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.
1991 Geo Storm GSi, AutomaticFACTORY OEM SERVICE PROCEDURE
Electronic Ignition Timing Advance
- Electronic Spark Timing (EST) is controlled by ECM. Ignition module sends a reference signal on Yellow/Red wire to ECM when engine is cranking. With engine speed less than 400 RPM, ignition module controls ignition timing.
- With engine speed greater than 400 RPM, ECM applies 5 volts on by-pass line (Yellow/Green wire) to switch timing control to EST mode. In EST mode, ECM controls the ignition timing. If problem exists with EST, a Code 42 will be set. See TESTS W/CODES article in the ENGINE PERFORMANCE Section.
Factory Service Tools
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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.
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