TRUCK ALTITUDE ADJUSTMENTS REQUIRED MODIFICATIONS
Model and Year ALL 1984 TRUCKS WITH GASOLINE ENGINES
TO: ALL CHEVROLET DEALERS
This bulletin has been divided into two (2) procedures. Procedure I is applicable to Low Altitide (NA5) trucks taking up permanent residence in a high altitude county. Procedure II is applicable to High Altitude (NA6) trucks and/or "exempt high altitude" trucks.
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, 4.1L, 5.0L, and 5.7L engines.
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.
The labels can be ordered from GM Parts, P/N 14057201. Labels are no longer supplied through Helm order directly from WDDGM (see Figure 1).
HIGH ALTITUDE COUNTIES
The following is a list of high altitude counties:
State of Colorado
Adams Fremont Montezuma 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 Routt Custer La Plata Saguache Delta Larimer San Juan Denver Las Animas 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 San Juan 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 1984 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 LOW ALTITUDE. IF RE-ADJUSTED TO LOW ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.
PROCEDURE I
To be used for all the following 1984 low altitude (NA5) engines used primarily at high altitudes.
Light Duty Federal Emission Packages
1.9L and 2.0L Federal
1. Advance timing to a maximum of 4 DEG over specification if no or little detonation occurs.
2. Reset idle speed to specs.
5.0L and 5.7L Federal
1. Advance timing to a maximum of 4 DEG over specification if no detonation exists.
2. On vehicles equipped with automatic transmissions, increase choke vacuum break setting from 24 DEG to 26 DEG. Manual transmission vehicles do not require adjustment.
3. Adjust idle speeds to:
- On all 5.0L automatic transmission models, adjust curb idle speed from 500 RPM in D to 600 RPM in D.
- On all 5.7L automatic transmission models, adjust curb idle from 550 RPM in D to 600 RPM in D.
- On all manual transmission models, only adjust A/C solenoid idle speed from 800 PRM in N to 700 RPM in N.
2.8L Federal
1. Advance timing to a maximum of 20 over specification if no or little detonation occurs.
2A. Automatic Transmission.
- Adjust curb idle speed to 700 RPM in D.
2B. Manual Transmission.
- Reset curb idle speed to 700 RPM in N.
3. Reset A/C solenoid speed to 850 in D (automatic) and N (manuals). Fast idle speed to 2100 RPM in P or N -- P (automatic) and N (manual).
4.1L Federal
1. Advance timing to a maximum of 4 DEG over specification if no or little detonation occurs.
2A. Adjust primary choke vacuum break setting from 23 DEG to 25 DEG required to improve cold start at high altitude on automatic transmission models.
2B. Adjust primary choke vacuum break setting from 26 DEG to 28 DEG on manual transmission models if required to improve cold start at high altitude.
3. Adjust idle speed to 600 RPM in D for automatics and N for manuals.
Heavy Duty Federal Emission Packages
1. Advance timing to a maximum of 4 DEG if no detonation exists.
2. Set idle speed to specification.
Exempt Vehicles (see Figure 2 for label sample)
For 1984, all light duty "exempt" trucks are equipped with high altitude (NA6) engines therefore no adjustments are required for operation in high altitude areas.
1984 High Altitude (NA6) Trucks
Most high altitude trucks equipped with a gasoline engine may receive adjustments to improve performance in low altitude areas. The only exceptions are the California vehicles equipped with closed loop (Computer Command Control) system which is self compensating for altitude.
This portion of the bulletin is applicable to high altitude and "exempt" (NA6) trucks which will principally be operated in low altitude areas. A statement on the tune-up label will identify a "high altitude" and/or "exempt" high altitude designed vehicle (see Figures 2 and 3) .
VEHICLE EMISSION CONTROL INFORMATION UPDATE LABEL
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.
The label can be ordered as outlined in GM Standard Parts Catalog #89, P/N 14055121 (see Figure 4).
DEALER AND CUSTOMER INFORMATION
Owners of 1984 high altitude vehicles taking up permanent residence in 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 HIGH ALTITUDE PERFORMANCE. IF RE-ADJUSTED TO HIGH ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.
PROCEDURE II
To be used for all the following 1984 high altitude (NA6) engines used primarily at low altitudes.
Light Duty Federal Emission Packages
1.9L Federal
1. Remove vacuum delay valve from the diverter valve vacuum line. (Refer to Emission Hose Routing on Emission control Information Label.)
2. Adjust idle speed to specifications.
2.0L Engine
1. Retard timing as much as 4 DEG from the basic specification if knock occurs at low altitude.
2. Re-adjust idle speed to specifications.
2.8L Federal
1. Retard ignition timing as much as 2 DEG from specification if spark knock occurs at low altitude.
2A. Automatic Transmission.
- Adjust curb idle speed from 700 RPM in D to 650 RPM in D.
2B. Manual Transmission.
- Reset curb idle speed to 700 RPM in N.
3. Reset A/C solenoid speed to 850 RPM in D automatic and N manual. Fast idle speed to 2100 RPM in P or N automatics and N manuals.
4.1L Federal
1. Retard ignition timing as much as 4 DEG from specification if spark knock occurs at low altitude.
2A. Adjust the primary choke vacuum break from 24 DEG to 22 DEG on automatic transmission models.
2B. Adjust the primary choke vacuum break from 24 DEG to 22 DEG on manual transmission models.
3A. Adjust idle speed from 600 RPM to 525 RPM in D on automatic transmission models.
3B. Adjust idle speed from RPM to 550 RPM in N or manual transsion models.
5.0L and 5.7L Federal
1. Adjust choke vacuum from nominal 26 DEG to 24 DEG on automatic transmission vehicles. Manual transmission vehicles do not require adjustment.
2. Do not adjust curb idle on any models.
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.
