单项选择题
A.excessive fuel oil temperature
B.long periods of engine overload
C.metallic contact between nozzles and cylinder heads
D.hard carbon deposit and varnish on the nozzles
单项选择题 Critical speed in diesel engines occurs when engine torque pulsations become （）
单项选择题 Air receivers installed in starting air systems are to be （）.
单项选择题 Which of the conditions listed may occur in an operating diesel engine if air pockets form within the cylinder head circulating water passages? （）