RUNS ROUGH BETWEEN 25-30 MPH SPEED ONLY

[rovinrivers]

Hi. Haven't been here in a while but sure do value this forum. My 03 GT now has 237,000 miles on it and still going strong. Have owned it since 07. Manual transmission with 6 speeds. But recently as I am shifting up 3rd to 4th (around 25 to 30 mph speed) it feels/sounds rough. Then is gone once at higher speed. First I replaced spark plugs, but no change. Any ideas? I did have #4 injector replaced about a year ago.

[tpollauf]

Based on your brief description, it is ONLY running rough in 4th gear? If so than that gear is on its way out. Then when in 5th or final 6th it is smooth again? More info please

[rovinrivers]

Yes, the roughness is happening in 3rd gear and then disappears once in higher gears. What did you mean when you say "the gear is on its way out".

[SeattleJeremy]

In 3rd and 4th gear between 25mph and 30mph what RPM is the engine at? Are you trying to accelerate at that speed in these gears?

I ask because in my five speed car, 25mph to 30mph is where I shift from first to second. I think you're shifting too early, and lugging the engine.
https://www.roadandtrack.com/car-cultur ... or-engine/

Yes, the roughness is happening in 3rd gear and then disappears once in higher gears. What did you mean when you say "the gear is on its way out".

How does the engine sound/feel between 4k and 5k RPM in third and fourth gear?

[rovinrivers]

You could be right about the lugging which I try not to do. Let me drive it some more and note my RPMs at shift changes and see exactly what I am doing. Thanks.

[zbyers]

Is the CEL on? If so, please provide us with the codes, too.

I am curious that you're in 4th at 25-30mph? I guess I never pay attention, but I think I am barely getting into third at that speed most of the time.

As tpollauf and others suggested, it could be that you're lugging it.