that's just the turbo venting off excess boost.
I had exactly the same issues; sputtering, rough idle, P0303 code. Mine turned out to be a bad plug. Took me a few days to figure it out because the plugs had been running flawlessly for 3k miles. Check the gap on the plugs. Gap them all to 0.026. See if problem goes away. Make sure you keep track of each plug and coilpack. Don't get them mixed up, otherwise it'll take you longer to diagnose the problem. For instance, when you pull the plugs and gap them, you can try putting the #3 plug into the #2 cylinder and see if you get a P0302 code (misfire in #2 cylinder). Pretty good indication that plug is bad. You can also try swapping #3 coilpack with #4 coilpack. P0304 code could mean coilpack is bad. Don't make too many changes at once though. Makes it hard to figure out which change made the difference. And, if your memory is bad as mine, write down every change you make.
No. I figured it out myself. I don't take my cars to the dealer unless I can't find the problem and fix it myself, or I know what's wrong and know I can't fix it myself, or the problem is covered under warranty.
Yeah, it would if the plugs that were in my Stang at the time the factory plugs. Should have mentioned in my original reply that I had replaced the factory plugs with Denso plugs. I like Denso plugs. I've used them for over 20 years and never had one fail before.I was pretty surprised when I figured out it was a bad plug. Put in all new plugs and car has been back to running beautifully now for 2 weeks.
