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 MANUALSGMC1975MAGNAVAN 6.6L ENG VIN M, AUTOMATIC TRANSREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETUNE-UPTUNE-UP PROCEDURES - V8IDLE MIXTUREPROPANE ENRICHMENT PROCEDURE1978-79 LIGHT DUTY MODELS
1975 GMC Magnavan 6.6L Eng VIN M, Automatic Trans
1978-79 Light Duty Models
1975 GMC Magnavan 6.6L Eng VIN M, Automatic TransSECTION 1978-79 Light Duty Models
- With engine at normal operating temperature, choke fully open and A/C off (if equipped), set parking brake and block drive wheels. Disconnect and plug hoses as directed by engine compartment Emission Control Tune-Up decal.
- Connect tachometer to engine. Disconnect vacuum advance and set timing to specification. Reconnect vacuum advance. Disconnect crankcase ventilation tube from air cleaner. Insert hose with Rubber Stopper (J-26911) from propane valve into PCV tube opening in air cleaner.
- Propane cartridge must be in vertical position. Slowly open propane control valve until maximum engine speed is reached with automatic transmission in Drive and manual transmission in Neutral.NOTE: Too much propane will cause engine speed to drop.
- Observe propane flow meter to ensure propane cartridge is full. With propane flowing, adjust idle speed screw or solenoid to specified enriched RPM. Readjust propane flow to be certain of maximum engine speed and adjust idle speed if necessary.
- Turn off propane. Place transmission shift lever in Neutral and run engine at 2000 RPM for 30 seconds. Let engine return to idle. Place shift lever in Drive (automatic transmission). Check idle speed. If speed meets specifications, idle mixture is correct. Proceed to step 8 ).
- If idle speed is too low, carefully remove cap(s) from mixture screw(s) and back out screws (richen) 78 turn at a time until correct speed is reached. If speed is too high, turn screw(s) in (leaner) 1/8 turn at a time until correct speed is reached.
- Turn propane on again to check maximum engine idle speed. If speed does not meet specifications, readjust idle speed screw or solenoid screw to obtain specified enriched RPM with propane flowing. Turn off propane, place transmission shift lever in Neutral and accelerate engine to 2000 RPM for 30 seconds. Recheck idle speed. Speed should meet specifications. If not, repeat procedure starting with step 6).
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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.