What are the reasons for the loud engine noise in the Previa?
2 Answers
Carbon deposits and transmission overload operation can cause loud engine noise. Carbon deposits: Carbon deposits can cause incomplete combustion of gasoline, leading to engine shaking. Engine carbon deposits have a significant impact on the car: first, they reduce engine power, resulting in uneven and gradually diminishing power output, making the car feel increasingly sluggish. Second, they increase fuel consumption, adding to your financial burden. Third, they make cold starts difficult, meaning the engine is hard to ignite. In severe cases, combustion chamber carbon deposits can even cause cylinder knocking, producing noises during low-speed acceleration, damaging pistons and crankshafts, and leading to engine overheating, which severely compromises vehicle safety. Transmission overload operation: Transmission overload operation can degrade the quality and friction characteristics of transmission fluid, increasing friction and wear, which leads to transmission noise. At the same time, sludge (impurities or dirt) can also form inside the transmission (e.g., in the valve body), causing issues such as jerking, gear slipping, impact, slipping, abnormal noises, delays, high temperatures, and oil leaks.
After repairing the Previa for so many years, the most common cause of loud engine noise is either an exhaust pipe leak or a damaged muffler, which produces a puffing sound when accelerating. Additionally, aging and slipping engine belts can cause a sharp squealing noise, especially in older cars over ten years old—if the alternator belt is loose, it needs to be replaced immediately. Carbon buildup is another factor; prolonged city driving in traffic jams can lead to deposits, causing incomplete combustion in the cylinders and louder engine operation. Oil-related issues should not be overlooked either—either the wrong viscosity grade was used (too thin), or the oil pump isn’t supplying enough oil, leading to insufficient lubrication. Lastly, the engine mounts should be checked; when the rubber pads age, vibrations transfer to the body, making the cabin feel particularly noisy. All these issues need to be systematically inspected.
