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Q: What would cause the car to lose rpm and try to stall when the a.c. turned on? on 2003 Ford Mustang GT

fuel pressure was erratic, around 30psi or less. with A.C. on fuel pressue drops to 20 or less and engine stumbles. Changed fuel pressure sensor and fuel pump. now the fuel pressure is steady 30 psi but when the air is turned on the engine still stumbles like it is running out of fuel. does this car have a fuel pressure regulator? cant find one.
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fuel pressure controlled by fuel pump driver module. if only happens with a/c on, make sure a/c compressor not trying to lock up.
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