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 MANUALSCHEVROLET1986C 10 P/U 2WD V8-350 5.7LREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTEMISSION CONTROL SYSTEMSTECHNICAL SERVICE BULLETINSALL TECHNICAL SERVICE BULLETINSENGINE PERFORMANCE - ALTITUDE ADJUSTMENTPROCEDURE II
1986 Chevrolet C 10 P/U 2WD V8-350 5.7L
Procedure II
1986 Chevrolet C 10 P/U 2WD V8-350 5.7LSECTION Procedure II
To be used for the following 1986 high altitude (NA6) engines used primarily at low altitudes.
^ 4.3L Federal All Except "M" Vans
1. Retard base timing up to 4~ from specification if detonation occurs at low altitude.
2a. Manual Transmission
Reset idles to specifications on the Emission Control Information Label.
2b. Automatic Transmission
1. Set fast idle speed to 2200 RPM in N (neutral).
2. Reset all other idles to specifications on the Emission Control Information Label.
^ 5.0L/5.7L Federal
1. Reset base timing to specification on the Emission Control Information Label.
2a. Manual Transmission
Do not adjust the carburetor vacuum break.
2b. Automatic Transmission
Reset carburetor vacuum break to 24~
3. Reset base idle speed to specification on the Emission Control Information Label.
4. Reset fast idle speed to specification on the Emission Control Information Label.
^ LIGHT DUTY FEDERAL DIESEL ENGINE (LH6) Vehicles designed for principal use at high altitude that perform satisfactorily at low altitude without excessive smoke levels need not be adjusted for extended operation at low altitude.
DEALERS AND CUSTOMER INFORMATION Owners of the 1986 high altitude vehicles taking up permanent residence in one of the 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 PERFORMANCE. IF RE-ADJUSTED TO HIGH ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.
^ 4.3L Federal All Except "M" Vans
1. Retard base timing up to 4~ from specification if detonation occurs at low altitude.
2a. Manual Transmission
Reset idles to specifications on the Emission Control Information Label.
2b. Automatic Transmission
1. Set fast idle speed to 2200 RPM in N (neutral).
2. Reset all other idles to specifications on the Emission Control Information Label.
^ 5.0L/5.7L Federal
1. Reset base timing to specification on the Emission Control Information Label.
2a. Manual Transmission
Do not adjust the carburetor vacuum break.
2b. Automatic Transmission
Reset carburetor vacuum break to 24~
3. Reset base idle speed to specification on the Emission Control Information Label.
4. Reset fast idle speed to specification on the Emission Control Information Label.
^ LIGHT DUTY FEDERAL DIESEL ENGINE (LH6) Vehicles designed for principal use at high altitude that perform satisfactorily at low altitude without excessive smoke levels need not be adjusted for extended operation at low altitude.
DEALERS AND CUSTOMER INFORMATION Owners of the 1986 high altitude vehicles taking up permanent residence in one of the 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 PERFORMANCE. IF RE-ADJUSTED TO HIGH ALTITUDE SPECIFICATIONS, THE SUPPLEMENTAL LABEL SHOULD BE REMOVED.
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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.