Hi,
Just to double check, this is not a later model Pony with drive by wire, correct?
Any EEC codes? May be present in the absence of a CEL, recommend checking.
This is isolated to an idle issue, the engine otherwise runs & drives normally & doesn’t bog out under varying throttle levels & will stay running at idle by manually holding the throttle open?
Typical issues:
1) IACV idle air bypass valve faulty or gasket behind is leaking.Verify function/seal.
2) Throttle position sensor voltage setting is out of range. Adjust if necessary.
3) Chech for any vacuum lines that may be loose, damaged or landed incorrectly. New gasket used under throttle body on debris-free surfaces torqued to specification to ensure no T.B./Intake vacuum leaks present.
Feel free to post any questions and/or findings.
Good luck!
-John