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 MANUALSCHEVROLET1987V 20 P/U 4WD V8-350 5.7LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSTECHNICAL SERVICE BULLETINSALL TECHNICAL SERVICE BULLETINSENGINE CONTROL - ALTITUDE ADJUSTMENTSPROCEDURE II
1987 Chevrolet V 20 P/U 4WD V8-350 5.7L
Procedure II
1987 Chevrolet V 20 P/U 4WD V8-350 5.7LSECTION Procedure II
All carbureted H.D. Gasoline Powered Vehicles should be adjusted to low altitude specifications.
^ 5.7L (LT9)
1. Reset base timing to specification on the low altitude Emission Control Information Label.
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 Information Label.
^ HEAVY DUTY GASOLINE EMISSION PACKAGES (CARBURETED MODELS ONLY) 4.8L (L25) AND 7.4L (LE8) ENGINES
1. Reset base timing to specification on the low altitude Emission Control Information Label.
2. Reset idle speed to specifications on the low altitude Emission Control Information 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.
^ 5.7L (LT9)
1. Reset base timing to specification on the low altitude Emission Control Information Label.
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 Information Label.
^ HEAVY DUTY GASOLINE EMISSION PACKAGES (CARBURETED MODELS ONLY) 4.8L (L25) AND 7.4L (LE8) ENGINES
1. Reset base timing to specification on the low altitude Emission Control Information Label.
2. Reset idle speed to specifications on the low altitude Emission Control Information 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.
RENDER: 1.0x
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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.