Compression Test 3A-85 - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23
2. If used piston is not satisfactory, determine if a new piston can be selected to fit cylinder
3. If cylinder bore must be reconditioned, measure new piston diameter (across centerline
4. Mark piston to identify cylinder for which it was fitted.
Compression Test
NOTE: Compression pressure is 100 psi (690 kPa) minimum. Minimum compression re-
corded in any one cylinder should not be less than 70% of the highest recorded cylinder.
1. Disable ignition system and remove all spark plugs.
NOTE: Use a fully charged battery when performing the following procedure.
2. Open throttle plate completely.
3. Starting with the compression gauge at "0," crank the engine through 4 compression
4. Record compression for each cylinder.
5. If some cylinders have low compression, use a pump type oil can to inject 3 squirts of
6. No cylinder should read less than 100 psi (690 kPa). Compression recorded on any one
7. 4 characteristics may be evident:
Index
90-861326--1 MARCH 1999
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bore within acceptable range.
of piston pin), then hone cylinder bore to correct clearance (preferable range).
strokes (4 puffs).
oil into each combustion chamber through the spark plug hole. Do a second compres-
sion check on each cylinder.
cylinder should not be less than 70% of the highest cylinder. For example, if the highest
measurement is 150 psi (1035 kPa), the lowest allowable pressure for any other cylinder
would be 105 psi (725 kPa). (150 x 70% = 105 psi)
Normal: Compression builds up quickly and evenly in each cylinder.
Piston Rings Leaking: Compression low on first stroke then increases on following
strokes, but does not reach normal. Compression improves considerably with 3
squirts of oil.
Valves Leaking: Compression low on first stroke and does not build up on following
strokes or with 3 squirts of oil.
If 2 Adjacent Cylinders Have Lower Than Normal Compression: Inject 3 squirts of oil
into the cylinders. If the compression does not increase, the cause may be a blown
head gasket between cylinders.
454 CID (7.4L) / 502 CID (8.2L)
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