System Check - Idle Air Control (IAC )
Idle Air Control (IAC) ValveThe scan tool displays Idle Air Control (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 System Diagnosis, in order to diagnose the IAC system. Refer to Rough, Unstable, or Incorrect Idle, Stalling, for other possible causes of idle problems.
System Diagnosis - Idle Air Control (IAC)
Rough, Unstable, or Incorrect Idle, Stalling
Throttle Position (TP) Sensor
The Throttle Position (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 Description and Operation / Information Sensors.
Mass Air Flow (MAF) Sensor
The Mass Air Flow (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 Description and Operation / Information Sensors.
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.
System Diagnosis - Fuel System Pressure Test
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 property. 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.