Several available methods can help you identify an engine vacuum leak. The first step is to look at your vacuum hose diagram; this information can sometimes be found under the hood on a sticker. If it is peeled away or wasn’t there to begin with, you can find the diagram in a repair manual. Then, you can choose the best method for your needs so that you can find the vacuum leak. Depending on the method you choose, the leak might show itself as a change in idle smoothness or engine speed. Along with such, small leaks may only reveal themselves on a scan tool reading when the STFT meter fluctuates.
The best way to start is with a visual check, especially if you think the leak may be in a tube or vacuum hose. When the plastic vacuum tubes and rubber vacuum hoses are exposed to high temperatures under the hood with oxygen present, they can become brittle or stiff, which causes breakage or cracking. Along with such, engine intake tubes made of rubber can also crack and become brittle, which opens the way for too much air to get into the system. If you run the engine while manipulating these components, you might find the leak.
The water test is one of the least expensive and easiest methods of finding a vacuum leak because it requires a spray bottle filled with water. When your engine is running, spray some water around the areas where you suspect a vacuum leak, including intake manifold gaskets, vacuum hose fittings, or throttle plate bushings. If a leak is present, it sucks the water inside, which ‘seals’ the leak for a moment. Such a small amount of water doesn’t damage the engine.
Using intake cleaner spray or carburetor cleaner can also help you find the leak. However, you should be aware that carburetor cleaners are flammable; keep a fire extinguisher nearby while working and use caution. You should spray the cleaner sparingly around areas you think might have a vacuum leak while the engine is on and idling. If you find a leak, the cleaner is going to be sucked in and seal the leak for a while, and your engine may run more smoothly because the flammable mixture can make up for your lean air-to-fuel ratio.
The propane method for finding vacuum leaks has been tested and used many times by mechanics and DIY people. It works similarly to the carb cleaner method. You need a rubber hose and a small propane torch (unlit). With the engine idling, poke the hose end around the areas you think may have a vacuum leak and place the propane torch (on but unlit) at the other end of the hose. If leaks are found, the engine is likely to smooth out or speed up because the flammable propane gas compensates for your lean AFR. However, remember that propane is also flammable.
If you have access to one, you can use a mechanic’s stethoscope and remove the probe; poke around on suspected areas that leak while your engine is running. You should make sure to check the vacuum brake booster, both behind the brake pedal and in the engine bay. While small leaks can be hard to pinpoint, trained ears can pick up the whistling or hissing sound caused by a vacuum leak.
If you own an air compressor with a regulator, you can add up to two psi into your intake while the engine is off. If you add more than two psi, you run the risk of damaging valves and sensors or creating new leaks. Seal the exhaust and throttle body and spray the engine with a soap-water mixture; the vacuum leak is revealed when the mixture bubbles at the leak site.