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 MANUALSGMC1984RALLY G2500, 5.7 L, STANDARDREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMFEEDBACK CARBURETOR TESTS W/CODESDIAGNOSISCHART A-3, DWELL FIXED OVER 50°
1984 GMC Rally G2500, 5.7 L, Standard
Chart A-3, Dwell Fixed Over 50°
1984 GMC Rally G2500, 5.7 L, StandardSECTION Chart A-3, Dwell Fixed Over 50°
- This test determines whether problem is related to engine or electronics. Normal response is dwell decrease, this indicates that O2 sensor, harness and ECM are okay; problem is a rich engine.NOTE: If engine is very rich, a large air leak may be required to lean mixture. When mixture is lean enough, engine will begin to run rough.
- If plugging the PCV or bowl vent vacuum hose causes the dwell to decrease, that hose leads to the source of the problem.
- This test checks ECM response to a "lean" O2 signal. Normal response to this test is low dwell. No dwell change indicates a defective ECM. This test also eliminates the possibility of an open sensor wire. An open wire would cause open loop operation and may set Code 13.
- This test checks for excessive voltage in O2 line. If under .55 volt, wire and ECM are okay, fault is in O2 sensor. If over .55 volt, wire is shorted to battery voltage or ECM is faulty.
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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.