 SENSOR SIGNAL CIRCUIT


NOTE: The scan tool will display all O2 Sensor voltage readings approximately 5.0 volts when only one O2 Sensor's Signal circuit is shorted to voltage.


NOTE: The scan tool will display one O2 Sensor voltage close to zero and the others will read lower than normal when one O2 Sensor Signal circuit contains excessive resistance.



Is the voltage above 4.8 volts?

Yes


No
  • Check all the O2 Signal circuits for a short to ground, open, or short to voltage. Inspect the O2 Sensor connector and the PCM harness connector. If OK, replace and program the Powertrain Control Module per Service Information.
  • Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

11.(K902) O2 RETURN UPSTREAM CIRCUIT






Is the voltage at 2.5 volts?

Yes


No
  • Check the (K902) O2 Return Upstream circuit for a short to ground, open, or short to voltage. Inspect the O2 Sensor connector and the PCM harness connector. If OK, replace and program the Powertrain Control Module per Service Information.
  • Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

12.PCM

NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.




Were there any problems found?

Yes


No

13.RESTRICTED FUEL SUPPLY LINE

WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released. Failure to follow these instructions can result in personal injury or death.







NOTE: Fuel pressure specification is 407 kPa +/- 34 kPa (59 psi +/- 5 psi).



Is the fuel pressure within specification?

Yes


No
  • Go To 14

    CAUTION: Stop All Actuations.


14.FUEL PUMP INLET STRAINER

WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released.





Is the Fuel Inlet Strainer plugged?

Yes


No

15.FUEL PUMP MODULE


If there are no possible causes remaining, view repair.

Repair