K5 car's throttle and fuel nozzle have been cleaned, so the idle speed is unstable and vacillating. What is the reason?

The common reasons are: the timing belt is installed in the wrong position, which changes the opening and closing time of the valves of each cylinder, resulting in inaccurate valve distribution phase and abnormal combustion of each cylinder. There is too much carbon deposit on the valve working face and valve seat ring, and the valve seal is not strict, resulting in inconsistent compression pressure of each cylinder. The cam of the camshaft is worn, and the inconsistent wear of the cams of each cylinder leads to inconsistent air intake of each cylinder. Faults of valve-related parts, such as valve push rod wear or bending, rocker arm wear, valve sticking or air leakage, and valve spring fracture.

I have encountered two cases of intermittent idling jitter caused by valve spring fracture, and the reasons can not be determined by using various instruments. The cause of the failure was discovered only after the valve spring was removed. In addition, for engines equipped with hydraulic tappets, the pressure reducing valve is installed in the oil passage leading to the cylinder head. When the pressure is higher than 300 kPa, open the valve. If the valve is blocked, the hydraulic tappet will be stretched too much due to too high pressure, resulting in the valve not closing normally. There is a large amount of carbon deposit on the back of the intake valve, which makes the newly injected fuel absorb when the car is cold, but it cannot enter the cylinder. Because the mixture is too thin, the fast idle speed of the car is unstable.

Engine cylinder block, piston connecting rod mechanism

These failures will reduce the power of a single cylinder too much, thus making the power of each cylinder unbalanced. Common causes are: the cylinder liner is ablated or damaged, resulting in air leakage between one cylinder or two cylinders; The end clearance of piston ring is too large, butt joint or fracture, and the piston ring loses elasticity; Excessive carbon deposition in piston ring groove; Piston and cylinder wear, cylinder roundness and cylindricity out of tolerance; Because water enters the cylinder, the connecting rod bends and the compression ratio changes; Carbon deposition in combustion chamber will change the compression ratio, which will seriously lead to unstable idle speed.

Other reasons

The dynamic balance of rotating parts such as crankshaft, flywheel and crankshaft pulley is unqualified, the engine footstool is broken and damaged, and the engine bottom guard plate is deformed and collides with the oil pan. These reasons will only lead to engine vibration, and will not affect the speed.