Finding vacuum leaks
An engine having a vacuum leak will tend to have an uneven, "hunting" idle speed. If the vacuum leak is sufficient, it may even run poorly under light throttle settings.
Unless the vacuum leak is very large, the symptoms will disappear with higher engine speeds due to the relatively small amount of unmetered air entering the engine under those conditions.
- Split, cracked or broken vacuum lines
- Leaking intake manifold or carburetor gaskets
- Leaking carb gasket or loose carb-to-intake fasteners
- Open carb or intake manifold vacuum ports
- Loose/missing carb body/throttle plate screws
- Faulty vacuum devices like:
- AT modulator
- Brake booster quite commonly un or misdiagnosed
[little harder brake pedal remove booster vac line plug & idle will improve]
- Vacuum advance canister
- Carb choke pull-off
- HVAC controls
- Emissions equipment
- Check valves
- Vacuum switches
- Charcoal canister and hoses
- PCV hose, valve or grommet
 Finding a vacuum leak
There are several methods for locating vacuum leaks. If you suspect a vacuum leak on a carbureted or TBI engine, you may be able to verify your suspicion by performing a simple check. With the air cleaner removed and the engine at idle speed, use the choke blade to partially block the airflow into the engine. Or you can place a wadded-up rag lightly over the throttle body throat to partially choke off the air supply. If this artificial richening of the air/fuel mixture causes the engine rpm to increase, you may well have a vacuum leak.
Often a vacuum leak will cause a whistling or hissing noise. Use an automotive stethoscope or even a length of vacuum or fuel hose to listen for the vacuum leak with the engine idling.
A stethoscope is self-explanatory. To use a piece of hose, put one end of the hose by your ear and use the other end to probe around for the leak.
 ATF method
Using ATF mixed with a flammable solvent in a squirt can, apply to the suspected areas. Listen for a change in the idle speed/quality, AND watch for the tell-tail white smoke caused by the ATF. There should be a distinctive odor associated with the burning ATF as well.
 Propane method
- Get a propane bottle and a soldering torch and valve attachment.
- Unscrew the end of torch so that you have a threaded pipe without the nozzle.
- Attach a 2-foot piece of hose to the end.
- Start the engine and let it warm up.
- Turn on the propane and run the hose all around the vacuum lines and the bottom of the carb.
- If the engine smooths out or revs up a bit, you have found your leak.
 Carburetor cleaner method
- With the engine running, use a can of carburetor cleaner to spray down the hoses and vacuum connections.
- If there's a change in the idle speed or if the engine runs smoother, you might have found the vacuum leak.
Note: You may also use engine starting fluid instead, as it does not leave residue.
Warning: Carb cleaner and starting fluid are extremely flammable! If this is attempted, have a fire extinguisher at hand. Keep away from sparks and any other sources of ignition. Even a cross firing plug wire could ignite such materials.
 Vacuum gauge method (internal)
Note: See Vacuum gauge engine diagnosis.
The following procedure may be used to determine an internal leak:
- Disconnect/remove the primary wire between the coil and distributor.
- Adjust the idle screw so the throttle plate in the carburetor is completely closed.
- Remove PCV, breathers, and all vacuum ports on carburetor/manifold and seal openings with plugs/tape.
- Connect vacuum gauge to a manifold vacuum source.
- Have assistant crank engine while you read vacuum. If gauge reading is 0-2", you may have an internal leak between the manifold/head.
 "Untraceable" vacuum leak
Occasionally on engines that use the intake manifold to seal the lifter valley, you may experience a vacuum leak from the lifter valley area into the intake ports. This may be due to a bad gasket seal or because the intake manifold-to-cylinder head interface was not machined parallel (usually done to correct for milling cylinder heads and/or block).
No amount of propane or carburetor cleaner will find such leaks. The best way to insure there's no such problem is to use quality intake gaskets installed correctly, and to double check any machining before final assembly of the engine.
- Carb vacuum port ID, including info on reading a vacuum gauge