Wet Compression Test - BMW 5251 Service Manual

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Cylinder Head Removal and Installation
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Release pressure at compression gauge valve, then remove
gauge from spark plug hole. Repeat test for each cylinder
and compare results with values given in Table a.
Table a. Compression testing specifications
Minimum compression
6-cylinder
10-1 1 bar (142-156 psi)
V-8
12-14 bar (174-203 psi)
Maximum difference between
cylinders (all engines)
0.5 bar (7 psi)
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Compression readings may be interpreted as follows:
Low compression indicates a poorly sealed combustion
chamber.
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Relatively even pressures that are below specification nor-
mally indicate worn piston rings andlor cylinder walls.
Erratic values tend to indicate valve leakage.
Dramatic differences between cylinders are often a sign of
a failed head gasket, burned valve, or broken piston ring.
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If readings are within specifications, reinstall spark plugs and
ignition coils.
NOTE-
Used spark plugs should be reinstalled in the same cylinder
from which they were removed.
Tightening torque
Spark plug to cylinder head 25 Nm (18 ft-lb)
lanition coils to cylinder head 10 Nm (7.5 ft-lb)
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Remainder of installation is reverse of removal. Be sure to re-
install all wires disconnected during test, especially ground
wires at coils and cylinder head cover (where applicable).
Wet compression test
To further help analyze the source of poorcompression,awet
compression test is the next step.
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Repeat compression test, this time squirt a teaspoon of oil
into each cylinder. The oil will temporarily help seal between
piston rings and cylinder wall, practically eliminating leakage
past rings for a short time.
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If this test yields a higher reading than "dry" compression
test, there is probably leakage between piston rings and
cylinder walls, due either to wear or to broken piston rings.
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Little or no change in compression reading indicates other
leakage, probably from valves.