DIESEL ALTITUDE ADJUSTMENT HIGH ALTITUDE / 6.2L

These adjustments are considered altitude performance adjustments and apply only to in-use vehicles. They should not be used to modify vehicles prior to sale.

1983 trucks equipped with the 6.2L diesel engine for low altitude operation may be modified if operation is changed to high altitude for an extended period or permanently. Dealers in high altitude areas should encourage owners who have moved to high altitude counties to purchase the service adjustment procedure outlined below which is considered an owner maintenance expense. High altitude is defined as 4000 feet and above. Vehicles designed for principal use at high altitude are identified by option number NA6 on Service Parts Identification label. All other vehicles are designed for principal at low altitude.

Operation of the 6.2L diesel at high altitudes without modification can result in excessive emission of black exhaust smoke due to low air density. 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 bulletins detailing the timing procedure to follow.

ALTITUDE ADJUSTMENT LABEL

When LOW ALTITUDE vehicles are adjusted for high altitude operation, a SUPPLEMENTAL emission control information label must be placed next to the existing underhood emission control information label. Wash off the area with soap and water, dry thoroughly, and apply the new label (P/N 14057201).

The label should be ordered as regular parts and accessories through the General motors Warehousing and Distribution Division 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 GMWDD, 6060 West Bristol Road, Flint, michigan 48554, attention Ship Direct Department.

After performing these adjustments, dealers should advise customers that, IF THE VEHICLE IS RETURNED TO CONTINUOUS, LOW ALTITUDE OPERATION, THE PUMP MAY BE RECALIBRATED TO LOW ALTITUDE SPECIFICATION PER THE ABOVE PROCEDURE AND THE SUPPLEMENTAL LABEL REMOVED.

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.