I have a 07 STX when I start it up it runs fine but when I back out of the driveway and go foward it hesitates and misfires and backfires. Then after a few seconds it smooths out and runs fine but traveling at highway speeds it has a light miss. But the funny thing is the check engine doesn't come on and all the gauges read normal. And I run good gas in it and serviced it every 3000 miles. It has a 17000 miles on it. mods are k&n drop in filter and flowmaster 44 muffler. Does anyone have this same problem or any advice?

 

You may want to try and run a higher octaine gasoline, or switch gas brands. Try adding that lucas brand fuel injection cleaner. I put some reg. unleaded a Texaco station one time and it made my truck run funny.