Compression Test

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Valve adjusting shims may also be used on OHC engines  for  the  cam-to-valve  clearance.  To  determine whether  shims  are  required,  measure  the  valve clearance with a feeler gauge. Then, if needed remove or change the shim thickness as necessary. Other OHC engines have an Allen adjusting screw in the cam followers. Turning the screw changes the valve clearance. Always refer to the manufacturer’s manual for detailed instructions. COMPRESSION  TEST A compression test is one of the most common methods for determining the mechanical condition of an engine. It should be done when symptoms (engine miss, rough idle, puffing noise in induction or exhaust) point to  major  engine  problems.  Measure  compression pressures of all cylinders with a compression gauge (fig. 3-76). Then compare them with each other and with the manufacturer's specifications for a new engine. This provides an accurate indication of engine condition. When  gauge  pressure  is  lower  than  normal, pressure is leaking out of the combustion chamber.  Low engine  compression  can  be  caused  by  the  following conditions: BLOWN  HEAD  GASKET  (head  gasket ruptured). PHYSICAL  ENGINE  DAMAGE  (hole  in piston, broken valve, etc.). BURNED VALVED SEAT (cylinder head seat damaged by combustion). BURNED VALVE (valve face damaged by combustion heat). Figure 3-76.—Cylinder compression tester. WORN RINGS OR CYLINDERS (part wear that prevents a ring-to-cylinder seal). VALVE TRAIN TROUBLES (valve adjusted with insufficient clearance. This keeps the valve from fully closing. Also, broken valve spring, seal, or retainer). JUMPED TIMING CHAIN OR BELT (loose or worn chain or belt has jumped over teeth, upsetting valve timing). To perform a compression test on a gasoline engine, use the following procedures: Remove all spark plugs so the engine can rotate easily.  Block  open  the  carburetor  or  fuel injection pump throttle plate. This prevents restricted air flow into the engine. Disable  the  ignition  system  to  prevent  sparks from arcing out of the disconnected spark plug wires.  Usually,  the  feed  wire  going  to  the ignition coil can be removed to disable the system. If  the  engine  is  equipped  with  electronic  fuel injection, it should also be disabled to prevent fuel from spraying into the engine. Check the manufacturer’s manual for specific directions. Screw the compression gauge into one of the spark plug holes. Some gauges have a tapered rubber-end  plug  and  must  be  held  by  hand securely in the spark plug opening until the highest reading is obtained. Crank the engine and let the engine rotate for about   four   to   six   compression   strokes (compression  gauge  needle  moves  four  to  six times). Write down the gauge readings for each cylinder and compare them to the manufacturer’s specifications. The compression test for a diesel engine is similar to that of a gasoline engine; however, do not use the compression gauge intended for a gasoline engine. It can  be  damaged  by  the  high-compression-stroke pressure. A diesel gauge must be used that reads up to approximately 600 psi. To perform a diesel compression test, use the following procedures: Remove all injectors or glow plugs. Refer to the manufacturer’s manual for instructions. 3-45

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