Causes of the tachometer's fluctuation during driving.

Although the tachometer fluctuates up and down during driving, it will not affect the driving of the vehicle for the time being, but hearing the roar of the engine fluctuating up and down will make people feel uneasy. The main reasons that cause the engine speed to go up and down during driving are: 1. The failure of charging circuit may be caused by generator failure, battery voltage sensor failure and loose battery pile head. Because the generator needs to charge the battery when the vehicle is running. When the battery can't be charged or is not fully charged, the engine ECU will increase the engine speed and let the generator charge the battery, causing the tachometer to go up and down while driving. 2. There is something wrong with the throttle. The fault of the throttle may be the fault of the throttle position sensor or the fault of the throttle control motor. Throttle position sensor, as its name implies, detects the opening of throttle to control the fuel injection quantity. The function of the throttle control motor is to open the throttle at a certain angle according to the depth of the throttle. When these two components fail, the air intake of the engine will change, causing the tachometer to fluctuate up and down when the vehicle is driving. 3. Failure of PCV valve or carbon canister solenoid valve The PCV valve is also a crankcase forced ventilation valve, which is used to make the combustible mixture that has escaped into the cylinder return to the intake pipe and be sucked into the cylinder for combustion; The function of the carbon canister solenoid valve is to send the gasoline vapor in the fuel tank to the intake pipe and suck it into the cylinder for combustion, thus reducing fuel consumption. When these two valve bodies fail, the fuel entering the cylinder for combustion will deviate from the setting of ECU, causing the tachometer to fluctuate up and down while driving. 4. Carbon deposition in engine throttle and combustion chamber will seriously affect the combustion of mixed gas. Throttle carbon deposition will affect the engine intake; However, carbon deposition in the combustion chamber will affect fuel injection and ignition, which will seriously lead to early combustion and deflagration. This will lead to unstable engine speed.