ALTITUDE ADJUSTMENT FOR ENGINE PERFORMANCE
MODELS: 1987-88 G/R/V 20-30 AND P20-40 W/4.8L(L25) 5.7L (LT9) 7.4L (LE8), AND 6.2L DIESEL (LL4 AND LH6)
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 vehicle performance and customer satisfaction with emissions specifications for trucks with gasoline carbureted fuel systems and 6.2L diesel engines. 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 trucks being used primarily in low altitude counties. The exceptions are fuel injected closed loop computer command controlled engines, which are self compensating for altitude changes.
H.D. Trucks Vehicle Emission Control Information Update Label ------------------------------------------------------------- Anytime a vehide 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 ballpoint pen in the space provided. Label, P/N 10123811 is applicable for low altitude vehicles being used primarily at high altitudes (See Figure 1).
Parts are currently available from GMSPO.
Dealer and Customer Information for Low to High Altitude Operation
Owners of carbureted 1987 and 1988 low altitude vehicles taking up permanent residence in one of the high altitude counties, should be encouraged to have these adjustments performed when vehicles are brought in for service.
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 Gunniuson Ouray Chaffee Hinsdale Park Cheyenne Huerfano Pitkin Clear Creek Jackson Pueblo Conejos Jefferson Rio Blanco Costillo 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 Hunboldt Nye Douglas Lander Perslling Elko Lincoln Storey Esmeraldo Lyon Washoe Eureka Mineral White Pine
State of New Mexico
Bemalillo Lincoln San Juan Catron Los Alamos San Miguel Colfax Luna Santa Fe Curry McKinley Sierra DeBaca Otero Socorro Grant Mora Taos Guadalupe Rio Arriba Torrance Harding Roosevelt Union Idalgo 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 SanJuan Weber Grand
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 PERFORMANCE. IF RE-ADJUSTED TO LOW ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.
Procedure I
To be used for the following carbureted 1987 and 1988 low altitude (NA5) H.D. engines used primarily at high altitudes.
- 5.7L (LT9)
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 ---------------------- Set carburetor vacuum break to 26 degrees.
3. Set base, A/C solenoid (if present) and fast idles to the specifications on Emission Tune-Up Label (P/N 10123811).
- Heavy Duty Gasoline Emission Packages 4.8L (L25) and 7.4L (LE8) (Carbureted Models Only).
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 per Emission Tune-Up Label (P/N 10123811).
- Federal 6.2L Diesel Engine (LL4 and LH6)
The high altitude modification involves the recalibration 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 Bulletin detailing the timing procedure to follow.
4. Install Emission Control Information Label (P/N 10123811).
Dealers and Customer Information for High to Low Altitude Operation
Owners of 1987 and 1988 high altitude vehicles taking up permanent residence in a low altitude county, should be encouraged to have their vehicle adjusted when 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 ADJUSTED TO HIGH ALTITUDE SPECIFICATIONS PER THE CORRECT CHASSIS SERVICE MANUAL, FOR IMPROVED PERFORMANCE.
Procedure II
To be used for all carbureted 1987-88 high altitude (NA6) H.D. Gasoline Powered Vehicles used primarily at low altitude.
- 5.7L (LT9) (VIN Code M)
1. Retard base timing up to 4 degrees from specification if detonation occurs at low altitudes.
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 low altitude Emission Control Information Label.
4. Reset fast idle speed to specification on the low altitude Emission Control Inforrnation Label.
- Heavy Duty Gasoline Emission Packages (Carbureted Models Only 4.8L (L25) and 7.4L (LE8) Engines
1. Retard base timing up to 4 degrees from specification if detonation occurs at low altitudes.
2. Reset idle speed to specifications on the low altitude Emission Tune-Up Label.
- Federal 6.2L Diesel Engine (LH6) and (LL4)
Vehicles designed for principal use at high altitude perform satisfactorily at low altitude without excessive smoke levels and need not be adjusted for extended operation at low altitude.
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.
