The 1990 grand marquis 5.0 has what they call an "AIC" motor. It's located on front left side of throttlebody. It has a PINCLE valve inside that gets dirty. Take it apart and clean with throttle body cleaner and disconnect the battery, wait 10 min, connect battery, start it up and let it idle for 5mins , then take it out for a brisk ride to let the CPU relearn.
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