procedure varies to some extent, depending on the valve construction. Except  in  emergencies,  it  is  advisable  to  shut  the engine down when troubles cause safety valve popping. Clogged or partially obstructed exhaust ports may also cause the cylinder safety valve to lift. This condition will  not  occur  often  if  proper  planned  maintenance procedures are followed. If it does occur, the resulting increase in cylinder pressure may be enough to cause safety valve popping. Clogged exhaust ports will also cause   overheating   of   the   engine,   high   exhaust temperatures, and sluggish engine operation. You can prevent clogged cylinder ports by removing carbon  deposits  at  prescribed  intervals.  Some  engine manufacturers  make  special  tools  for  port  cleaning. Round wire brushes of the proper size are satisfactory for this work You must be careful in cleaning cylinder ports to prevent carbon from entering the cylinder-bar the engine to such a position that the piston blocks the port. SYMPTOMS OF ENGINE TROUBLE In learning to recognize the symptoms that may help locate the causes of engine trouble, you will find that experience  is  the  best  teacher.  Even  though  written instructions are essential for efficient troubleshooting, the information usually given serves only as a guide. It is very difficult to describe the sensation that you should feel  when  checking  the  temperature  of  a  bearing  by hand; the specific color of exhaust smoke when pistons and rings are worn excessively; and, for some engines, the   sound   that   you   will   hear   if   the   crankshaft counterweights  come  loose.  You  must  actually  work with the equipment to associate a particular symptom with  a  particular  trouble.  Written  information,  however, can save you a great deal of time and eliminate much unnecessary  work.  Written  instructions  will  make detection  of  troubles  much  easier  in  practical  situations. A symptom that indicates that trouble exists may be in the form of an unusual noise or instrument indication, smoke, or excessive consumption or contamination of the lube oil, fuel, or water. Figure 3-43 is a general listing of various trouble symptoms that you may encounter. NOISES The unusual noises that may indicate that trouble exists or is impending may be classified as pounding, knocking, clicking, and rattling. Each type of noise must be associated with certain engine parts or systems that might be the source of trouble. Pounding or hammering is a mechanical knock (not to be confused with a fuel knock). It may be caused by a loose, excessively worn, or broken engine part. Gen- erally,  troubles  of  this  nature  will  require  major  repairs. Detonation  (knocking)  is  caused  by  the  presence  of fuel or lubricating oil in the air charge of the cylinders during  the  compression  stroke.  Excessive  pressures accompany detonation. If detonation is occurring in one or  more  cylinders,  stop  the  engine  immediately  to prevent  possible  damage. Clicking  noises  are  generally  associated  with  an improperly  functioning  valve  mechanism  or  timing gear. If the cylinder or valve mechanism is the source of metallic clicking, the trouble may be due to a loose valve stem and guide, insufficient or excessive valve tappet clearances, a loose cam follower or guide, broken valve springs, or a valve that is stuck open. A clicking in the timing   gear   usually   indicates   that   there   are   some damaged or broken gear teeth. Rattling  noises  are  generally  due  to  vibration  of loose  engine  parts.  However,  an  improperly  functioning vibration  damper,  a  failed  antifriction  bearing,  or  a gear-type  pump  operating  without  prime  are  also possible  sources  of  rattling  noises. When you hear a noise, first make sure it is a trouble symptom. Each diesel engine has a characteristic noise at any specific speed and load. The noise will change with a change in speed or load. As an operator, you must become familiar with the normal sounds of the engine. Investigate all abnormal sounds promptly. Knocks that indicate a trouble may be detected and located by special instruments or by the use of a “sounding bar,” such as a solid  iron  screwdriver  or  bar. INSTRUMENT INDICATIONS As an engine operator, you will probably rely more on the instruments to warn you of impending troubles than  on  all  the  other  trouble  symptoms  combined. Regardless  of  the  type  of  instrument  being  used,  the indications   are   of   no   value   if   the   instrument   is inaccurate.   Be   sure   an   instrument   is   accurate   and operating  properly  before  you  accept  a  low  or  high reading.  Test  all  instruments  at  specified  intervals  or whenever you suspect them of being inaccurate. 3-40