Is it normal for a short-term fuel trim to swing plus or minus 10% during driving? It happens during every acceleration and deceleration. I've got a new car as my last one had a bad engine.
Yes, this is normal when you accelerate and decelerate. Your vehicle will add fuel at times, and when you take your foot off the gas, it will subtract. This is normal for any vehicle, even my Toyota does this. Once you’re at a steady speed, the trim will usually be around plus or minus 1% to 2%, at 36,000 miles.
It’s the opposite when accelerating and decelerating in short periods, the short-term fuel trim will go up and down more noticeably. That’s why it’s called short-term fuel trim. The more important data to observe is analyzing the long-term fuel trim over time. That gives you a better idea of the overall health of your engine and your fuel injection system.