Checking Compression Pressure

Before checking compression, check the engine oil level and that the starter motor and battery are all in normal operating condition.
Start the engine and wait until engine coolant temperature reaches 80°C~95°C (176°F~205°F).
Stop the engine and disconnect the spark plug cables.
Remove the spark plugs.
Crank the engine to remove any foreign material in the cylinders.
Attach the compression gauge to a spark plug hole.
Depress the accelerator pedal to fully open the throttle.
Crank the engine and read the gauge.
Standard value: 12.2 kg/cm² (1220 kPa, 170 psi)

Limit: 10.7 kg/cm² (1050 kPa, 149 psi)

Fig 1: Checking Compression Pressure
Courtesy of KIA MOTORS AMERICA, INC.
Repeat steps 6- 8 on all cylinders, ensuring that the pressure differential for each of the cylinders is within the specified limit.
Limit:

Max. 1.0 kg/cm² (100 kPa, 14 psi) between cylinders
If a cylinder's compression or pressure differential is below the specification, add a small amount of oil through the spark plug hole and repeat steps from 6- 9.
1. 1) If the addition of oil brings the compression up, it is possible that there is wear between the piston ring and cylinder wall.
2. If compression remains the same, valve seizure, poor valve seating or a compression leak from the cylinder head gasket are all possible causes.
  Tightening torque
  
  Spark plug: 20-30 Nm (200-300 kg.cm, 14-22 lb.ft)