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.
1990 Mercury Grand Marquis LSFACTORY OEM SERVICE PROCEDURE
Electrical Vacuum Control Valve
- Electrical vacuum control valve may be either opened or closed at room temperature. It should be reversed (opened to closed or closed to opened) with engine at full operating temperature. While engine is cold, measure continuity across switch terminals. See Fig 1 . Note if switch is open or closed.
- Warm engine to normal operating temperature. Measure continuity across switch terminals again. Continuity should be opposite of cold condition. If switch continuity does not switch, replace valve. Check vacuum function of valve as described in VACUUM CONTROL VALVE above.
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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