At any speed, the engine can hear the metal tapping sound of "tap and tap". The sound is continuous and rhythmic. It is clear at idle and medium speed, and noisy at high speed.

When the engine temperature changes or the sound does not change when the fire breaks, the abnormal sound of the valve foot can be determined.
        The valve chamber cover can be removed during inspection. When the engine runs at information speed, the push rod is lifted by hand or a gauge is inserted into the gap between the valve foot to test the valves one by one. When inserted into a valve clearance, the sound disappears or decreases, that is, the valve foot clearance is too large, and should be adjusted according to technical specifications.
[Cause of valve knock abnormal noise failure]
        Adjusted valve clearance changes (lock is not firm, wear between valve rod and driving parts) or is not adjusted properly during maintenance; valve screw wear deviates, lock nut loosening, or valve clearance is too large or inconsistent; seat abrasion grooves, or valve rod and conduit wear too much.
[Fault Diagnosis of Valve Knock Abnormal Sound]
        When the engine is working, listen carefully to the sound and identify the sound location; try to insert a plug ruler into the valve clearance, if the sound decreases or disappears, the valve clearance is too large. At this time, the valve clearance should be adjusted according to the required value. The valve clearance can be judged by the sound and vibration of rocker spring. If the locking nut on the adjusting screw is loose, it should be locked. Serious parts should be replaced.
[troubleshooting of abnormal valve knocking noise]
        According to the specific requirements and regulations of the automobile manufacturer for valve clearance adjustment, attention should be paid to the temperature effect when adjusting.
The temperature of valve rocker arm and valve stem will affect the valve clearance. Generally speaking, adjusting valve clearance in hot engine should be smaller than that in cold engine. Some cars need to adjust in cold engine, and some cars can adjust in hot and cold state, but their clearance values are different. The valve clearance of each cylinder should be adjusted in accordance to avoid the unbalanced operation of the engine in operation. When adjusting the valve clearance, the adjusted valve should be completely closed, and then the adjusted clearance value is accurate.
[Notes for repair of valve knock abnormal sound]
        Before adjusting, pay attention to check the working face of the arm according to the data. In engine work, the arc working face of rocker arm head constantly collides with the end of valve rod and slides. In the case of poor lubrication, it will cause wear and tear out pits. In serious cases, the end of valve rod is stuck in pits and the rocker arm is broken. Therefore, new parts should be repaired or replaced according to the wear condition, so as to avoid affecting the accuracy of adjustment. This is the case. Adjustment of valve clearance is one of the important tasks to be accomplished in engine maintenance. Whether the adjustment is appropriate will directly affect the power and economy of the engine, which deserves the attention of drivers.