Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1987SUBURBAN 1/2 TON 4WD V8-454 7.4LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSTECHNICAL SERVICE BULLETINSALL TECHNICAL SERVICE BULLETINSENGINE CONTROL - ALTITUDE ADJUSTMENTSOVERVIEW
1987 Chevrolet Suburban 1/2 Ton 4WD V8-454 7.4L
Engine Control - Altitude Adjustments: Overview
1987 Chevrolet Suburban 1/2 Ton 4WD V8-454 7.4LSECTION Overview
88chevy18
Number: 88-326-6E
Section: 6E
Date: AUGUST, 1988
Subject: ALTITUDE ADJUSTMENTS FOR ENGINE PERFORMANCE
Model and Year: 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/LH6)
TO: ALL CHEVROLET DEALERS
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 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 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:
Number: 88-326-6E
Section: 6E
Date: AUGUST, 1988
Subject: ALTITUDE ADJUSTMENTS FOR ENGINE PERFORMANCE
Model and Year: 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/LH6)
TO: ALL CHEVROLET DEALERS
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 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.
FIGURE 1:
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:
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.