1999 mustang gt - Stalling

Tsnyder124

New Member
Jan 7, 2019
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Delaware
I have a 1999 mustang gt. When I press the brakes and the car is in neutral, the car will drop RPMS and stall. Sometimes even when the car is sitting the RPMS will drop really low and then rev back up. The car has a check engine light, and the code reads that it is running lean, and my brake light is also on. I replaced the IAC, EGR. I’m thinking there might be a vacuum leak but I searched for it and couldn’t find one.
 
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And also, my passenger side long tube header is rusted a little bit, with some holes. Maybe that could have the O2 sensors mis-reading and that could be the issue, not sure.
^^^THIS would be my vote. The unmetered air in the exhaust will make the PCM "think" the motor is running lean. The PCM will respond by adding fuel. Since the AF mixture isn't really lean, the extra fuel will make the air fuel mixture RICH. Recommend fixing the outstanding DTC before replacing a ton of parts.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

After the first round of repairs, please post the full DTC codes. There are 1,000's of them and they each have a story to tell.
 
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My dads truck which has the 4.6 liter motor was doing the exact same thing. He had an exhaust leak which was throwing the oxygen sensor off and it was adding extra fuel. It would idle up then down then stall. Fix your exhaust leaks before you do anything else otherwise your chasing your tail.