Ok, so it's an 04GT mustang... it has a CAI and a 75mm thorottle body. This car ran perfect when I bout it with 13k miles, only 2k miles have been put on it. When I first got the car I brought it to a ford dealership and they said it had an 01GT computer in it. I had them put an 04GT computer in it and it lost a ton of power, night and day difference. It still ran great so I bought a diablo sport pred tuner and put their tune on it and it didn't help much with power and it ran great for about a month. Now it is running terrible, on startup the car keeps jumping it's idlespeed from 600rpms-1000 very quickly and it sounds like crap. If you give it gas to try n make it run better it will backfire a bit. After it has been started and running it seems to run and drive fine but with less power than before. Could this be a fouled plug or do you have any ideas?

 

take it to the dealer it may still have factory warranty depending on when the warranty was started.
have you tried taking the tune off and see how it runs? also have you cleaned the eng recently sometimes water will get into a spark plug whole and may cause a miss. is the eng light on ?

 

Aside from the computer and CAI assembly what else has been done...? I'm curious 1) why the car had an 01 computer in it and 2) If the dealer knew that why didn't they either a) swap it before selling it or b) find out the reason for themselves then decide if it was necessary to swap it?

 

It has no factory warrenty left because it has a rebuilt title. I don't believe water could have gotten in the plugs but i'm guessing even tho it only has 13k miles it's still an 04' with original plugs and maybe one could be fouled? I bought it off of ebay so I can't figure out why it had the 01 computer but the dealership inspected the whole car and said nothing was wrong but the ABS sensor and that was what was causing my check engine light. The odd thing is that with the new computer my diablosport tuner won't read codes back and my brother works at auto-zone and their machine wouldn't read the codes off of the new computer either. I'll try resetting the tune tonight and let you know how that goes.

 

Myself and my brother both think resetting to stock tune added power! but it still hasn't helped the idle

 

your idle air valve might be shot. take it off and spray it out with some brake cleaner then give it a shot.

 

Only 15,000 miles but no factory warranty left, a computer that didnt belong to the car, and has a rebuilt title? Somethin aint kosher on that.

Im curious, why does it have a rebuilt title?