How to Find Vacuum Leak in Engine

Finding a Leak in Engine 

Nobody likes the word leak. It becomes double dreadful when one hears it from a close friend who is a car enthusiast and knows so much more about cars than you. You are having a cold one with him on a saturday night and sharing how your 5 year old honda is not the same as it was when you first pressed the gas pedal. And he goes; ‘may be there is a vacuum leak in engine. Do you know how to find a vacuum leak in engine?’ You just stare blankly. Vacuum. Leak. Engine. You know the words. But, how do they relate to each other and what do they have to do with your honda?

Not to Worry at All

In this article, I am going to rob you of that blank stare which you just gave your buddy after hearing the words ‘vacuum leak in engine’. Now, before I go all sciency on you, I need to make sure that you, dear reader, understand what is meant by vacuum leak in engine. 

Vacuum Leak in Engine

Vacuum is the absence of matter. For the purpose of this article, it is absence of air pressure at the intake manifold of an engine. You see, the intake manifold of the engine has a butterfly like valve at its mouth. When it is fully closed, no air enters into it.And when it is open, air enters into it depending on the movement of the valve around its central axis. More movement translates to more air intake. Now, the air entering the valve when it is fully closed, that is the problem named ‘vacuum leak in engine’. 

The Problem is the Air Entering

You see, air entering the engine, when it should not, is a problem. A big one. A car engine is a beautiful piece of mechanical artistry. It takes a fixed amount of air and mix it with a fixed amount of gas to give you, the driver, a fixed amount of power to navigate the road in a way you need to. If you are idling at a red light, you require the least amount of the mixture. But, a maximum amount of air to fuel ratio is required, when you are taking your spouse to the hospital for the delivery of your first baby.

Nostalgic Vibes

Vacuum at a fully closed valve is required to run a lot of small auxiliary equipment. For example, less effort on  your part when pressing the brake pedal. In the old days of car manufacturing, which your buddy also reminisces, there were only just three vacuum lines in the whole of the classic muscle cars. Enter the era of speed and one finds dozens of vacuum pipes and connections. Each hose and its connection has now become a potential source of vacuum leak in engine.

EPA to the Rescue

The Environmental Protection Agency regulates car manufacturing as well. You know, because burning fuel, even though to watch baseball, is an atmospheric hazard. The vacuum created in the engine system, when the engine is shut off and the car parked outside the stadium, allows the fuel to remain within the engine and it is not emitted out. These emissions are a fire hazard and are not very fun to inhale.

Manifestation of Vacuum Leak in Engine

Vacuum leak in engine is manifested as erratic idle or decreased fuel efficiency and even check engine light on the dashboard.

Tools of Trade

The most important tool for finding the vacuum leak in engine is the vacuum hose diagram located either under the hood or in your car manual. Make a copy of the diagram and locate all the endings points. Now go through one line at a time and highlight it on the copy as you inspect it. You will look for end fittings and the length of the hose for any visible defects. You will look for the hose elasticity and its grip at the end.

Old Cars

If you car is older than you or your teenage daughter, replace all vacuum lines and their plastic junctions. It may take only 10 feet of hose and half an hour of your time, but it totally saves you from the trouble of finding the individual leaks on the old car


Carburetor cleaner is a hydrocarbon rich liquid which comes in a pressurized can. It can be sprayed onto the suspected leak, while idling, causing the engine to have an influx of rich hydrocarbons and making it go WAAAAA!. Keep your ears open to hear sounds and your eyes closed or at a safe distance from the point of spray as there is a chance that the aerosol spray can hit an already hot piece of engine causing it to flare up.

Propane Torch

Another tool is an unlit propane torch. The principle to find a vacuum leak is the same as for a carb-cleaner. You spray short bursts of propane on to those parts of vacuum hose, which you may think has a leak and keep an eye and ear out for a sudden increase in the engine speed while idling.


Again, just like carb-cleaner and propane torch, this aerosol spray can also be used to detect vacuum leaks in engine. Its effect may not be as strong or discernible as with the other two, but it will get the job done. It will tell you the presence of a leakage and its approximate position in the vacuum hose system.


I confess, of all the methods shared above, this one is the costliest. You take mineral oil and vaporize it to make smoke which is then pumped into the vacuum hose system until it is smoke rich and then it is just a game of finding  where the smoke is coming from. You just have to make sure that the most obvious points of leak such as the intake manifold have been properly sealed before plugging the smoke generator into the hose system.


A single, tiny leak of less than a hundredth of an inch can cause severe ill effects on car performance. So, next time, when you are wondering why my car’s performance is not up to par, vacuum leak of engine might be the reason.


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