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Gauge
Ignition Timing
ENGINE DIAGNOSTICS CONT.
15
LEAKING PISTON RING
10
• If you experience leaking rings the vacuum at idle will be low
with the needle holding steady between 12 and 16 in. of Hg.
5
1. To test open the throttle.
2. Increase the engine speed to about 2000 RPM.
0
Hg
3. Then close the throttle quickly and watch the needle of the
Vacuum
Gauge
gauge. It should jump 2 to 5 in. of Hg above its lowest reading.
A lesser gain may indicate faulty rings, and a complete cylinder
leakage or compression test should be done.
Broken Valve Spring
IDLE AIR/FUEL MIXTURE
15
• A sweeping change of 4 to 5 in. of Hg back and forth at idle
10
indicates a rich fuel mixture. With a lean mixture the needle
will drop irregularly over about the same range.
5
0
Hg
Vacuum
Gauge
TESTING PROCEDURES
Fuel mixture
INTAKE MANIFOLD LEAK
1. With the vacuum pump connected to an intake manifold
15
vacuum port.
20
10
2. Start engine, bring to normal temperature and let idle.
3. Look at the needle's movement on the gauge.
5
25
• If you have a bad gasket, or cracked hose this could
0
30
cause air to leak into the intake system. If this happens,
Hg
Hg
Vacuum
the needle will read about 4 to 9 in. of Hg below normal,
Gauge
and will remain steady.
Vacuum Solenoid
EXHAUST GAS RECIRCULATION (EGR)
1. With the vacuum pump connected to an intake manifold
vacuum port.
2. Start engine, bring to normal temperature and let idle.
3. Note the needle's movement on the gauge.
• The needle should read 16-22 in. of Hg and hold steady.
and will remain steady.
4. Increase the engine speed to approx. 2000 RPM.
• As the speed increases slowly, so should the vacuum.
This indicates the EGR valve is operating properly.
• If the needle drops, moving toward zero you have a
malfunctioning EGR valve.
• To test:
1. Remove the EGR valve from the manifold.
2. Connect your vacuum tester to the diaphragm on the
EGR valve.
3. Pump the trigger of your tester building vacuum. The
needle should not move and hold steady. If the
needle starts to drop, your EGR valve has a crack/
hole in the diaphragm.
6
10
20
5
25
0
30
Hg
Hg
Leaking Valve
10
20
5
25
0
30
Hg
Hg
Leaking Piston Ring
15
10
5
0
Hg
Vacuum
Gauge
Gauge
0
Hg
Mis re
Vacuum
Gauge
15
Broken Valve Spring
20
25
15
10
30
Hg
Vacuum
5
Gauge
0
Hg
Vacuum
Gauge
Fuel mixture
15
20
15
25
20
10
30
Hg
25
5
Vacuum
Gauge
0
30
Hg
Hg
Vacuum
Gauge
Vacuum Solenoid
20
10
25
5
30
0
Hg
Hg
Vacuum
Gauge
ELECTRICAL /VACUUM SOLENOID
FUEL PRESSURE REGULATOR
Gauge
30
Clogged Exhaust
Hg
15
20
10
5
5
20
0
30
Hg
5
Hg
Vacuum
25
Gauge
30
Worn Valve Guides
Hg
Broken Valve Spring
15
20
10
Leaking Piston Ring
5
0
30
15
5
Hg
Vacuum
10
Gauge
5
Blown Head Gasket
0
Hg
Vacuum
Gauge
15
10
20
5
5
25
0
30
Hg
Hg
Vacuum
Vacuum Solenoid
5
5
EXHAUST GAS RECIRCULATION (EGR)
5
0
30
Hg
Hg
Vacuum
Ignition Timing
Gauge
Leaking Valve
15
20
10
15
25
25
20
10
0
30
25
Hg
Hg
Vacuum
Gauge
0
30
Hg
Hg
Vacuum
Gauge
15
25
20
10
25
Hg
20
10
0
30
25
5
Hg
Hg
Vacuum
Gauge
30
0
Hg
Hg
Fuel mixture
15
15
20
10
20
25
25
0
30
30
Hg
Hg
Vacuum
Hg
Gauge
Gauge
15
20
10
25
0
30
Hg
Hg
Vacuum
Gauge
Nornal
15
20
10
25
0
30
Hg
Hg
Vacuum
Gauge
Ignition Timing
15
20
10
25
5
1
5
15
20
0
H
30
Hg
Vacuum
Leak
Gauge
15
10
5
0
Hg
Vacuum
Gauge
5
L
5
1
5
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