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. 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. 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. 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.

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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