My 2003 ford mustang is sputtering and the check engine light is blinking no code
You can get a free scan from Autozone, Oreilys, or Advance auto parts. This should indicate a problem. If the standard test scanner does not verify a problem, the Dealers have more advanced scanners which can capture intermittent problems.
There are so many possibilities that you need more tests to find the problem. Offhand, if this is a V6 engine, there were problems with intake Manifold runners and sandwich gaskets for 2 piece intake manifolds.
Stuck manifold runners could provide lousy low rpm performance and perfect high rpm performance just like you describe whether a V6 or V8. The 2 piece intakes were used on V6 engines and once you get above midrange in Rpm, it could run acceptably at higher Rpm.
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