I have a 2000 Ford Crown Victoria. For a couple of months, sometimes when I am slowing down, the idle speeds way up, like I'm applying the throttle, but I'm not.I'm braking! The engine's running very fast, and to slow it down, I have to put the car in neutral when slowing down, the engine speeds way up, like I'm applying the throttle, but I'm not. I have to put the car in neutral for the engine racing to stop. The dealer changed the IAC but that did not work. Now they say they have no idea what the problem could be, that it does not return any faults or trouble codes, and it does not act that way when they drive it.
DO YOU HEAR A HISSING SOUND (MAYBE FAINT) AROUND THE BRAKE BOOSTER (UNDER HOOD OR IN CAR) WHILE BRAKES ARE APPLIED AND ENGINE IS RUNNING?
IF SO, EVEN IF NOT SO, THE BOOSTER MAY HAVE A VACUUM LEAK INTERNALLY
ANOTHER TEST YOU MIGHT TRY IS TO REMOVE THE VACUUM FITTING GOING INTO THE BOOSTERCAP IT WITH YOUR FINGER OR OR CAP, ETC.----INSURE IT IS CLOSED OFF COMPLETELYCRANK IT UP AND HAVE YOUR BUDDY APPLY THE BRAKES. DOES IT STILL REV UP? (THE BRAKES WILL BE STIFF, WITHOUT THE LINE CONNECTED. DON'T DRIVE IT LIKE THAT!)
IF IT DOES NOT REV, MOST LIKELY YOU NEED A NEW BOOSTER
AN ENGINE S. UCKING "OUTSIDE AIR" FROM AN OPEN VACUUM LINE, OR A MESSED UP VACUUM COMPONENT WILL SPEED UP THE ENGINE. SOMETIMES A BOOSTER IS "AIR TIGHT" (AS IT SHOULD ALWAYS BE) UNTIL YOU PRESS THE PEDAL AND "OPEN UP" THE LEAK