ALTITUDE PERFORMANCE ENGINE ADJUSTMENTS

MODELS: 1982 LOW ALTITUDE (NA 5) TRUCKS WIT

Any low altitude truck equipped with a gasoline engine may receive performance adjustments to improve performance in high altitude areas. The only exceptions are the California closed loop (Computer Command Control) system which is self compensating for altitude and the 2.8L automatic transmission engines which meet the altitude requirements without adjustment. Both of the engines are available in the 1982 "S" truck.

This bulletin is applicable to low altitude (NA5) trucks, both in use and new trucks classified as "exempt" under the current high altitude regulations. GMC recommends that exempt vehicles should receive the applicable adjustments prior to customer delivery. A statement on the tune-up label will identify an "exempt" vehicle. Sample is shown on page 3. Adjustments to high altitude (NA6) trucks which will be principally operated at low altitude will be provided in a separate bulletin.

VEHICLE EMISSION CONTROL INFORMATION UPDATE LABEL

The label should be affixed in a readily visible position in the engine compartment and where possible beside the 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.

The label can be ordered direct from Helm Industries as outlined in GM Standard Parts catalog #89, P/N 14057201.

DEALER AND CUSTOMER INFORMATION

Owners of 1982 low altitude trucks 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 VEHICLE PERFORMANCE AT LOW ALTITUDE. IF READJUSTED TO LOW ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.

PROCEDURE

To be used for all the following 1982 low altitude engines used primarily at high altitudes.

Light Duty Federal Emission Packages

1.9L - All 2.8L - Manual Trans. Only 4.1L - All

1. Advance timing to a maximum of 4 DEG over specification if no or little detonation occurs.

2. Reset idle speed to specifications.

5.0L and 5.7L Federal

1. Advance timing to a maximum of 4 DEG over specification if no detonation exists.

2. Increase choke vacuum break setting from 24 DEG to 26 DEG.

3. Adjust idle speeds to:

(a) Automatic transmission - idle speed should be adjusted from 500 to 600 in drive.

(b) Manual transmission - curb idle is now 700 RPM (was 600) solenoid speed is now 700 RPM (was 750).

5.0L and 5.7L California

1. Advance timing to maximum of 4 DEG if no detonation exists.

2. Set idle speed to 600 RPM.

Heavy-Duty Emission - Gasoline

1. Advance timing a maximum of 4 DEG if no detonation exists.

2. Set idle speed to specifications.

COUNTIES WHICH ARE DESIGNATED HIGH-ALTITUDE AREAS

ARIZONA MONTANA

Apache Navajo Beaverhead Madison Cochise Yavapai DeerLodge Meagher Coconino Gallatin Park Jefferson Silver Bow COLORADO Judith Basin Wheatland Powell Adams Kit Carson Alamosa Lake NEBRASKA Arapahoe LaPlata Archuleta Larimer Banner Kimball Boulder Las Animas Cheyenne Sioux Chaffee Lincoln Cheyenne Mesa NEVADA Clear Creek Mineral Conejos Moffat Carson City Lyon Costilla Montezuma Douglas Mineral Crowley Montrose Elko Nye Custer Morgan Esmeralda Pershing Delta Otero Eureka Storey Denver Ouray Humboldt Washoe Dolores Park Lander White Pine Douglas Pitkin Lincoln Eagle Pueblo Elbert Rio Grande NEW MEXICO El Paso Routt Fremont Saguache Bernalillo Mora Garfield San Juan Catron Rio Arriba Gilpin San Miguel Colfax Roosevelt Grand Summit Curry Sandoval Gunnison Teller DeBaca San Juan Hinsdale Washington Grand Sans Miguel Huerfano Weld Guadalupe Santa Fe Jackson Rio Blanco Harding Sierra Jefferson Hidalgo Socorro Lincoln Taos IDAHO Los Alamos Torrance Luna Union Bannock Franklin McKinley Valencia Bear Lake Fremont Otero Bingham Jefferson Blaine Lemhi OREGON Booneville Madison Butte Minidoka Harney Lake Camas Oneida Klamath Caribou Power Cassia Teton TEXAS Clark Valley Custer Jeff Davis Hudspeth Parmer

UTAH

Beaver Box Elder Cache Carbon Daggett Davis Duschesne Emery Garfield Grand Iron Juab Kane Millard Morgan Piute Rich Salt Lake San Juan San Pete Sevier Summit Tooele Unitah Utah Wasatch Wayne Weber

WYOMING

Albany Campbell Carbon Converse Fremont Goshen Hot Springs Johnson Laramie Lincoln Natrona Niobrara Park Platte Sublette Sweetwater Teton Vinta Washakie Weston

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.