| Engine Vacuum |
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Engine vacuum is produced by the piston drawing air from the intake manifold
during the intake stroke. The throttle plate keeps air from freely flowing into the
manifold creating manifold vacuum. When the throttle body is opened there is a
collapse in vacuum. This collapse in vacuum is actually an increase in air pressure.
A quick snap of the throttle plate should drop the vacuum to around 5 Hg and then
it should recover to a steady 21 Hg. When this air is mixed with atomized fuel from
the fuel injectors engine speed is increased.
To check manifold pressure with a vacuum gauge you need to locate a port in the
manifold or throttle body. Manufacturers put ports on their manifolds for lots of
different reasons: Brake Booster, PCV tube, EGR Switch, A/C vents, and on and
on. You simply need to find one small enough for the vacuum gauge line to slide
onto firmly.
Gauge Readings:
Normal Engine: On a normal engine accelerate to around 2000 rpm and then
quickly release the throttle plate. The engine should snap right back to a steady
17- 21 Hg.
Steady low between 5-10 Hg: This indicates that the engine has a leak in the
intake manifold or the intake gasket. This leak should be pretty easy to find
because it would be making a loud hissing noise.
Steady low between 10-15 Hg: This reading indicates late valve timing. There's
a good chance the vehicle has jumped timing. I would check the timing belt or
chain depending on the application.
Steady low between 15-18Hg: This just low reading indicates retarded ignition
timing. I would advance the timing on the distributor to correct this problem.
Fluctuating Needle: A fluctuating needle indicates there's a problem with a valve
or a there's an engine misfire.
Needle drops during acceleration: If the needle drops steady during
acceleration there's a restriction in the exhaust or intake. This is almost always
caused by a clogged CAT or muffler.
Engine vacuum is produced by the piston drawing air from the intake manifold
during the intake stroke. The throttle plate keeps air from freely flowing into the
manifold creating manifold vacuum. When the throttle body is opened there is a
collapse in vacuum. This collapse in vacuum is actually an increase in air pressure.
A quick snap of the throttle plate should drop the vacuum to around 5 Hg and then
it should recover to a steady 21 Hg. When this air is mixed with atomized fuel from
the fuel injectors engine speed is increased.
To check manifold pressure with a vacuum gauge you need to locate a port in the
manifold or throttle body. Manufacturers put ports on their manifolds for lots of
different reasons: Brake Booster, PCV tube, EGR Switch, A/C vents, and on and
on. You simply need to find one small enough for the vacuum gauge line to slide
onto firmly.
Gauge Readings:
Normal Engine: On a normal engine accelerate to around 2000 rpm and then
quickly release the throttle plate. The engine should snap right back to a steady
17- 21 Hg.
Steady low between 5-10 Hg: This indicates that the engine has a leak in the
intake manifold or the intake gasket. This leak should be pretty easy to find
because it would be making a loud hissing noise.
Steady low between 10-15 Hg: This reading indicates late valve timing. There's
a good chance the vehicle has jumped timing. I would check the timing belt or
chain depending on the application.
Steady low between 15-18Hg: This just low reading indicates retarded ignition
timing. I would advance the timing on the distributor to correct this problem.
Fluctuating Needle: A fluctuating needle indicates there's a problem with a valve
or a there's an engine misfire.
Needle drops during acceleration: If the needle drops steady during
acceleration there's a restriction in the exhaust or intake. This is almost always
caused by a clogged CAT or muffler.