PROPER ENG. ADJUSTMENT EFFECTS POWER BRAKE PEDAL EFFORT

It is important that service technicians understand the need for properly setting engine adjustments to published specifications to ensure good vehicle performance. All vehicle tune-up label instructions should be carefully followed.

Specific items for attention are:

1. Spark Timing

Improperly set initial timing can result in generally poor engine performance, surge, spark knock, poor fuel economy, etc. Refer to the Vehicle Emission Control Label for proper adjustment procedure and specifications.

2. Idle Speeds

All idle speeds should be set accurately, using only the procedures and specifications on the Vehicle Emission Control Label. The tendency to increase idle speeds above specifications during "break-in" should be avoided.

NOTE: An accurately calibrated tachometer is essential in making a correct idle speed adjustment.

High Idle Settings Can Result In:

1. Poor fuel economy.

2. Engine dieseling.

3. Higher brake pedal effort due to reduced available manifold vacuum for power assist. (Drivers who brake with their left foot prior to completely releasing accelerator pedal will further reduce available vacuum.)

4. Slight tip-in hesitation due to premature EGR opening during acceleration.

Engine initial "break-in" has an effect on idle speeds. Idle speeds will often increase by approximately 10% during the first 1,000 miles of driving. Idle speeds slightly below specifications should be considered normal for a new vehicle and would be expected to increase to specification after initial "break-in". A new dealer-stock vehicle should have idle speeds increased to specified RPM only if an operational problem exists or if the vehicle is clearly out of specification. If the idle speed must be set on a new dealer stock vehicle, it should never be set above speicification.

When setting idle speeds, the throttle should be opened slightly and allowed to snap closed several times after each adjustment to "settle" the adjustment. Slightly opening the throttle is very important when setting "solenoid-activated" idle speeds since many solenoid plungers will not extend unless throttle return pressure is momentarily removed.

NOTE: EGR should not operate at either curb or solenoid-active (A/C "ON") idle settings. EGR at idle will reduce idle quality and idle manifold vacuum, and result in increased brake pedal effort on power brake equipped vehicles.

After idle adjustments are completed, the EGR valve vacuum supply hose should be momentarily disconnected and reconnected, while the engine is at curb idle, and again at solenoid-active idle, to assure that EGR is not being introduced due to high idle speed or an improperly connected EGR system. There should be no RPM changes during this check since EGR should not be operating at specified idle speeds (except fast idle).

A functional check to assure an adequate vacuum supply can be made by checking engine manifold vacuum at the A/C solenoid-activated idle speed setting but with the A/C compressor clutch disengaged. This reading should be greater than 12 inches. If reading is less, check for and correct any vacuum leaks. If no leaks are found and vacuum is still low, run a propane enrichment check and adjust idle mixture if not within specifications. Idle speeds must be reset after any changes.

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.