1、Valve sticking. Gum or carbon deposits on the valve stem will cause valve sticking. Excessive valve-stem clearance speeds up valve deposits.
2、Valve body – #1 ball check or LR switch valve sticking (may also set code 50 or give no reverse)
3、"Another cause of valve sticking is warped stems. This Could result from Overheating, an eccentric seat (which throws side pressure on the valve), or from a cocked spring or retainer (which tends to bend the stem )."
