ALTITUDE ADJUSTMENTS FOR ENGINE PERFORMANCE

THE ADJUSTMENTS SPECIFIED WITHIN THIS BULLETIN ARE INTENDED FOR VEHICLES CURRENTLY IN CUSTOMER USE AND DO NOT APPLY TO NEW VEHICLES IN DEALER INVENTORY. THE ADJUSTMENTS ARE PROVIDED FOR THE PURPOSE OF VEHICULAR PERFORMANCE AND CUSTOMER SATISFACTION OF EMISSIONS SPECIFICATIONS FOR THE INVOLVED VEHICLES. THIS BULLETIN HAS BEEN DIVIDED INTO TWO (2) PROCEDURES. PROCEDURE I IS APPLICABLE TO LOW ALTITUDE (NA5) TRUCKS TAKING UP PERFORMANCE RESIDENCE IN A HIGH ALTITUDE COUNTY. PROCEDURES II IS APPLICABLE TO HIGH ALTITUDE (NA6) TRUCKS BEING USED PRIMARILY IN LOW ALTITUDE COUNTIES. THE ONLY EXCEPTION IS THE CALIFORNIA CLOSED LOOP (COMPUTER COMMAND CONTROL) SYSTEM WHICH IS SELF COMPENSATING FOR ALTITUDE. APPLICABLE CALIFORNIA ENGINES WHICH REQUIRE NO ADJUSTMENT ARE THE 1.9L, 2.8L, 4.3L, 5.0L AND 5.7L. VEHICLE EMISSION CONTROL INFORMATION UPDATE LABEL ANYTIME A VEHICLE IS MODIFIED FOR AN ALTITUDE CHANGE, A LABEL IS NEEDED. THE LABEL SHOULD BE AFFIXED IN A READILY VISIBLE POSITION IN THE ENGINE COMPARTMENT AND WHERE POSSIBLE BESIDE AN EXISTING EMISSION CONTROL INFORMATION (TUNE-UP) LABEL AND SHOULD NOT BE AFFIXED TO ANY EQUIPMENT THAT CAN BE EASILY DETACHED FROM THE VEHICLE. PRIOR TO INSTALLATION OF THIS LABEL, THE DEALER CODE NUMBER AND DATE OF MODIFICATION MUST BE TYPED (WITH OR WITHOUT RIBBON) OR LETTERED WITH BALLPOINT PEN IN THE SPACE PROVIDED. LABEL, P/N 14091953, IS APPLICABLE FOR LOW ALTITUDE VEHICLES BEING USED PRIMARILY AT HIGH ALTITUDES (SEE FIGURE 1). VEHICLE EMISSION CONTROL INFORMATION UPDATE LABEL (P/N 14091953) FIGURE 1 LABEL, P/N 14091954, IS APPLICABLE FOR HIGH ALTITUDE VEHICLES BEING USED PRIMARILY AT LOW ALTITUDE (SEE FIGURE 2). VEHICLE EMISSION CONTROL INFORMATION UPDATE LABEL (P/N 14091954) FIGURE 2 THE LABELS CAN BE ORDERED DIRECTLY FROM WDDGM. DEALERS AND CUSTOMER INFORMATION OWNERS OF THE 1985 LOW ALTITUDE VEHICLES TAKING UP PERMANENT RESIDENCE IN ONE OF THESE HIGH ALTITUDE COUNTIES, SHOULD BE ENCOURAGED TO HAVE THESE ADJUSTMENTS PERFORMED WHEN VEHICLES ARE BROUGHT IN FOR SERVICE. AFTER PERFORMING THESE ADJUSTMENTS, DEALERS SHOULD ADVISE CUSTOMERS THAT, IF THE VEHICLE IS RETURNED TO CONTINUOUS, LOW ALTITUDE OPERATION, THE ENGINE SHOULD BE READJUSTED TO LOW ALTITUDE SPECIFICATION PER THE CORRECT CHASSIS SERVICE MANUAL FOR IMPROVED PERFORMANCE. IF READJUSTED TO LOW ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED. PROCEDURE I TO BE USED FOR THE FOLLOWING 1985 LOW ALTITUDE (NA5) ENGINES USED PRIMARILY AT HIGH ALTITUDES. LIGHT DUTY FEDERAL EMISSION PACKAGES 1.9L FEDERAL 1. ADVANCE BASE TIMING TO A MAXIMUM OF 4 DEGREE OVER SPECIFICATION IF LITTLE OR NO DETONATION OCCURS AT HIGH ALTITUDE. 2. ADJUST IDLE SPEED TO SPECIFICATIONS ON THE EMISSION CONTROL INFORMATION LABEL. 2.8L FEDERAL MANUAL TRANSMISSION 1. ADVANCE BASE TIMING TO A MAXIMUM OF 2 DEGREE OVER SPECIFICATION IF LITTLE OR NO DETONATION OCCURS AT HIGH ALTITUDE. 2. RESET CURB IDLE SPEED TO 700 RPM IN N (NEUTRAL). 3. RESET IDLE STOP SOLENOID TO 1100 RPM IN N (NEUTRAL). 4. RESET FAST IDLE SPEED TO 2300 RPM IN N (NEUTRAL). AUTOMATIC TRANSMISSION 1. RESET BASE TIMING TO 12 DEGREE BTDC (BEFORE TOP DEAD CENTER) IF LITTLE OR NO DETONATION OCCURS AT HIGH ALTITUDE. 2. RESET BASE IDLE SPEED TO 700 RPM IN D (DRIVE). 3. RESET A/C SOLENOID SPEED TO 850 RPM IN D (DRIVE). 