IDLE SPEED CONTROL DIAGNOSIS IDLE AIR CONTROL VALVE

1985 S/T TRUCKS AND M VAN - 2.5L L4 ENGINE WITH ELECTRONIC FUEL INJECTION (EFI)

Incorrect diagnosis and/or misunderstanding of the Idle Speed Control Systems used on 2.5L EFI engines may lead to unnecessary replacement of the Idle Air Control Valve.

Engine idle speed is controlled by the Electronic Control Module (ECM) which changes the idle speed by moving the Idle Air Control Valve. The ECM adjusts idle speed in response to fluctuations in engine load (air conditioning, power steering, electrical loads, etc.) to maintain acceptable idle quality and proper exhaust emission performance. The following is provided to assist the technician to better understand the system and correctly respond to the following customer concems:

1. Rough Idle 2. Low Idle Speed 3. High Idle Speed 4. Warm-up Idle Speed; No "Kickdown"

ROUGH IDLE/LOW IDLE SPEED ------------------------- The ECM will respond to increases in engine load, which would cause a drop in idle speed, by moving the Idle Air Control Valve to maintain proper idle speed. After the induced load is removed the ECM will return the idle speed to the proper level.

During A/C compressor operation (max, bi-level, norm or defrost mode) the ECM will increase idle speed in response to an "A/C-On" signal, thereby compensating for any drop in idle speed due to compressor load. The ECM will also increase the idle speed on 1985 models in response to high power steering loads.

During periods of especially heavy loads (A/C- On plus parking maneuvers) significant effects on idle quality may be experienced. These effects are more pronounced on 4 cylinder engines. Abnormally low idle, rough idle and idle shake may occur if the ECM does not receive the proper signals from the monitored systems.

HIGH IDLE SPEED/NO "KICK-DOWN"

Engine idle speeds as high as 2100 RPM may be experienced during cold starts to quickly raise the catalytic converter to operating temperature for proper exhaust emissions performance. The idle speed attained after a cold start is ECM controlled and will not drop for 45 seconds regardless of drivers attempts to "Kick-Down". On 1985 models, after 45 seconds following cold start, depressing the accelerator pedal will rotate the Throttle Position Sensor, signaling the ECM to reduce the idle speed in programmed, fixed steps.

It is important to recognize that EFI engines have no accelerator pump or choke. Idle speed during warm-up is entirely ECM controlled and cannot be changed by accelerator "Kick-Down" or "Pumping".

DIAGNOSIS

Customer concerns regarding idle speed or idle quality can be successfully diagnosed with information provided in section 6E of the S Truck and M Van Light Duty Service Manuals.

Abnormally low idle speeds are usually caused by an ECM system controlled or monitored irregularity, while the most common cause for abnormally high idle speed is an induction (intake air) leak.

The idle air control valve may occasionally get "lost" and has an ECM programmed method of "relearning" the correct idle position. This reset, when required, will occur the next time the vehicle exceeds 35 mph. At this time the ECM seats the Idle Air Control Valve pintle in the throttle body to determine a reference point and then backs out a fixed distance to maintain proper idle speed. No replacement of the Idle Air Control Valve should be made until this reset procedure has occurred.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.