Will and open header cause my 3.8 mustang not idle car runs no idle
No. You put a header on it, so you are looking for free flowing exhaust, so I assume you must have added hp. to the engine. You still need to fine tune.
Open header will not cause it to not idle.
To add to my previous comment. Unless upstream O2 sensors are not installed. If you have an open header and O2 sensor is installed I would think it should idle, but if the O2 sensor is missing from the equation it is possible it might not idle correctly.
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