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Trucks Of The Month
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Vacuum
Leaks:
Few
things are more annoying than an engine that 'hunts' and basically runs
poor at idle and light throttle. A vacuum leak is air that
enters the engine unmetered. Cracked or broken vacuum lines,
leaking intake manifold or carburetor gaskets, open carburetor fittings,
and loose or missing carburetor screws are just a few of the causes of
vacuum leaks. These leaks are often difficult to detect,
especially those bothering intake manifold gaskets.
Perhaps
the easiest way to determine if an engine has a vacuum leak is to cup
your hand over the choke housing while the engine is idling. This
artificial choke will create a richer mixture. If the idle speed
increases with the richer mixture, there is a vacuum leak lurking about.
Snap-On, Mac, and other tool emporiums sell inexpensive automotive
stethoscopes to locate vacuum leaks. If these tools are
unavailable, you can substitute a length of vacuum hose.
Place
one end of the hose by your ear and use the other end to search for the
leak while the engine is running. Be careful not to come too close to
the fan or fan belts. If the engine is not a polished show piece,
mix cleaning solvent with automatic transmission fluid in a squirt oil
can. Shoots this mixture around the intake gasket. If
there's a leak, engine speed should more than likely increase, but the
most obvious sign will be white smoke from the ATF coming from the
tailpipes. Other tricks include shooting aerosol carb cleaner
around the suspect gaskets while the engine is running. When the
carb cleaner hits the leak, engine rpm usually increases, thus
pinpointing the source. The hardest part of fixing an unmetered
air leak is finding it. These few tips should help.
Using
Vacuum Gauges:
A
good tool to have (especially on a carbureted engine) is a vacuum gauge.
A vacuum gauge can tell you a lot about what's going on in your engine.
I have an Auto Meter vacuum gauge along with other gauges in my dash.
The vacuum gauge readings are measured in inches of mercury at sea
level. In all 4-cylinder engines, a slight fluctuation will always
be noticeable. This can be overcome by pinching the gauge hose
slightly to enable a steadier reading.
Below
you will find (8) animated vacuum gauges that will demonstrate various
readings/indications. You may have to refresh the page to get them
all to load.
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NORMAL
MOTOR: Steady reading of 17-21 when motor idling. |
NORMAL
MOTOR: When throttle is opened and closed rapidly needle
falls to 2 and swings back to 24 or 25, falling back to normal
idle reading Indicates rings and valves ok
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POOR
RINGS: Motor idling, hand reading steadily, but 2-4
points lower than normal. This may also indicate poor or
contaminated engine oil, late ignition timing, a leaky vacuum hose
and a leaking gasket between the intake and carb or throttle body.
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POOR
RINGS/POOR OIL: When throttle is opened and closed
rapidly needle falls to 0 and rises to only 24 or less.
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STICKING
VALVE: Needle drops occasionally about 4 points at idle
speed. May also be caused by ignition misfire. Read
the spark plugs.
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BURNT
VALVE: Needle drops regularly by several points at idle
speed.
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LEAKY
VALVE: Needle drops 2-4 points when valve should close.
short circuiting individual spark plugs will indicate cylinder in
which the valve is defective, when engine idling.
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WORN
VALVE GUIDES: Rapid fluctuation of needle between 17 and
21, when engine idling. Exhaust smoke may also be present.
If engine speed goes up with fluctuation, check for a leaking
intake manifold gasket, head gasket, weak valve springs, burned
valves, or ignition misfire.
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Also not listed:
If
the needle moves slowly through a wide ranger, check for a clogged PCV
system, incorrect idle fuel mixture or intake manifold gasket leaks.
A
slight needle fluctuation, say one inch up or down, may mean ignition
problems. Check all the usual tune up items.
If
there is a large fluctuation of the needle, perform a compression test
to look for a weak or dead cylinder or a blown head gasket.
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