Thank you in advance.
I know there are many posts on this already but nothing I have seen has been quite like my issues, I have tried everything I have read so far.
We transplanted a code free good running 3.9 5-speed from a 2004 mustang into a 1999 3.8 5-speed body.
The engine is all 2004 with the exception of the RS exhaust manifold and the intake upper.
We removed the cold air intake from the 99 in favor of the OEM airbox from the 2004.
The RPMs will climb to 3500 between shifts, 2500...

1999 IAC code issue