30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
1 2 3
READING: 16-21 inHg with
needle steady.
CAUSE: Normal.
READING: Extremely low but
steady.
CAUSE: Leaking intake manifold
system, faulty manifold gasket,
carburettor base gasket, split
vacuum hose, EGR valve seized.
READING: When the throttle is
suddenly opened then released
the needle should drop to below
5inHg then bounce up to approx.
25inHg settling back to original
reading. CAUSE: Normal.
4 5 6
READING: Low but steady.
CAUSE: Pulled back ignition
timing. Confirm using the timing
light and reset to the
manufacturer’s specification.
READING: Regular fluctuation
between normal and low
readings. CAUSE: Blown head
gasket between two adjacent
cylinders. Carry out a cylinder
leakage test.
READING: Slightly low and
fluctuating slowly.
CAUSE: Over lean or rich
mixture. Check manufacturer’s
specifications and reset.
7 8 9
READING: Slightly lower
reading than a normal engine
including when the throttle is
suddenly opened then released.
CAUSE: Worn piston rings.
Carry out compression test.
READING: Normal when started
but drops rapidly when revs are
held at 3000rpm.
CAUSE: Restriction in the
exhaust system.
READING: Regular drop
between normal and low reading.
CAUSE: Burnt valve.
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
30
25
20
15
10
5
0
in Hg
76
70
60
10
20
30
50
40
04
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