ALTITUDE ADJUSTMENT FOR ENGINE PERFORMANCE
MODELS: ALL 1986 TRUCKS EXCEPT HD DIESEL
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 IMPROVED VEHICULAR PERFORMANCE AND CUSTOMER SATISFACTION ONLY, AND IN NO WAY CONSTITUTE RECERTIFICATION 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 permanent residence in a high altitude county. Procedure II is applicable to High Altitude (NA6) trucks being used primarily in low altitude counties.
The vehicles which do not require adjustments are those with the closed loop (Computer Command Control) system, which is self compensating for altitude. Applicable 1986 engines that require no adjustments are the 2.5L TBI, 2.8L TBI (S/T Trucks), 4.3L TBI (M Van) and the following engines with Califomia Emissions: 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 ball-point pen in the space provided. Label, P/N 14101439 is applicable for low altitude vehicles being used primarily at high altitudes (see Figure 1). Label, P/N 14101440, is applicable for high altitude vehicles being used primarily at low altitude (see Figure No. 2). The labels can be ordered directly from WDDGM.
HIGH ALTITUDE COUNTRIES
The following is a list of high altitude countries:
State of Colorado
Adams Fremont Montezum Alamosa Garfield Montrose Arapahoe Gilpin Morgan Archuleta Grand Otero Boulder Gunnison Ouray Chaffee Hinsdale Park Cheyenne Huerfano Pitkin Clear Creek Jackson Pueblo Conejos Jefferson Rio Blanco Costilla Kit Carson Rio Grande Crowley Lake Roud Custer LaPlata Saguache Delta Larimer San Juan Denver Las Anunas San Miguel Dolores Lincoln Summit Douglas Mesa Teller Eagle Mineral Washington Elbert Moffat Weld El Paso
State of Nevada
Carson City Humboldt Nye Douglas Lander Pershing Elko Lincoln Storey Esmeralda Lyon Washoe Eureka Mineral White Pine
State of New Mexico
Bernalillo Lincoln SanJuan Catron Los Alamos San Miguel Colfax Luna Santa Fe Curry McKinley Sierra De Baca Otero Socorro Grant Mora Taos Guadalupe Rio Arriba Torrance Harding Roosevelt Union Hidalgo Sandoval Valencia
State of Utah
Beaver Iron Sanpete Box Elder Juab Sevier Cache Kane Summit Carbon Millard Tooele Daggett Morgan Uintah Davis Piute Utah Duchesne Rich Wasatch Emery Salt Lake Wayne Garfield San Juan Weber Grand
DEALERS AND CUSTOMER INFORMATION
Owners of the 1986 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 RE-ADJUSTED TO LOW ALTITUDE SPECIFICATION PER THE CORRECT CHASSIS SERVICE MANUAL FOR IMPROVED PERFORANCE. IF RE-ADJUSTED TO LOW ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.
PROCEDURE I
To be used for the following 1986 low altitude (NA5) engines used primarily at high altitudes.
- 4.3L Federal All Except "M" Vans
1. Advance base timing to a maximum of 4 degrees 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 a maximum of 4 degrees 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 degrees.
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 degrees over specification if little or no detonation occurs at high altitude.
2. Set idle speed to specifications in accordance to Emission Control Information Label.
- LIGHT DUTY FEDERAL DIESEL ENGINE (LH6)
The high altitude modification involves the re-calibration of the fuel injection pump to a different fuel rate which will compensate for the lower air density at high altitudes. The injection pump cannot be modified on the vehicle. The following procedure should be used:
1. Remove the injection pump per Service Manual procedures. Be sure to note the relationship of the timing marks on the pump and front housing. The pump must be reinstalled to its exact previous position.
2. Send the pump to an ADS (Association of Diesel Specialists) Service Center for the altitude performance adjustment. The ADS Service Centers will be advised by Stanadyne Corporation Bulletin as to injection pump recalibration procedures.
3. Reinstall the pump per Service Manual procedure. The pump must be installed to its original timing setting. If the original timing relationship was not retained, refer to the appropriate Service Manual.
PROCEDURE II
To be used for the following 1986 high altitude (NA6) engines used primarily at low altitudes.
- 4.3L Federal All Except "M" Vans
1. Retard base timing up to 4 degrees 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 degrees.
3. Reset base idle speed to specification on the Emission Control Information Label.
4. Reset fast idle speed to spedfication on the Emission Control Information Label.
- LIGHT DUTY FEDERAL DIESEL ENGINE (LH6)
Vehicles designed for principal use at high altitude that perform satisfactorily at low altitude without excessive smoke levels need not be adjusted for extended operation at low altitude.
DEALERS AND CUSTOMER INFORMATION
Owners of the 1986 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 RE-ADJUSTED TO HIGH ALTITUDE SPECIFICATION PER THE CORRECT CHASSIS SERVICE MANUAL FOR IMPROVED PERFORMANCE. IF RE-ADJUSTED TO HIGH ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REM
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.
