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How to Test For Vacuum Leaks With Smoke – Know It All!

A vacuum leak is a problem that can occur in any type of vehicle. To solve the problem, one should know the basic mechanics of a car.

Luckily, not all vacuum leaks are serious problems that need a mechanic. In most cases, you can check your car using basic tools like a smoke machine.

To know how to test for vacuum leaks with smoke, first, let’s understand how the engine works. Then, you will be able to understand why gas might leak and solve the problem.

Any car engine uses fuel to operate. The fuel needs oxygen to undergo the burning process to produce energy. Thus, there are vacuum connectors, inlets, and gaskets to maintain this airflow to supply oxygen.

Any changes to the airflow can cause problems with the combustion process. By changes here we mean less or more air than usual.

“When there is a leak, the required amount of air is not reaching the engine. Hence, the engine won’t work properly.”

What causes a vacuum leak?

As we have just mentioned, the engine bay has vacuum parts like hoses, gaskets, and connectors. Most of these parts are rubber or any other material that can withstand heat. However, due to excessive use, these parts might wear out.

Also, if low-quality replacement parts are used, they will tear quickly. These parts are consumed all the time as long as the car is on. So, they are subjected to tear and wear quickly. Any small, even an inch, or tear in one of these parts can cause a leak.

Indicators of a vacuum leak

Luckily, the vacuum leak has many signs that indicate there is a leak to solve the problem as soon as possible. Although these signs might indicate other problems, they call for an inspection. Thus, whenever you notice any of these signs, check the engine bay for problems.

Engine light

Usually, this is the first sign of vacuum leaks. When there are any problems with the engine, the light comes on. This light is not necessarily indicating vacuum leaks. Yet, it means there is a problem that needs checking.

Problems with acceleration

Acceleration means more burning of fuel which needs more oxygen. If the vacuum is leaking, insufficient oxygen will be reaching the engine. So, you won’t be able to accelerate the car as usual.

Changes in RPM

RPM has a constant average that every car rider knows about his car. When there is a leak, the vehicle won’t be able to measure the correct airflow. So, the RPM average gives fault readings.

Hissing sound

Leaks mean there is a torn part around the engine. As the air flows and passes through this part, it will produce this hissing sound. To hear this sound, the driver needs to turn on the car, open the front panel and listen carefully.

Problems with starting the engine

Similar to acceleration, the engine needs oxygen to start. As the air is not flowing properly to the engine, it won’t start as normal. So, check for the leaks if you experience hard starts.

Reduced gas mileage

Finally, the gas mileage which may indicate tens of problems. A poor fuel economy alone cannot serve as an indicator for leaks. However, it is an alarming change to check the whole engine bay.

How to test for a vacuum leak?

Ideally, to make sure there is a vacuum leak within the engine, you should go to the mechanic. However, there are some basic tests you can do before you use a smoke machine or go to the mechanic.

The soapy water test

A common method most of us used to do for checking gas leaks of any apparatus. You use some soap mixed with water to check if there is gas coming out or no.

Put some liquid soap in a spray and add a little amount of water. Shake them well and spray around the vacuum in the car’s engine bay. If you see the water bubbling around, this indicates a leak.

However, the problem with this test is that it is not accurate enough to detect any leak. So, you cannot rely on it all the time to detect leaks. Another problem is that too much water around the engine bay is not a good thing at all. There are sensors and electronic parts in this area. Surely, we don’t want to damage other parts of the car to check for a leak.

Flammable liquid test

The second test is to use a flammable liquid. You can get this at any automotive accessory store. Simply, spray this liquid around the vacuum lines and listen for any sounds. If you heard the sound of the engine revving up, this means a leak.

To detect where exactly the gas is leaking, spray on a certain area and listen. Do the same on each small part of the vacuum connector and lines until you hear the rev-up sounds. Once you hear a change in the sound, you know this is the spot where the gas is leaking.

Although this test is more accurate than the above one, it is not 100% perfect. The reason for this is it has some safety concerns. In the end, you are spraying a flammable substance on a hot engine which may cause a fire.

A pro tip is to have safety gloves and glasses. Also, always have a fire extinguisher with you whenever you decide to use any flammable substance. If things went wrong, you will need this fire extinguisher.

Another problem is that these liquids are not good for rubber. Since the vacuum connections are all rubber, too much of the liquid can cause it to wear out.

Propane test

A final method to check for vacuum leaks is to use propane. Propane is a flammable gas. Yet, it disperses in the air. So, this method is safer than the flammable liquid one.

To use this method, get a propane cylinder connected to a hose. You can create this apparatus at home. Just purchase the propane gas tank and connect it to any suitable hose.

It looks like a fire extinguisher with a long hose to feed it into any part of the engine bay. Therefore, you won’t be spraying all over the engine bay but into direct parts. So, it is safer than the two other methods.

Moreover, propane doesn’t dry out rubber or affect the paint. Thus, the propane test is more effective and causes less damage.

How to test for vacuum leaks with smoke?

If you have a smoke machine, forget about all the above tests. This is much better and way more accurate.

The first encounter with a smoke machine may be overwhelming. It comes with many parts that connect to generate smoke to test for leaks. However, connecting these pieces is not an issue using the manual. Once the smoke machine is ready, inspecting for leaks shouldn’t take more than a few minutes.

To use a smoke machine for inspecting gas leaks, follow these steps:

  • Take off the air filter to free the space into the manifold intake. Use a screwdriver to unscrew the air filter and put it aside.
  • Insert the smoke machine’s intake into the manifold intake. Make sure they are connecting tightly and the connection is sealed. You can use the rubber bands that come with the machine to tightly seal this connection.
  • Take the two battery ports of the smoke machine and hook them up to the battery.
  • Push the button on. You should see the red light on. Wait for a few minutes till the smoke starts to come out from the smoke output.
  • Plug the smoke output into the other end of the intake you just inserted into the manifold. Wait for a few minutes till the smoke goes through the whole engine bay.
  • Now observe the gas emissions coming out from different parts. Use a flashlight to notice where the leak comes from. Check every part, the hoses, tees, gaskets, and all connections.
  • Once you determine what is exactly leaking, mark this part to know what you are going to change.

Any of the damaged parts can be changed easily. After doing the necessary changes, you can recheck using the same method to ensure it is solved.


“Although vacuum leaks are not common and don’t occur every day, their consequences can be very bad. If the leak is not managed, it can cause serious damage to other parts. Further, it affects the fuel and the whole engine.”

Car drivers who just rely on their observation won’t determine the exact leaking part every time. So, knowing how to test for vacuum leaks with smoke is better to permanently solve the problem.

Whenever you experience any of the above indicators, check for leaks. Re-check again after you replace whatever needs replacing. If you can’t reach a smoke machine nor you can do the other inspection tests, seek professional help immediately.

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