HIGH ALTITUDE ADJUSTMENTS ADJ. TO IMPROVE PERFORMANCE

MODELS: 1983 TRUCKS WITH GASOLINE ENGINES

Any low altitude truck equipped with a gasoline engine may receive adjustments to improve performance in high altitude areas. The only exception is the California closed loop (Computer Command Control) system which is self compensating for altitude. Applicable California enines which require no adjustment are the 1.9L, 2.8l, 3.1L, 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 equipement 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 should be ordered as regular parts and accessories through the Warehousing and Distribution Division of General Motors system. When ordering these labels via rapid entry, use order type CSD. Orders also may be placed on a PC 66 and mailed directly to WDDGM, 6060 West Bristol Road, Flint, Michigan 48554. Attention Ship Direct Department Label P/N 14057201.

Dealers and Customer Information

Owners of the 1983 low altitude vehciles 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 OPERATIONS, THE ENGINE SHOULD BE READJUSTED 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

To be used for all of the following 1983 low altitude 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 over specification if no detonation exists.

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

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 not 700 RPM (was 600) solenoid speed is now 700 RPM (was 750).

2.8L Federal

A. Automatic Transmission

1. Idle speed should be adjusted from 650 to 700 RPM in D.

B. Manual Transmission

1. Advance timing to a maximum of 2 over specification if no or little detonation occurs.

2. Idle speed should be adjusted from 650 to 700 in N.

4.1L Federal

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

2. Adjust choke vacuum break setting from 26 to 28 if required to improve cold start at high altitude.

3. Adjust idle speeds to:

a. Automatic Transmission - idle speed should be adjusted from 550 to 525 in D on all models except C15 pickups.

C15 Pickups - automatic should be adjusted from 550 to 475 in D.

b. Manual Transmission - idle speed should be adjusted from 500 to 600 in N.

Heavy Duty Federal Emission Packages

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

2. Set idle speed to specification.

Exempt Vehicles (identification sample shown on Page 3)

For 1983, all light duty "exempt" trucks are equipped with high altitude (NA6) engines; therefore, no adjustments are required for operation in high altitude areas.

1983 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. Applicable California engines which require no adjustment are the 1.9L, 2.8L, 4.1L, 5.0L and 5.7L.

This 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. Samples are shown on Page 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 equipemnt 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 should be ordered as regular parts and accessories through the Warehousing and Distribution Division of General Motors system. When ordering these labes via rapid entry, use order type CSD. Orders also may be placed on a PC 66 and mailed directly to WDDGM, 6060 West Bristol Road, Flint, Michigan 48554. Attention Ship Direct Department Label P/N 14055121.

Dealer and Customer Information

Owners of 1983 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

To be used for all the following 1983 high altitude 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 from the basic specificataion if knock occurs at low altitude.

2. Readjust idle speed to specifications.

2.8L Engine

1. Retard timing as much as 4 from the basic specification if knock occurs at low altitude.

2. Adjust idle speed:

a. Automatic Transmission - idle speed should be adjusted from 700 to 650 in D.

b. Manual Transmission - idle speed should be adjusted from 700 to 650 in N.

4.1L Engine

1. Retard timing as much as 4 from the basic specifications if knock occurs at low altitude.

2. Adjust choke vacuum break settin from 28 to 26 if required to improve cold start at low altitude.

3. Adjust idle speeds to:

a. Automatic Transmission - idle speed should be adjusted from 525 to 550 in D on all models except C15 pickups.

C15 Pickup automatic should be adjusted from 475 to 550 in D.

b. Manual Transmission - idle speed should be adjusted from 600 to 550 in N.

5.0L and 5.7L Engine

1. Adjust choke vacuum break from nominal 26 to 24.

2. On all automatic transmission models, adjust curb idle to 600 RPM in D.

3. On all manual transmission models, adjust curb idle speed from 700 RPM in N to 600 RPM in N.

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.