ENGINE ADJUST HIGH ALTITUDE INFORMATION (NA6)
Subject: ALTITUDE PERFORMANCE ADJUSTMENTS FOR 1982 HIGH ALTITUDE (NA6) TRUCKS WITH GASOLINE ENGINES
Most high altitude trucks equipped with a gasoline engines 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 and vehicles equipped with the 2.8L engines and automatic transmission, which meets the altitude requirements without adjustments. Both of the engines are available in the 1982 "S" truck. This bulletin is applicable to high altitude (NA6) trucks which will principally be operated in low altitude areas. A statement on the tune-up label will identify a "high altitude" designed vehicle. Sample is 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 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 14055121.
DEALER AND CUSTOMER INFORMATION
Owners of 1982 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 ABEL SHOULD BE REMOVED.
PROCEDURE
To be used for all the following 1982 high altitude engines used primarily at low altitudes.
Light Duty Federal Emission Packages - 1.9L L4 "S" Truck
o Remove vacuum delay valve from the diverter valve vacuum line (refer to Emission Hose Routing on Emission Control Information Label).
o Adjust idle speed to specifications shown on the Emission Control Information Label.
o The altitude switch/solenoid valve assembly is not removed from automatic transmission equipped vehicles since it will automatically provide carburetor calibration changes in response to barometric pressure changes at lower altitudes.
2.8L - Manual Transmission Only
o Retard ignition timing as much as 40from the basic specification if spark knock occurs at low altitude.
o Adjust the idle speed to basic specifications.
4.1L Engine
o Retard ignition timing as much as 40 from the basic specification if spark knock occurs at low altitude.
o Adjust the idle speed to basic specifications.
o Adjust choke vacuum break from nominal 28deg to 26deg if required to improve cold start at low altitude.
5.0L and 5.7L Engine
o Adjust choke vacuum break from nominal 26deg to 24deg
o On all automatic transmission models, adjust curb idle to 600 RPM in D.
o On all manual transmission models, adjust curb idle speed from 700 RPM in N to 600 RPM in N.
"EXEMPT" VEHICLES
Vehicles which are in the "exempt" category and have been modified to the appropriate high altitude adjustments, but due to customer relocation will now be principally operated in a low altitude area should be readjusted to low altitude specifications and the supplemental high altitude label should be removed. This statement is also applicable to heavy duty gasoline engines which have been modified for high altitude usage. See Bulletin 82-I-43 for additional truck altitude performance adjustment information.
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.
