Following install of an exhaust and intake, I had my 2002 4.6L GT tuned on a dyno a year or so ago. It ran great for many months, then lately started to misfire, gradually getting worse over time to the point where I couldn't drive at a cruising speed without the car jerking. Returning the computer to stock tune did not help. After testing the fuel pressure my mechanic identified a failing fuel pump. Upon replacing the fuel pump, the car started running even more rough, with a lot of booming misfire and "popping" felt and heard through the exhaust. He thinks it is now running rich. I took the car on a 50 mile drive on the highway to try and give the computer a chance to adjust to the new fuel pump, but to no avail. Any ideas? I'm bringing her back to my mechanic in the morning but any guidance from someone with similar experience would be appreciated.

 

check the fuel filter, air filter, MAF sensor for dirt, IAT for dirt/malfunction, and spark plugs. That would be a good start.

If the car ran great on your custom tune for months, its not the tune, put it back in.

 

Bring it to a mechanic who can do more than "think it's running rich" and knows how to properly diagnose the EEC-V system--"rough", "jerking", and "misfire" are described several times in the factory troubleshooting guide; along with the components listed by 2000GT4.6 referenced as possible causes, and pinpoint tests for each.[/align][/align]It sounds like the guy you are going to is guessing, and planning on replacing parts 'til he maybe gets lucky, and in the meantimeyour wallet gets empty and the car doesn't get any better...[/align]

 

+1111