Mechanics

Vacuum Leak Test

How to Locate an Engine Vacuum Leak

An engine generates vacuum when running, this vacuum is used to operate several accessories from the power brake system and temperature control systems (heater, air conditioner) including the cruise control. The pistons create vacuum on their downward slide which allows air and fuel to be delivered into the combustion chamber. Once delivered, the mixture is ignited, there is an explosion, and the piston is forced back down creating power. For the engine to run properly, the correct air and fuel mixture needs to be delivered to the combustion chamber. When you have a vacuum leak, the air/fuel ratio becomes incorrect and can cause many problems. Most common problems are, low idle, high idle, poor acceleration, cylinder misfire and stalling at idle

There are many components that require vacuum, however, most commonly, vacuum leaks are usually the result of a dry rotted or disconnected rubber hose or a failed intake gasket. Testing a vacuum source is simple, remove the vacuum line from any giving accessory and attach the vacuum gauge. Start engine and allow to idle, the gauge should read between 14 and 18 inches of vacuum. If a low or no vacuum exits there is blockage or breakage of a vacuum line. Inspect all vacuum line integrity, then remove vacuum line from the engine source and check for blockage, repair as needed and recheck the system. If you are experiencing a problem with the air flow direction of the ventilation system, make sure to check for vacuum leaks under the dashboard of the vehicle. 

Additionally, if you are concerned about using carburetor and choke cleaner to locate a leak, you can try pinching the different individual hoses to see if there are changes to the engine idle speed. When checking for a vacuum leak use an aerosol carburetor cleaner like Gumout or Berryman can be useful. When an engine burns this additional flammable liquid it will raise the engine idle slightly. When the cleaner is completely digested into the engine it will return to its normal, malfunctioning condition. These cleaners are flammable so use them with caution.

Start with your car parked on level ground with the engine warm but not hot. Set the parking brake to prevent the car from rolling, also wear protective eyewear, gloves and clothing. 


Tools Needed

1. Carburetor/Choke cleaner

2. Fire extinguisher

3. Replacement vacuum hose

4. Needle nose pliers

5. Screwdriver set


Directions

Step 1 - Open the hood and do a visual inspection. Check for broken or disconnect rubber vacuum hoses. Also, some vehicles actually use a small plastic hose which commonly break.

Step 2 - If the vehicle passes the visual inspection, start the engine and listen. You may hear a hissing sound generated from a vacuum leak that will help you pinpoint the malfunction.

Step 3 - If you are unable to hear any noise or locate it directly, you will now need to begin using the carburetor cleaner. WARNING: Carburetor and choke cleaner is flammable. If the product makes contact with a component, such as an exhaust manifold, that is hot enough, it can start a fire. Keep a fire extinguisher near you while you perform this test for safety.

Step 4 - Using the carburetor cleaner, spray around the intake manifold gasket and on all vacuum lines. Also, make sure to spray the PCV valve vacuum supply and all EVAP vacuum system vacuum hoses.

Step 5 - When the carburetor spray reaches the affected vacuum leak, the engine RPM’s will smooth out and change. They may increase. If that happens, pinpoint where the spray is going to identify where the leak is coming from and repair.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-11)