For 1990-2009 cars

### Fuel Metering System

Always start with Powertrain On Board Diagnostic (OBD) System Check . This will reduce diagnosis time and prevent unnecessary parts replacement.

Some failures of this system will result in an Engine Cranks But Will Not Run symptom. If this condition exists, refer to Engine Cranks ut Does Not Run. This will determine if the problem is caused by the ignition system, the PCM, or the fuel pump electrical circuit.

Refer to Fuel Pump Electrical Circuit Diagnosis for the fuel system wiring schematic.

If there is a fuel delivery problem, refer to the Fuel System Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) . The Fuel System Pressure Test diagnoses the fuel injectors, the fuel pressure regulator, and the fuel pump. If a malfunction occurs in the fuel metering system, it usually results in either a rich HO2S signal or lean HO2S signal. 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 than 0%. Refer to DTC P0171 Fuel Trim System Lean or DTC P0172 Fuel Trim System Rich .

### Throttle Body Assembly

(1) | MAF Sensor
(2) | Throttle Body Air Inlet Screen
(3) | Throttle Position Sensor
(4) | IAC Valve

### Idle Air Control (IAC) Valve

The scan tool displays IAC pintle position in counts. 0 counts indicates the PCM is commanding the IAC pintle to be driven all the way into a fully seated position. This is usually caused by a vacuum leak.

The higher the number of counts, the more air is being commanded to bypass the IAC pintle. Refer to Idle Air Control (IAC) System Diagnosis in order to diagnose the IAC system. Refer to Rough, Unstable, or Incorrect Idle and Stalling for other possible causes of idle problems.

### Throttle Position (TP) Sensor

The TP sensor is mounted on the side of the throttle body opposite the throttle lever. It senses the throttle valve angle, and relays that information to the PCM. Knowledge of the throttle angle is needed by the PCM to generate the required injector control signals (pulse). For further information, refer to Information Sensors/Switches Description .

### Mass Air Flow (MAF) Sensor

The MAF sensor is mounted to the front of the throttle body, and is used to measure the amount of air entering the engine. The PCM uses this information to determine the operating condition of the engine and to control fuel delivery. For further information, refer to Information Sensors/Switches Description .

### Fuel System Pressure Test

A fuel system pressure test is part of several of the diagnostic charts and symptom checks. To perform this test, Refer to Fuel System Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) .

### Methylcyclopentadienyl Manganese Tricarbonyl (MMT)

If a vehicle is certified to meet California Emission Standards (indicated on the under-hood tune-up label), it is designed to operate on fuels that meet California specifications. If such fuels are not available in states adopting California emissions standards, a vehicle will operate satisfactorily on fuels meeting federal specifications, but emission control system performance may be affected. The malfunction indicator lamp on the instrument panel may turn on. The vehicle may also fail a smog-check test.

Some gasolines that are not reformulated for low emissions contain an octane-enhancing additive called methylcyclopentadienyl manganese tricarbonyl (MMT). General Motors does not recommend the use of such gasolines. If fuels containing MMT are used, spark plug life may be reduced, and emission control system performance can be affected. The malfunction indicator lamp on the instrument panel can turn on.

To provide cleaner air, all gasolines are now required to contain additives that will help prevent deposits from forming in the engine and fuel system. These additives will allow the emission system to function properly. Therefore, there does not need to be anything added to the fuel. In addition, gasolines containing oxygenates, such as ethers and ethanol, and reformulated gasolines may be available in the area to help clean the air. General Motors recommends that the above mentioned gasolines be used if they comply with specifications described earlier.