The gases burn the valve and seat and may cause a leak between  these  two  surfaces  even  though  the  original causes of the sticking are eliminated. Completely  disassemble  and  inspect  a  leaking valve. It is subject to a resinous deposit similar to that found in a pressure-actuated air valve. Use a specified cleaning compound to remove the deposit. Be sure the valve stem is not bent. Check the valve and valve seat surfaces  carefully.  Eliminate  scoring  or  discoloration  by lapping  with  a  fine  lapping  compound.  You  may  use jewelers’  rouge  or  talcum  powder  with  fuel  oil  for lapping. From  the  preceding  discussion,  you  have  learned that the air starting system may be the source of many troubles that will prevent an engine from cranking even though it can be barred over. You will avoid a few of these  troubles  by  following  prestarting  and  starting instructions.   One   such   instruction,   sometimes overlooked, is that of opening the valve in the air line. Obviously,  with  this  valve  closed  the  engine  will  not crank. Recheck the instructions for such oversights as a closed  valve,  an  empty  air  storage  receiver,  or  an engaged jacking gear before starting any disassembly. ELECTRIC  START  MALFUNCTION Electric  starting  system  malfunctions  fall  into  the following   categories: 1.  Nothing  happens  when  the  starter  switch  is closed. 2.   The starter motor runs, but it does not engage the engine. 3.   The starter motor engages, but it cannot turn the engine. If   nothing   happens   when   you   close   the   starter switch,  there  is  a  failure  in  the  electrical  system.  The failure  could  be  an  open  circuit  caused  by  broken connections or burned out components. Test the circuit continuity to make sure the relay closes and the battery provides  sufficient  voltage  and  current  to  the  starter circuit. If the circuit is complete, there may be resistance through   faulty   battery   connections.   Considerable current  is  needed  to  operate  the  solenoid  and  starter motor. If the starter runs without engaging, it will produce a distinctive hum or whine. The lack of engagement is usually  caused  by  dirt  or  corrosion,  which  keeps  the solenoid or Bendix gears from operating properly. If the starter motor engages the flywheel ring gear but is not able to turn the engine or cannot turn it quickly enough to obtain starting speed, the cause may be lack of battery power or, more likely, a mechanical problem. If  the  engine  can  be  barred  over,  there  is  excessive friction in the meshing of the starter pinion and the ring gear. Either the teeth are burred, or the starter pinion is out of alignment. Either case would have been preceded by noise the last time the starter was used. A major repair may  be  necessary. Other   problems   and   malfunctions   of   electric starting   systems   are   discussed   in   association   with gasoline engines at the end of this chapter. ENGINE CRANKS BUT FAILS TO START Even when the starting equipment is in an operating condition, an engine may fail to start. Most troubles that prevent  an  engine  from  starting  are  associated  with  fuel and the fuel system. However, defective or inoperative parts or assemblies may be the source of some trouble. Failure  to  follow  instructions  may  be  the  cause  of  an engine failing to start. The corrective action is obvious for such items as leaving the fuel throttle in the OFF position  and  leaving  the  cylinder  indicator  valves  open. If an engine fails to start, follow the prescribed starting instructions and recheck the procedure. Foreign Matter in the Fuel Oil System In the operation of an internal-combustion engine, cleanliness   is   of   paramount   importance.   This   is especially true in the handling and care of diesel fuel oil. Impurities  are  the  prime  source  of  fuel  pump  and injection  system  troubles.  Sediment  and  water  cause wear,  gumming,  corrosion,  and  rust  in  a  fuel  system. Even though fuel oil is generally delivered clean from the  refinery,  handling  and  transferring  increase  the chances that fuel oil will become contaminated. Corrosion often leads to replacement or at least to repair of the part. You must continually take steps to prevent water from accumulating in a fuel system, not only  to  eliminate  the  cause  of  corrosion  but  also  to ensure proper combustion in the cylinders. Centrifuge all  fuel,  and  drain  the  fuel  filter  cases  periodically  to prevent excessive collection of water. Water in fuel will cause irreparable damage to the entire fuel system in a short time. It corrodes the fuel injection   pump,   where   close   clearances   must   be maintained, and also corrodes and erodes the injection nozzles.  The  slightest  corrosion  can  cause  a  fuel injection pump to bind and seize which, if not corrected, 3-27