Fault diagnosis and elimination of unstable speed of automobile idle engine

The speed of idle engine of automobile is unstable.

After starting, the car runs at idle speed, and the engine speed is unstable, sometimes high and sometimes low, and the engine shakes.

[Failure reason]

(1) The idle switch of the throttle position sensor is not turned off.

(2) Idle speed control valve (ISC) is faulty.

(3) The air inlet pipe leaks.

(4) The fuel injector leaks or is blocked.

(5) The exhaust system is blocked.

[Troubleshooting]

(1) Idle switch is not turned off.

Diagnosis: Turn on the air conditioner or turn the steering wheel when idling. If the engine speed does not rise, this fault can be proved.

Troubleshooting: Adjust, repair or replace the throttle position sensor.

(2) Idle speed control valve (ISC) failure

Diagnosis: Check the starting sound of idle speed control valve. If there is no starting sound, the idle speed control valve will fail.

Troubleshooting: Clean or replace the idle speed control valve, and set the basic idle speed with a special decoder.

(3) The air inlet pipe leaks.

Diagnosis: If you hear the sound of air leakage in the intake pipe, it proves that the air intake system is leaking.

Troubleshooting: Find the leakage, reseal or replace the phase parts.

(4) The fuel injector leaks or is blocked.

Diagnosis: Check whether the fuel injector is injected by an external power supply and measure the fuel injection quantity of the fuel injector. If the fuel injector does not inject fuel or the fuel injection quantity exceeds the standard, the fuel injector is faulty.

Troubleshooting: Clean the injectors and check the fuel injection quantity of each injector to ensure that there is no blockage or leakage.

(5) The exhaust system is blocked

Diagnosis: there is a sense of tension during acceleration, which can be determined as a fault.

Troubleshooting: Replace the three-way catalytic converter.