Always start with the Powertrain On Board Diagnostic (OBD) System Check .

The Powertrain OBD System Check table will identify other tables for specific checks, such as:

If there is no fuel pressure check all component and circuit grounds.

Fuel system deposits can cause various driveability problems. Deposits usually occur during hot soaks after key Off. Poor fuel quality or driving patterns such as short trips followed by long cool down periods can cause injector deposits. This occurs when the fuel remaining in the injector tip evaporates and leaves deposits. Leaking injectors can increase injector deposits. Deposits on fuel injectors affect their spray pattern, which in turn could cause reduced power, unstable idle, hard starts and poor fuel economy.

Intake valve deposits can also be related to fuel quality. While most fuels contain deposit inhibitors, some do not and the effectiveness of deposit inhibitors varies by manufacturer. If intake valve deposits occur, fuel may be suspected. These deposits can cause symptoms such as excessive exhaust emissions, power loss and poor fuel economy.

The fuel pump relay is located in the Multi-Use Relay Center. Refer to Driveability and Emissions for the locator view. For diagnosis of the fuel pump relay circuit, refer to DTC P0231 Fuel Pump Feedback Circuit Low Voltage .

If there is a fuel delivery problem, refer to Fuel System Pressure Test . This test diagnoses the fuel injectors, the fuel pressure regulator, and the fuel pump. If a malfunction occurs in the fuel metering system, a rich HO2S signal or lean HO2S signal is a result. This condition is indicated by the HO2S voltage, causing the PCM to change the fuel calculation (fuel injector pulse width) based on the HO2S reading. Changes made to the fuel calculation will be indicated by a change in the long term fuel trim values which can be monitored with a scan tool. Ideal long term fuel trim values are around 0%; for a lean HO2S signal, the PCM will add fuel, resulting in a fuel trim value above 0% and for a rich HO2S signal the PCM will take away fuel, resulting in a fuel trim value below 0%. Some variations in fuel trim values are normal due to power train design. If the fuel trim values are excessively greater or lower then 0%, refer to DTC P0171 Fuel Trim System Lean Bank 1 , DTC P0172 Fuel Trim System Rich Bank 1 , DTC P0174 Fuel Trim System Lean Bank 2 , or DTC P0175 Fuel Trim System Rich Bank 2 .

Always start with the Powertrain On Board Diagnostic (OBD) System Check .

If a malfunction occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the HO2S, and causes the PCM 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 can be monitored by a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. Average fuel trim values are around 0%, but vary slightly from engine to engine.

Important: When using a scan tool to observe 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.

Important: If both fuel trim values are fixed well below 0%, refer to items which can cause the system to run rich. If a driveability symptom exists, refer to the particular symptom in for additional items to check.

Refer to 98 E/Ksp 4.6L Eng Sch - PSP/Coolant Lvl Switch and IAC for an electrical diagram of this circuit.

The Idle Speed Control Check verifies proper IAC valve operation.

Reviewing the Failure Rec. may help diagnose an intermittent failure by showing how long ago and under what conditions a DTC has set.

The numbers below refer to the step numbers on the Diagnostic Table.

3. This check test the IAC valve to make sure that it is moving in and out without binding. When the IAC valve is commanded the PCM adjusts the IAC position and spark advance for a desired RPM.