I recently bought a mildly wrecked 2002 mustang GT to use as a donor car for a 2004 V6. Both cars were 5 speed manual. The 2002 drove great before being swapped into the v6. But once the motor and all the necessary wiring was swapped it runs awful. Very choppy and bogged down. It also stays at around 2300 RPM’s when the clutch is pressed in and when it’s not it accelerates at a slow pace without me touching the gas pedal. I’ve already changed the IAC and the MAF sensor. It should be noted that this problem isn’t prevalent 100% of the time, just very frequently. Any ideas on what’s going wrong?

 

My guess is a vacuum leak. And a big one. Maybe around the injectors or intake someplace.

 

Agree, sounds like a vacuum leak.