Why does the car speedometer fluctuate?

[LaAndres]

Fluctuating car speedometer and unstable engine speed may be caused by excessive carbon deposits in areas such as the throttle body, idle speed motor, intake tract, intake valves, combustion chamber, and fuel injectors. It is necessary to clean the fuel injectors and intake manifold, as well as inspect and clean the throttle body. Below are the specific methods: 1. Clean the fuel injectors and intake manifold; 2. Inspect and clean the carbon deposits inside the throttle body and combustion chamber. During cold starts, carbon deposits can absorb a certain amount of the air-fuel mixture, leading to an overly lean mixture at startup, poor mixture atomization, and consequently uneven engine combustion. 3. Issues with the carbon canister or carbon canister solenoid valve can cause an overly rich mixture at startup, making it difficult to start; 4. Check the condition of the spark plugs.