Tools: Carburetor cleaner spray; Length of vacuum hose
Sport trucks are meant to be enjoyed. So when it comes to driving your street dream, it must run properly. Few things are more aggravating than an engine that hunts, sags, and generally runs poorly at idle and light throttle. Short of an engine that won't run at all, vacuum leaks are the worst.
Before you search for that elusive bugger, it's best to know what you're looking for. Vacuum leaks are what Bosch calls "false air," or 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 that affect the intake-manifold gaskets. Nick Jennings (Jennings Dyno Shop, Dept. ST, 9519 Sepulveda Blvd., North Hills, CA 91343, 818/894-3811) gave us a few quick and easy tips for ferreting out those imps.
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's 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 use a length of vacuum hose. Place one end of the hose to 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, Jennings mixes cleaning solvent with automatic transmission fluid in a squirt oil can and shoots this mixture around the intake gasket. If there's a leak, engine speed should 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. While fixing a vacuum leak won't make your car run quicker in the quarter-mile, it will certainly be more fun to drive.
The easiest way to determine if your engine is suffering from a vacuum leak is to cup your hand over the carburetor. This creates an artificial choke and will make it run richer. If engine rpm increases, it has a vacuum leak. Now all you have to do is find it. | 
Here's a quick and dirty stethoscope made from a length of vacuum hose. Place one end of the hose near your ear and the other end anywhere you think there may be a vacuum leak. You should be able to hear a tell-tale whistling noise. |
A vacuum gauge identifies a leak by fluctuating wildly. A consistently low reading can also indicate the presence of unmetered air. This can be harder to identify if the engine is equipped with a long duration camshaft, which will also indicate low intake-manifold vacuum. | |