Why did jeep herdsman suddenly turn from low speed to high speed?

Hello, according to your description, the engine idle speed control is poor. It should be that there is too much carbon deposit in the throttle valve, which leads to the inability of the engine computer to accurately control the air intake and poor idle speed control. You need to clean the throttle and match it with the computer. I'm happy to answer your question. I hope I can help you. Have a nice trip! If there is something wrong with your car, ask the car mechanic. 4S shop professional technician, 10 minutes to solve.