What Causes a Sudden Increase in Idle Speed While Driving?
2 Answers
Reasons for a sudden increase in idle speed while driving: 1. Automatic transmission downshifting: For automatic transmission cars in D mode, the computer programming sets the gear-shifting sequence. After downshifting, the idle speed naturally increases to maintain speed continuity, which is a normal phenomenon; 2. Manual transmission cars going downhill: As the speed increases, the vehicle's motion is transmitted to the engine, restraining the engine speed from rising, which is also normal; 3. Cars with clutches may experience varying degrees of idle speed increase when the clutch is disengaged, and turning on the air conditioning can also cause a slight rise in idle speed—both are normal; 4. It cannot be ruled out that the issue stems from car malfunctions or other causes, such as a dirty throttle, excessive carbon buildup, transmission failure, or poor fuel quality. If these abnormal causes are suspected, it is necessary to visit a professional auto repair shop for inspection and maintenance.
Last month while driving in the city, my car's idle speed suddenly surged, with the tachometer jumping sharply to over 2000 RPM, which really scared me. After taking it to the repair shop, the technician said the throttle body was stuck due to heavy carbon buildup, causing restricted airflow, so the ECU compensated by increasing RPM. There was also a small crack in a vacuum hose causing air leaks, allowing extra air into the engine. He also reminded me that faulty sensors like coolant temperature detection errors can cause similar issues, especially common in older vehicles. Now during regular maintenance, I always have them clean the throttle body and inspect the hoses to avoid roadside scares. The lesson is: never ignore unusual engine sounds, address issues immediately for safety and to save on repair costs.
