Some failures of this system will result in an Engine Cranks But Won't Run. If this condition exists, refer to Engine Cranks but Does Not Run . This table will determine if the problem is caused by the ignition system, the ECM, or the fuel pump circuit. If you determine that this is a fuel problem, refer to Injector Circuit Diagnosis , Fuel Pump Electrical Circuit Diagnosis or Fuel System Diagnosis . These tables include the injectors, the pressure regulator, the fuel pump, and the fuel pump relay diagnosis. The fuel system wiring schematic diagram is covered on the facing page of Engine Cranks but Does Not Run and Fuel Pump Electrical Circuit Diagnosis .
If a problem occurs in the fuel metering system, the system usually results in either a rich or lean exhaust condition. The Oxygen Sensor (O2S) senses this condition and causes the ECM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short term fuel trim and long term fuel trim values, which a scan tool can monitor. A short term change to the fuel calculation is indicated by the short term fuel trim value, while a long term change is indicated by the long term fuel trim value. Average short term fuel trim and long term fuel trim values will be around 128 but will vary slightly from engine to engine.
Important: When using a scan tool to observe the short term fuel trim and long term fuel trim values, remember that if the system is in control, no action is required unless a driveability symptom is present.
Listed below are examples of lean and rich conditions with the system in control and out of control.
If both the short term fuel trim and long term fuel trim values are fixed well above 128, refer to DTC 44 Lean Exhaust Indicated for items which can cause a lean system.
If the short term fuel trim and long term fuel trim values are fixed well below 128, refer to DTC 45 Rich Exhaust Indicated for items which can cause the system to run rich. If a driveability symptom exists, refer to Symptoms for additional items to check.
Testing the fuel injector circuit is in Engine Cranks but Does Not Run with additional diagnosis in Injector Circuit Diagnosis .
A fuel injector which does not open may cause a no-start condition. An injector which is stuck partly open, could cause loss of pressure after sitting, so long crank times would be noticed on some engines. Also, dieseling could occur because some fuel could be delivered to the engine after the key is turned OFF.
Testing the pressure regulator is in Fuel System Diagnosis .
If the pressure regulator in the TBI supplies pressure which is too low (below 179 kPa or 26 psi), poor performance could result. If the pressure is too high, excess emissions and unpleasant exhaust odor may result.
The diagnosis of the Idle Air Control (IAC) valve can be found on Idle Air Control System Check table.
If you connect or disconnect the IAC valve with the engine running, the idle RPM may be wrong. In this case, you may reset the IAC valve by turning the ignition switch ON for ten seconds and then OFF.
The IAC valve affects only the idle characteristics of the engine. If it is open fully, too much air will be allowed to the manifold and idle speed will be high.
If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes.
The minimum idle speed is set at the factory with a stop screw. The stop screw should not be adjusted unless a replacement throttle body assembly (that has not been preset at the factory) is installed or the minimum idle speed does not meet specifications.
An incorrect readjustment with a high idle speed will cause the IAC valve pintle to constantly bottom on its seat and may result in early IAC valve failure. An idle speed that is too low may result in a no-start condition in cold weather, a stall after start or a stall during deceleration, because of poor air/fuel distribution through the throttle bore. Throttle valve sticking may also occur.
Vacuum leaks will cause the IAC valve pintle to be stopped closer to the seat or to be closed against its seat in an attempt to maintain controlled idle speed.
Refer to Symptoms for additional fuel control diagnosis.
Refer to Fuel System Diagnosis for fuel pump diagnosis.
An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance. Refer to Fuel System Pressure Test .
Refer to Fuel Pump Electrical Circuit Diagnosis for fuel pump relay diagnosis.
An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The oil pressure switch will turn ON the fuel pump as soon as oil pressure reaches about 2 kPa (4 psi).
Refer to Fuel Pump Electrical Circuit Diagnosis for oil pressure switch diagnosis.
The diagnosis of gasoline odor may be a condition of a leaking fuel feed, or return pipe or hose. Fuel pipes that are pinched, plugged, or mis-routed may cause restricted fuel delivery.
The diagnosis of gasoline odor may be a condition of leaking fuel tank, filler neck, or filler cap or may indicate a malfunction in the fuel balance control system if equipped with dual fuel tanks.
A malfunction of the fuel balance control system may result in a Cranks But Will Not Run condition, caused by an inaccurate fuel gauge. Diagnosis of the fuel gauge can be found in the Electrical Diagnosis section.
Check for correct cable routing, or binding, and correct as necessary.
