Engine Will Now Start & Run Normally At Part Throttle
WARNING: This page does not describe the selected car, but rather 11 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, 2003 Lincoln Town Car, and 2003 Lincoln LS. However, it is still accessible from the selected car via links, so may be relevant.
If engine starts and runs only at part throttle, check Idle Air Control (IAC) system. Go to TEST KE, step 2) under SYSTEM TESTS in SELF-DIAGNOSTICS - EEC-V - CNG, FLEX-FUEL & GASOLINE article.