EFFECT OF PROPER ENGINE ADJUSTMENTS ON BRAKE PEDAL

POWER BRAKE PERFORMANCE

Reports of hard brake pedal feel, increased brake pedal effort, or reduced brake effectiveness is often related to low manifold vacuum. A low engine vacuum condition, sometimes caused by a driver who brakes with his left foot while not completely releasing the accelerator pedal, may result in a manifold vacuum supply that is insufficient to replenish the brake booster reserve.

Intake manifold vacuum levels are greatly affected by curb idle speed, A/C idle speed, ignition timing, and carburetor air/fuel mixture ratio. Misadjustments in any of these areas can cause unacceptable decreases in the manifold vacuum level. The vacuum level in the manifold, under the operating conditions specified during the adjustment procedure, must be AT LEAST 12" Hg. Less than 12" Hg may result in reduced power assist, especially under cold fast idle operating conditions.

(When performing the adjustments and checks in this diagnosis, do not use sheet metal screws or ball bearings to plug vacuum lines. Golf tees and stub pencils can be used.)

A. Check the intake manifold vacuum level with a known accurate vacuum gauge. Tbe engine should be at normal operating temperature, tbe A/C switch ON, the compressor disconnected, the cooling fan in the OFF cycle and tbe transmission in Drive.

1. If vacuum is at or above 12" Hg, the condition is caused by something other than low manifold vacuum. Refer to the appropriate service manual for Brake System Diagnosis.

2. If less that 12" Hg of vacuum is available, go to Step B.

B. Check the routing of all vacuum hoses, especially the EGR hose, against either the Emission Control Label or the appropriate service manual. Also make sure that all the hoses are in good condition; free from cracks, kinks, and leaks. Correct as necessary.

1. If corrections are necessary, check their effect on the manifold vacuum following procedures in Step A. If it is now at or above 12"Hg, further diagnosis is not necessary.

2. If no corrections were necessary, or if vacuum is still below 12" Hg, go to Step C.

C. Check the ignition timing. The L-4 timing specification is 10 DEG B.T.C. at 650 RPM in Drive. Two (2) things should be noted here:

First, proper engine timing is essential. Ignition timing which is retarded a small amount can cause unacceptable decreases in manifold vacuum. So accurate timing is a must. Second, the EPA has approved an allowable reset tolerance of +/-2 DEG for vehicles classified low altitude, and +/-4 DEG for high altitude vehicles. In general, maximum vacuum levels are reached when the timing is set as high as possible, without causing severe spark knock, while staying within specification tolerances.

1. If timing adjustments are made, recheck the manifold vacuum as in Step A. If it is now 12" Hg or more, further diagnosis is not necessary.

2. If no timing adjustments are necessary, or if the vacuum is still below specification go to Step D.

D. Check curb idle speed. The curb idle speed specification for the L-4 engine is 650 RPM, with the transmission in Drive, the engine at normal operating temperature, and the cooling fan in the OFF cycle. You must use a known accurate tachometer when setting the idle speed. Use tool J-23040 or an equivalent to calibrate your tachometer. Refer to the Carburetor Adjustment Section for the curb idle speed adjustment procedure.

1. If idle speed adjustment was necesaary, recheck the manifold vacuum as in Step A. If vacuum is now at or above the 12" Hg specification, no further diagnosis is necessary.

2. If no adjustment was necessary, or if the manifold vacuum is still not up to specification, go to Step E.

E. Check the A/C idle speed. This is done with the transmission in Drive, the A/C solenoid engaged, the A/C on, the compressor clutch disconnected, and the cooling fan in the OFF cycle. The EPA has approved lowering the A/C idle speed from 950 RPM to 850 RPM. But, in deciding where in this range to set the speed, driveability, and effect on manifold vacuum are the determining factors. To ensure good driveability, you must make sure the main metering fuel discharge nozzle is flowing. To help insure proper manifold vacuum, you must make sure the EGR valve is not coming in too early. (This can happen if the A/C idle speed is set too high.) The best A/C idle speed setting, in terms of driveability and EGR operation, can be found by following the A/C Idle Speed Adjustment Procedure in the Carburetor Adjustment Section, with the following modifications:

Attach a vacuum gauge to the EGR vacuum hose at the valve. With the transmission in Drive, set the A/C idle speed at 950 RPM. Then, back off the idle speed until no vacuum is present at the gauge but there is still drip from the nozzle, staying within the 850 RPM to 950 RPM specification.

After adjustment, check the intake manifold vacuum, as in Step A.

1. If vacuum is now 12" Hg or more, further diagnosis is not necessary.

2. If the vacuum is still below the 12" Hg specification, go to Step F.

If, during the A/C idle speed adjustment, you:

1. go below the 850 RPM minimum specification and still have EGR vacuum, go to Step F; or,

2. get the idle speed within the 850 to 950 RPM specification with no vacuum at the EGR valve, but there is no drip at the main metering fuel discharge nozzle, check the float level adjustment. (You can use External Float Gauge J-9789-135 to check the float level.) Refer to Float Level Adjustment in the Carburetor Adjustment Section for checking and adjustment procedure. If the float level is correct, go to Step F. If you must adjust the float level, do so, then repeat Step E.

F. If at least 12" Hg vacuum is still not available in the manifold, perform the Propane Enrich Idle Speed Adjustment (refer to the Carburetor Adjustment Section). After performing the Propane Adjustment procedure, check for at least 12" Hg manifold vacuum with the engine at normal operating temperature, the A/C on, the compressor disconnected, and the transmission in Drive.

If:

1. the vacuum is now within specification, further diagnosis is not necessary; or,

2. you do not have 12" Hg of vacuum, disconnect the EGR valve while observing the manifold vacuum gauge. If:

a. manifold vacuum increases to specification, indicating vacuum is available to the EGR valve, you must replace the carburetor throttle body (refer to the appropriate service manual); or,

b. manifold vacuum does not increase, indicating no signal is available to the EGR valve, perform the idle mixture adjustment as described in the service manual using the specifications below.

PROPANE ENRICHED VIN ENGINE BBL. TRANS. USAGE IDLE RPM --- ------ ---- ------ ----- --------

5 2.5 L-4 2 MAN. LOW ALT. 1150* 5 2.5 L-4 2 AUTO. LOW ALT. 750 5 2.5 L-4 2 MAN. CALIF. ** 5 2.5 L-4 2 AUTO. CALIF. **

*CARBURETORS 17059623,'625 ONLY; 1250 RPM **SEE ADJUSTMENT PROCEDURES IN MANUAL

If:

1. the vacuum is now within specification, further diagnosis is not necessary; or

2. you do not have 12" Hg of vacuum, repeat adjustment procedures beginning at Step B.

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.