Because the PCM went into default mode when you unplugged it. When you plugged it back in, the PCM tried to adjust and reset. Opening the IAC motor causing to much air to come in and killed the engine. Fairly normal response for the engine. Since you modified the engine, did you modify the PCM? Can't do one with out the other.
Yes I did
What was the modifications? Did you do it? Have a shop do it? Either way it should be put on a scanner with the modifications specs on hand to reference numbers. If you are just trying to modify the PCM with a program and lap top. Good luck to you. They assume people know how computers on cars work. Or people just don't follow instructions. Tell me what was done to the car...
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I suggest you go to the Mustang Owner's forum to get your answer. You modified car presents changes to a basic system that is beyond a general forum like this one. My guess is: since you have a modified car and everything is connected properly, then you probably have modified the car beyond it's pre-set parameters for operation. You need a new chip programming to allow the modified parts to operate with your ECU under the new parameters.
It ran fine with all the mods for a long time I didn't just put them in
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SOURCE: rough idle, dies when trying to give it gas
it'll either be your ignition system (spark plugs, distributor cap, etc) or more likely your fuel system (most likely your fuel pump or your fuel filter)
SOURCE: CAR HAS ROUGH IDLE, THEN DIES, HOWEVER, WILL START
IS THE CHECK ENGINE LIGHT ON?IF SO GET THE CODES READ AND THEY SHOULD LEAD YOU TO THE PROBLEM.IF THERE ARE NO CODES THEN YOU NEAD TO CHECK OR REPLACE THE FUEL FILTER,OR YOU MAY NEED A THROTTLE POSITION SENSOR.
SOURCE: 1997 Mustang Cobra (4V, 4.6L), rough idling, won't stay running
silly question, however did you check the air box for nests. it is not uncommon for rodents to climb in and make themselves at home. i would also say to get a maf from ford. check ALL around the engine compartment and underneath for evidence of rodents and their leavings. youll be very suprised what you may find.cheers
SOURCE: 1999 mazda 626 4cyl runs rough at idle , engine
You may have to dig deeper for a vacuum leak. It's possibly the intake manifold gasket. Try spraying some starting fluid around the intake manifold. Also try disconnecting the brake booster and plugging that hose, the diaphram in the booster could be ruptured, causing a vacuum leak. Check the PCV valve and hose too.
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