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ENGINE DIAGNOSTICS
NOTE: Measurements and illustrations are for reference only.
Your test results may vary.
NORMAL ENGINE
1. With the vacuum pump connected to an intake manifold
vacuum port.
2. Start engine, bring to normal temperature and let idle.
3. Look at the needle's movement on the gauge. At idle,
the needle should read 16-22 in. of Hg and hold steady.
This is your base line, indicating everything
functioning normal.
WEAK OR BROKEN VALVE SPRING
• A sign of weak valve springs are when the pointer on the
vacuum pumps gauge fluctuates rapidly between 10 and 21
in. of Hg at idle. As the engine speed increases the
fluctuations will increase too.
• A broken valve spring will still cause the needle to fluctuate
rapidly at regular intervals. The difference can be seen every
time the cylinder fires and the valve attempts to close.
STICKING VALVE
• A valve that's sticking will exhibit a rapid drop intermittently
from a normal pointer indication. This is because a sticking
valve or valves don't always stick on each cycle of the
engine. To pin-point a sticking valve apply lightweight oil
directly to each valve guide using a syringe with a fine tip. As
the oil coats the guide the sticking valve will temporarily
function normal. The needle on the gauge will be steady until
the oil burns off.
BURNED OR WORN-LEAKING VALVE
• At idle, the pointer on the gauge will drop then return to
normal. This would happen at regular interval. Every time the
cylinder fires with the bad valve it will cause the needle to
drop to a low reading. The needle will drop from 1 to 7 in. of
Hg, then return to a normal reading.
WORN VALVE GUIDES
• A worn valve guide lets air into the cylinder, this changes the
air/ fuel mixture. The needle on the gauge will run lower than
normal, approx. 3 in. of Hg and will fluctuate rapidly. When
you increase the speed of the engine the needle will stop
fluctuating.
BLOWN CYLINDER HEAD GASKET
• At idle, the needle will fluctuate from normal to a low reading.
Broken Valve Spring
Each time the affected cylinder reaches firing position the
Broken Valve Spring
needle will drop sharply. You should see about 10 in. of Hg
drop from normal, then return.
10
5
0
Hg
Ignition Timing
10
5
0
Hg
15
20
10
15
Broken Valve Spring
20
10
5
25
5
25
0
30
10
Hg
Hg
0
30
Vacuum
Gauge
Hg
Hg
Vacuum
Gauge
5
Nornal
0
Nornal
Hg
15
Fuel mixture
20
10
15
20
10
25
5
15
5
25
10
0
30
Hg
Hg
0
30
Vacuum
Gauge
5
Hg
Hg
Vacuum
Gauge
0
Ignition Timing
Hg
Vacuum
Ignition Timing
Gauge
15
Vacuum Solenoid
20
10
15
20
10
25
5
25
5
0
30
Hg
Hg
0
30
Vacuum
Gauge
Hg
Hg
Vacuum
Gauge
15
20
15
10
20
10
25
5
5
25
0
30
Hg
Hg
0
30
Vacuum
Gauge
Hg
Hg
Vacuum
Gauge
Fuel mixture
15
20
25
30
Hg
Vacuum
Gauge
15
20
25
30
Hg
Vacuum
Gauge
15
10
15
10
5
5
15
0
20
Hg
0
Vacuum
Gauge
Hg
Vacuum
Gauge
25
Leaking Intake
30
Leaking Intake
Hg
Vacuum
Gauge
15
10
15
10
5
5
20
10
0
Hg
0
Vacuum
Gauge
25
5
Hg
Vacuum
Gauge
30
0
Mis re
Hg
Hg
Mis re
15
10
15
10
5
5
0
Hg
0
Vacuum
Gauge
Hg
Vacuum
Gauge
Leaking Valve
Leaking Valve
15
10
15
10
5
5
0
Hg
0
Vacuum
Gauge
Hg
Vacuum
Gauge
Leaking Piston Ring
Leaking Piston Ring
15
20
10
25
5
0
30
Hg
Hg
Vacuum
Gauge
Mis re
5
15
20
10
25
5
0
30
Hg
Hg
Vacuum
Gauge
Leaking Valve
20
20
25
5
5
25
5
15
30
20
10
Hg
30
Hg
Broken Valve Spring
5
25
0
30
Hg
Hg
Vacuum
Gauge
Leaking Piston Ring
20
20
25
5
5
15
25
5
20
30
Hg
30
25
5
Hg
30
Clogged Exhaust
5
Hg
Vacuum
10
Broken Valve Spring
Gauge
5
20
0
20
Hg
25
5
Broken Valve Spring
5
25
5
30
Hg
30
Hg
Worn Valve Guides
Worn Valve Guides
5
20
10
20
25
5
25
30
0
Hg
30
Hg
Hg
10
5
Vacuum Solenoid
Hg
Hg
Vacuum
Gauge
10
Nornal
5
0
Hg
15
20
10
15
25
5
Clogged Exhaust
20
10
0
30
Hg
Hg
25
Vacuum
Gauge
0
30
10
Hg
Hg
Vacuum
Ignition Timing
15
Gauge
5
20
10
Nornal
0
5
25
Hg
15
0
30
20
10
Hg
Hg
Vacuum
Worn Valve Guides
Gauge
15
5
25
15
20
10
15
20
10
20
10
Nornal
0
30
10
25
Hg
Hg
Vacuum
25
Gauge
25
5
0
30
Hg
Hg
0
30
0
30
Vacuum
Gauge
Hg
Hg
Hg
Hg
15
Vacuum
0
Vacuum
Gauge
Gauge
Hg
20
10
Valve Timing
Valve Timing
Ignition Timing
25
5
15
Blown Head Gask
20
10
0
30
Hg
Hg
Vacuum
5
25
Gauge
15
15
20
10
15
20
10
0
30
20
10
Ignition Timing
Hg
Hg
Vacuum
25
Gauge
25
15
25
20
10
0
30
0
30
Hg
Hg
Fuel mixture
0
30
Vacuum
Hg
Hg
15
Vacuum
Gauge
25
Hg
Hg
Vacuum
Gauge
20
10
Gauge
0
30
15
25
Hg
Hg
Clogged Exhaust
Vacuum
20
Gauge
0
30
25
Hg
Hg
Vacuum
15
Gauge
15
20
10
15
30
20
10
20
10
Hg
Vacuum
Gauge
25
25
25
0
30
0
30
Hg
Hg
Vacuum Solenoid
0
30
Vacuum
15
Hg
Hg
Gauge
Vacuum
Hg
Hg
Vacuum
Gauge
20
10
Gauge
25
Fuel mixture
15
0
30
20
15
10
Hg
Hg
Vacuum
20
10
Gauge
15
25
5
20
5
25
0
30
Fuel mixture
25
Hg
Hg
0
30
Vacuum
Gauge
Hg
Hg
Vacuum
30
Gauge
5
15
Hg
Vacuum
Blown Head Gasket
Gauge
20
Blown Head Gasket
25
15
20
L
25
30
Hg
Vacuum
Gauge
5
5
15
20
25
L
30
5
Hg
Vacuum
Gauge
5
15
L
20
5
25
30
Hg
Vacuum
Gauge
5
1
5
0
H
5
10
Leak
5
0
15
5
Hg
10
5
0
1
Hg
Vacuum
L
Gauge
5
0
H
1
5
Leak
0
H
15
10
Leak
5
0
15
Hg
Vacuum
Gauge
10
5
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