INJURED RT. KNEE WHEN LIFTING ENGINE HOOD DURING A PRE-OPERATIONAL INSPECTION OF A FUEL/LUBE TRUCK. THE PREFERRED LOCATION TO RAISE THE HOOD IS DIRECTLY IN FRONT OF THE TRUCK. WHEN EE PULLED THE HOOD FORWARD TO OPEN THE HOOD, IT TRAVELED PAST NORMAL OPEN POSITION FORCING EE TO THE GROUND. THIS WAS DUE TO A RETAINING PIN THAT APPARENTLY VIBRATED OUT OF THE CABLE/SPRING DEVICE.