umm mustangs surge becuase of a faulty o2 sensor, or the o2 sensor picking up flase air in the system, its a major ford porblem. check for exhaust leaks from the manifold back to the cat , false air ( air that enters after the combustion chamber) reaches the o2 sensor the computer is programed to richen the fuel mixture when too much 02 is left after combustion the rich signal is sent to the injectors the rpm rises and exhaust pressure rises and the false air cant be drawn in through the leak and the o2 sensor thinks the mixture is really rich so it cuts the fuel the sudden drop in rpm causes negative pressure in the exhast system (vaccum) and air is drawn in through the hole and the mixture is read as lean again. This goes back and forth till the car basicly surges itself to a stall. Most people say the problem is only when the motor is cold, and this is true cause when the motor warms up the exhast pipe expands and often seals the leak if its not too bad.
Also an 02 sensor giving an erratic pattern can cause the same thing. o2 sensor testing is very simple and can be done with a volt meter so if you want info on that gimme a shout at mustanger82@hotmail.com
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