Car miss at 40 and 68 mph

[dj31974]

Hey guys, hope you can shed some light. Just got my car retuned after adding a power pipe. Now the car misses at 40 when in 4th gear and at 67-69 when in 5th gear. When giving throttle it don't miss, run smooth as can be, I am puzzled as to what causes this. It just started days after the tune. Also was wondering what is the best plug for Boosted cars. Running NGK TR6 right now. Was wondering if this is part the cause. Have read on other forums of guys having the same problem sounds like to me and when changed plugs, smoothed out. Can I run the cobra (03-04) motorcraft plug in my 04 mach?

[Sonic Blue 85]

Sounds like whoever tuned it should recheck the tune under light load at the rpm that the problem exists. Sounds like they got it a bit too lean at one spot in the MAF curve. It may not be, but if I tuned it I would prefer to data log the car at those points just to make sure and correct the problem if it was the tune. I do like to tune cars lean under light load myself, sometimes I get them too lean, usualy when I try to gess at the difference in load from the dyno to actualy being on the road. If I tune the car only on the dyno and get it where it looks perfect on the dyno it will always be rich when actualy driving on the road.

[dj31974]

Wow...nice man, I think I will bring it back. Your advice sounds spot on. Thanks a bunch.