GM Chevrolet engine vibrates seriously, how to solve?

Fault:

A GM Chevrolet Limousine engine has serious vibration after accelerating and rapidly decelerating with the engine running and transmission in neutral. About half a minute, the vibration disappeared.

 

Diagnosis:

I applied GM Tech 2 diagnostic scanner, an authentic GM software in the Tech2 Flash, supporting for on-board diagnostics on all GM systems 1992 thru current year. What’s more, with TIS2000 software, it can program GM cars before 2007.

 

After testing, there appeared no trouble code. Therefore I went to check engine sensors and actuators, there also found no significant abnormalities. Moreover, the fuel system pressure is normal after measuring. But when I cleaned fuel injectors and idling motor to restart the car, the phenomenon still exists. Having heated engine and measured each cylinder, I found that the pressure of the first cylinder was low, others were normal; when cooling, I found the pressure of the first cylinder is substantially zero. Then I opened the valve-chamber cover and found out the valve springs at the bottom of the first cylinder has been broken, resulting in the oscillation of valve spring when released the accelerator pedal, and the valve was unable to close. When idling, the spring has a certain elasticity to meet the requirements of the opening and closing of valve, thus the car was still capable of stable operation; but when cooling, oil pressure has not come to being, so this phenomenon was more obvious.

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