Mitsubishi 3000GT 1991, Service Manual

Page: 65 / 1146

ENGINE Service Adjustment Procedures
COMPRESSION PRESSURE CHECK
(1) Before checking compression, ensure that engine oil, the
starter motor, and battery are all in normal operating
condition.
(2) Start the engine and wait until engine coolant temperature
has risen to 80 95°C (176 205°F).
(3) Remove the all spark plugs.
(4) Crank the engine to remove any foreign objects in the
cylinders.
Caution
Cover the spark plug holes with shop towel, etc., in
order to keep expelled foreign objects from flying out,
and keep away from the holes. When measuring
compression with water, oil, or fuel having entered the
cylinder through a crack, etc., these will come flying out
of the spark plug hole hot and fast, so be sure to take
the proper precautions.
(5) Set the compression gauge to the spark plug hole.
(6) Holding the throttle valve full open, crank the engine and
measure compression.
Limit:
<Non-Turbo>
min. 980 (139 psi)
<Turbo>
min. 810 (115 psi)
(7) Perform (5) and (6) above for all the cylinders, ensuring that
compression pressure differential for each of the cylinders
is within the specified limit.
Limit: max. (14 psi)
(8) If a cylinder’s compression or pressure differential exceeds
the limit, add a small amount of oil through the spark plug
hole and repeat steps (5) (7).
If the addition of oil brings compression up, it is possible
that there is friction between the piston ring
and cylinder wall.
If not compression up, valve seizure, poor valve seating,
or a compression leak from the gasket are all possible.
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