1968 Shelby GT500

Vacuum Leaks

Have you ever been confronted with an engine that just doesn’t seem to run right no matter what you’ve done or replaced? You may be dealing with a vacuum leak. On older carbureted engines, vacuum is needed to pull fuel into the engine. Fuel injected engines do not require intake vacuum to siphon fuel into the engine because fuel is sprayed directly into the engine under pressure through the injectors. Vacuum leaks can upset the carefully balanced air/fuel ratio by allowing “unmetered” air to enter the engine.

Few things are more annoying than looking for vacuum leaks, yet it can be very easy to diagnosis with the right tools and tricks. Cracked or broken vacuum lines, leaking intake manifold or plenum gaskets, injector o-rings, broken emissions solenoids, open vacuum ports are just a few of the causes of vacuum leaks. Use the under-hood emissions diagram and check every single vacuum circuit with a vacuum tester such as the Mityvac tool.


You can quickly find split hoses, small holes burned through the vacuum line, cracks at joints, and nipples that won�t seal. Don�t try to glue old lines back together, replace them with quality vacuum lines found at your local auto parts store. Most parts stores will have a tray or vacuum tee�s and connections.

A faster technique for finding intake manifold vacuum leaks is to get a bottle of propane and attach a length of rubber hose to the gas valve. Start the engine, be careful not to come too close to the fan or fan belts. Open the propane tank valve so you have a steady flow of gas. Then hold the hose near suspected leak points while the engine is idling. If there is a leak, the propane will be drawn in through the leak. The propane should cause a noticeable change in idle speed and/or smoothness. Always know where the closest fire extinguisher is and how to use it!

NOTE: Fuel injected engines also rely on intake vacuum to regulate the fuel pressure behind the injectors. Fuel delivery cannot be accurately metered unless a fairly constant pressure differential is maintained. So the fuel pressure regulator diaphragm is connected to a source of intake vacuum. Vacuum working against a spring-loaded diaphragm inside the regulator opens a bypass that shunts fuel back to the tank through a return line. This causes the fuel pressure in the injector rail to rise when engine load increases (and vacuum drops). Thus, the regulator uses vacuum to maintain fuel pressure and the correct air/fuel ratio. A vacuum leak changes the equation by causing a drop in vacuum and a corresponding increase in line pressure.

© 2002-2012 UNIVEX Automotive Institute.
FuelInjectedFord.com, is not affiliated with Ford Motor Company."