4. RESET FAST IDLE SPEED TO 2100 RPM IN N (NEUTRAL). 4.3L FEDERAL A. ADVANCE BASE TIMING TO A MAXIMUM OF 4 DEGREE OVER SPECIFICATION IF LITTLE OR NO DETONATION OCCURS AT HIGH ALTITUDE. 2A. MANUAL TRANSMISSION. RESET IDLES TO ORIGINAL SPECIFICATIONS ON THE EMISSION CONTROL INFORMATION LABEL. 2B. AUTOMATIC TRANSMISSION. SET FAST IDLE SPEED TO 1800 RPM IN N (NEUTRAL). 5.0L/5.7L FEDERAL 1. ADVANCE BASE TIMING TO MAXIMUM OF 4 DEGREE OVER SPECIFICATIONS IF LITTLE OR NO DETONATION OCCURS AT HIGH ALTITUDE. 2A. MANUAL TRANSMISSION. DO NOT ADJUST THE CARBURETOR VACUUM BREAK. 2B. AUTOMATIC TRANSMISSION RESET CARBURETOR VACUUM BREAK TO 26 DEGREE. 3. RESET BASE, A/C SOLENOID (IF PRESENT) AND FAST IDLES TO THE SPECIFICATIONS ON THE EMISSION CONTROL INFORMATION LABEL. HEAVY DUTY GASOLINE EMISSION PACKAGES 1. ADVANCE BASE TIMING TO A MAXIMUM OF 4 DEGREE OVER SPECIFICATION IF LITTLE OR NO DETONATION OCCURS AT HIGH ALTITUDE. 2. SET IDLE SPEED TO SPECIFICATIONS IN ACCORDANCE TO EMISSION CONTROL INFORMATION LABEL. PROCEDURE II TO BE USED FOR THE FOLLOWING 1985 HIGH ALTITUDE (NA6) ENGINES USED PRIMARILY AT LOW ALTITUDES. 1.9L FEDERAL (REFER TO FIGURE 3). 1. REMOVE VACUUM DELAY VALVE AND RUBBER HOSE 'A' CONNECTED WITH WHITE SIDE OF THE DELAY VALVE FROM RUBBER HOSE 'B'. 2. CONNECT RUBBER HOSE 'B' DIRECTLY WITH VACUUM PIPE WHERE THE RUBBER HOSE 'A' WAS CONNECTED. 2.8L FEDERAL MANUAL TRANSMISSION 1. RETARD BASE TIMING AS MUCH AS 2 DEGREE FROM SPECIFICATION IF DETONATION OCCURS AT LOW ALTITUDE. 2. RESET CURB IDLE SPEED TO 700 RPM IN N (NEUTRAL). 3. RESET IDLE STOP SOLENOID TO 1100 RPM IN N (NEUTRAL). 4. RESET FAST IDLE SPEED TO 2300 RPM IN N (NEUTRAL). AUTOMATIC TRANSMISSION 1. RESET BASE TIMING TO 12 DEGREE BTDC (BEFORE TOP DEAD CENTER). 2. RESET CURB IDLE SPEED TO 700 RPM IN D (DRIVE). 3. RESET A/C SOLENOID SPEED TO 850 RPM IN D (DRIVE). 4. RESET FAST IDLE SPEED TO 2100 RPM IN N (NEUTRAL). 4.3L FEDERAL 1. RETARD BASE TIMING UP TO 4 DEGREE FROM SPECIFICATION IF DETONATION OCCURS AT LOW ALTITUDE. 2A. MANUAL TRANSMISSION RESET IDLES TO SPECIFICATIONS ON THE EMISSION CONTROL INFORMATION LABEL. 2B. AUTOMATIC TRANSMISSION 1. SET FAST IDLE SPEED TO 2200 RPM IN N (NEUTRAL). 2. RESET ALL OTHER IDLES TO SPECIFICATIONS ON THE EMISSION CONTROL INFORMATION LABEL. 5.0L/5.7L FEDERAL 1. RESET BASE TIMING TO SPECIFICATION ON THE EMISSION CONTROL INFORMATION LABEL. 2A. MANUAL TRANSMISSION DO NOT ADJUST THE CARBURETOR VACUUM BREAK. 2B. AUTOMATIC TRANSMISSION RESET CARBURETOR VACUUM BREAK TO 24 DEGREE. 3. RESET BASE IDLE SPEED TO SPECIFICATION ON THE EMISSION CONTROL INFORMATION LABEL IN D (DRIVE). 4. RESET FAST IDLE SPEED TO SPECIFICATION ON THE EMISSION CONTROL INFORMATION LABEL IN N (NEUTRAL). DEALERS AND CUSTOMER INFORMATION OWNERS OF THE 1985 HIGH ALTITUDE VEHICLES TAKING UP PERMANENT RESIDENCE IN ONE OF THE LOW ALTITUDE COUNTIES, SHOULD BE ENCOURAGED TO HAVE THESE ADJUSTMENTS PERFORMED WHEN VEHICLES ARE BROUGHT IN FOR SERVICE. AFTER PERFORMING THESE ADJUSTMENTS, DEALERS SHOULD ADVISE CUSTOMERS THAT, IF THE VEHICLE IS RETURNED TO CONTINUOUS, HIGH ALTITUDE OPERATION, THE ENGINE SHOULD BE READJUSTED TO HIGH ALTITUDE SPECIFICATION PER THE CORRECT CHASSIS SERVICE MANUAL FOR IMPROVED PERFORMANCE. IF READJUSTED TO HIGH ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.

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.