When I take my foot off the gas, the car jerks and makes a sound like a crumbling plastic bag. I noticed that the transmission fluid is dirty, but was told not to flush because it's a high-mileage car.
I’m not sure about the "plastic bag noise". The jerking upon deceleration could have something to do with the transmission not downshifting properly; this could be due to internal transmission mechanical problems, or a problem with the electronic transmission controls. Flushing or changing the transmission fluid generally doesn’t help if you already have transmission problems. In fact, it usually makes things worse. I suggest transmission fluid exchanges (not flushes) as preventive maintenance for all vehicles. For your vehicle, I recommend that you have a professional, such as one from YourMechanic, inspect your Kia’s transmission, and run a diagnostic inspection of the noise you are hearing.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing