Vacuum Leak Test

Step by step automotive repair and troubleshooting guide on how to detect and fix an internal combustion engine vacuum leak. Most internal combustion engine's hold between 15 and 18 inches of vacuum when idling. This condition varies as the engine is loaded, the amount of vacuum is measured by a combination of sensors such as mass air flow (MAF) and throttle position (TPS) or directly using a manifold absolute pressure (MAP) sensor. Engine vacuum is a large factor in the equation of fuel management which is determined by the computer. To aid in the discovery of a vacuum leak an audible hiss or whistle sound might be heard which will help pinpoint the problem.

Difficulty Scale: 2 of 10

Begin with the engine cool on level ground with the emergency brake set, also wear protective eye wear, gloves and clothing. Carburetor cleaner is flammable so use caution when testing, have a fire extinguisher ready. (Note: Never spray exhaust manifolds.)

Step 1 - Inspect vacuum hoses for dilapidation or cracks, this will cause the engine to stall, idle poorly and hurt gas mileage.

Vacuum Leak

Step 2 - Vacuum lines can become hard due to under hood temperatures which produce an unwanted vacuum leak.

Broken Vacuum Line

Step 3 - After removing the old vacuum line and check for blockage, install a new replacement hose, this can be done by using vacuum or fuel hose.

Replacement Vacuum Line

Step 4 - If after inspection, no broken vacuum lines are found carburetor cleaner can be used to further inspect for leaks.

Carburetor Cleaner

Step 5 - After starting the engine and allowing it to idle, spray the cleaner near and around the throttle bore gasket, if the gasket has failed the engine will react to the spray by increasing the engine idle speed as the cleaner is burned. If the engine is idling rough it will temporarily smooth out as the cleaner is consumed.

Checking Throttle Bore Gasket


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2014-07-06)