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So, i'm new to this forum and I know that this question has probably been answered a million times but, what would be causing my 1991 5.0 to have surging idle (700-1250 Rpms) to the point where it dies or dies with the slightest touch of the pedal. The first i thought it might be was the IAC, but after replacing it i'm still having the same problems, the car has Heads, cam, and Intake. I was hoping that some the 5.0 experts here on the forum might know what my problem is. Any information is much appreciated guys!


Heres a video url to my car https://www.youtube.com/watch?v=XrkOQKA-9fQ&list=UUi_6AWbgpFRcrjPg00frZpA
 

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Fuel pump diagnosis: rent a pressure gauge from the part store and do a pressure test. Rev the engine and see if your pressure stays at a constant 40psi. It should not drop at ALL. IF so, you have a weak fuel pump. Let me know if you need some one to walk you through the pressure test. It is not hard.

http://www.allfordmustangs.com/foru...el-pump-diagnosis-101-1986-1993-mustangs.html

This was a useless link for your problem, they talk mostly about relays.Sorry. You might not have a problem with yours. Check your pressure and get back with us.
 

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You seem to have the exact same problem I am having but a lot worse here is my video it idles real low then returns to normal except now when cold I start it and dies ethier I gas it or start it a few times again to keep it alive. Let me know what happens but all info I've been searching its leading me to a low pressure pump what codes do you have ? Seems you have check engine light on have you ran the self Diagnostic's?

 

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If you have a voltmeter, you can disconnect the throttle position sensor, and check it. Since the sensor is a rheostat, it can wear out where it spends most of its time, at idle and just above it. There are 3 wires, one is 5volts, one is return or ground, and the other is voltage to the computer. With it disconnected, and a voltmeter in ohms, read across the black wire and the green wire, it should give a smooth linear return as you open and close it. It the reading jumps, it is probably shot. Mine reads about 543 ohms closed, up to 3.95 k ohms wide open.
The computer has no idea where the throttle is at, and does some rpm hunting. If you pulled the codes, the ecu might even tell you tps sensor.
 

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I would encourage the pressure test. You should be able to loan a tool from a part store and get the gauge. There should be a tire valve(schrader valve) on your silver fuel lines near the alternator. The gauge goes on this valve, could not be easier! Old days, you had to put the gauge inline with your fuel line. This is much easier.
 

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I just saw Fairmontdave. We must have posted about the same time. I didnt mean that I suggest the fuel pressure test over what he said.
 
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