Edmunds Answers

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  • zaken1 03/30/11 2:17 pm PST

    The screw you are talking about is not intended to be used to adjust the idle speed. This screw is machine adjusted at the factory with electronic emission measuring equipment to give the lowest emission readings. The idle speed in this motor is automatically adjusted by the computer; through electronically varying the opening of the idle air control valve. It is sometimes normal for the idle speed to increase to over 1,000 rpm when shifted to neutral. If the idle speed reading you posted is accurate (many factory tachometers are not reliable at idle speed) and the reading was obtained while the transmission was in drive; then the ignition timing should be checked and adjusted. BEFORE ADJUSTING THE TIMING; BE SURE TO FOLLOW THE FACTORY PROCEDURE FOR DISABLING THE ELECTRONIC SPARK ADVANCE (AS POSTED ON THE UNDERHOOD EMISSION LABEL). This will often bring the idle speed back into the normal range. But if someone turned the screw in, and broke the head off in that position; you'll either have drill the screw out and tap the hole to take a larger screw; or find another throttle body at a wrecking yard; or buy a remanufactured throttle body from a parts supplier. Rock Auto sells a remanufactured throttle body (Autoline # F19013) for this vehicle for $282.79, plus shipping and a $50 core deposit.

  • MrShift@Edmunds 03/30/11 5:48 pm PST

    There is no idle adjustment on this model. The idle is controlled by the ECM--so I don't know what screw you are looking at, but that's not your problem. You might want to check your Idle Air Control (IAC) valve. Sounds like it is stuck open. (see below).


    OPERATION
    All engine idle speeds are controlled by the control module through the Idle Air Control (IAC) valve mounted on the throttle body. The control module sends voltage pulses to the IAC motor windings causing the IAC motor shaft and pintle to move "IN" or "OUT" a given distance (number of steps) for each pulse (called counts). Inward movement of the pintle = Decreased RPM =Lower Counts. Outward movement of the pintle = Increased RPM = Higher Counts.

    This movement controls airflow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen using a Tech 1 scan tool. Zero (0) counts correspond to a fully closed passage, while 140 counts or more (depending on the application) corresponds to full flow.

    Actual or "controlled" idle speed is obtained by the control module positioning the IAC valve pintle. Resulting idle speed is generated from the total idle air flow (IAC/passage + PCV + throttle valve + vacuum leaks).

    Controlled idle speed is always specified for normal operating conditions. Normal operating condition is engine coolant temperature in operating range, the A/C is "OFF," manual transmission is in neutral or automatic transmission in drive with proper Park/Neutral Position (PNP) switch adjustment. A high or low engine coolant temperature, or A/C clutch engagement may signal the control module to change the IAC counts.

    The minimum idle speed is set at the factory with a stop screw. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts) from the seat during normal controlled idle operation. The IAC counts will be higher than normal on an engine with less than 500 miles, or an engine operating at high altitude or an engine with an accessory load such as the alternator, A/C, power steering or hydra-boost brakes activated.

    Results of Incorrect Operation
    If the pintle is stuck open, the idle speed will remain high. If the pintle is stuck closed, the idle speed will be too low, and stalling may occur. If the pintle is stuck partially open, the idle speed will be higher than normal, and there will be no response to changes in engine load such as A/C ("ON") or transmission in ("Drive").

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