What Is a Vacuum Leak? An Easy Beginner’s Guide

Leaks within the air intake system can result in higher fuel consumption, diminished power, and various other complications. At this point, it’s pretty clear that a vacuum leak is undesirable, they are a leading factor in poor performance and the notorious P0171 and P0174 “lean” codes. With all that said, you may be wondering, what is a vacuum leak exactly?

You are in the right place, in this article, I’ll explain what a vacuum leak is, broken down to the fundamentals, so you know everything you need to know about vacuum leaks.

I’ll cover everything you need to know, including symptoms, causes, and even a little diagnosis. Customers will learn what a vacuum leak is, and new technicians will gain the knowledge they need to diagnose this issue correctly.

Let’s get started…

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Understanding Vacuum Leaks: An Overview

Engines need air and fuel to run, air is mixed with the proper amount of fuel and then compressed before being ignited by the car’s ignition system. This is what creates power, in essence, this is the short version of how all internal combustion engines run (with some exceptions.)

Modern engines are in a constant battle to consume less fuel and keep efficiency and emissions at acceptable levels, to create the perfect balance, or “air/fuel mixture” to support efficient combustion.

The most efficient air/fuel mixture is referred to as a Stoichiometric Mixture

This is when all of the air and fuel are burned during a combustion cycle with nothing left over, thus creating optimal efficiency. The perfect stoichiometric mixture ratio is 14.7parts (air) to (1)part gasoline, (courtesy of Wikipedia.)

This is the perfect mix of air and fuel, when a stoichiometric mixture is achieved, combustion is at its most efficient peak.

This is what every car manufacturer is chasing.

How is this “perfect air/fuel ratio” mixture Achieved?

As you can probably imagine several sensors are working together in harmony to relay real-time information to the engine computer. This allows constant adjustment to the fuel injection rate and can be made on the fly.

All of the air entering the engine needs to be accounted for, if air is entering without being accurately measured by an airflow meter or MAP (manifold absolute pressure) sensor it is referred to as “false air.”

A good way to think about this is to imagine trying to bake a cake and accidentally adding too much flour. The recipe calls for a specific amount of flour, and any extra flour would throw off the balance and ruin the cake.

In the same way, any air entering the combustion chamber without being measured will disrupt the fuel mixture the computer is desperately trying to keep at stoichiometric.

What is a Vacuum Leak?

A vacuum leak is an unintended leak in the engine air intake, it may be a cracked hose, bad intake gaskets, or even a crankcase breather issue. It’s important to know that a vacuum leak is simply unmeasured air being pulled into the engine through a component that should be sealed.

The components of the air intake system are measuring data and sending it to the engine computer constantly, excess air entering the engine will drive the mixture “lean.”

After combustion, the oxygen sensors, (located in the exhaust downstream) read the exhaust gas and send the data to the engine computer. This is how the engine computer knows whether enough fuel and air are present for efficient combustion, in essence, whether the mixture is rich, lean, or just right. The oxygen sensors will pick up on the lean condition and eventually turn the check engine light on when an intake leak is present.

An active vacuum leak causes a discrepancy between what the air intake sensors are reading and what the o2 sensors are reporting on the other end. The oxygen sensors tell the computer the mixture is too lean and in turn, the injector pulse width is increased, adding more fuel into the combustion chamber to compensate for the extra air.

There is a limit to how much the computer deems acceptable for enrichment. This is recorded as “fuel trim.” There are two types of fuel trim, short-term and long-term ranging from -25 to +25 percent. If the vehicle has an intake air leak the short-term fuel trim will rise (the computer is adding fuel) as the realtime short-term fuel trim rises it will move the long-term up.

If the long-term fuel trim is driven past the designated specification the check engine light will come on for a “lean” condition. This indicates the engine computer has needed to add more and more fuel without being able to “richen” up the mixture enough.

The same is true for an overly rich running condition, the ECM pulls fuel back, and if the long term goes to -25 percent a rich code is set.

Ouick note: -25 and +25 are common tolerances, however, all vehicles have their own parameters.

Common Symptoms of a Vacuum Leak

Common symptoms of a vacuum leak include rough idling and poor performance. Depending on the size of the leak random misfiring can be felt, along with a “lean backfire.” If the leak is too large the engine may start and stall out right away.

Another concern is poor fuel economy, this is due to the larger amount of fuel being injected into the combustion chamber to compensate for the air leak. The check engine light will illuminate over time if the extra air drives the fuel trim too far to enrichment.

This indicates the computer is not happy with the lean mixture.

Common causes of Vacuum Leaks

Deteriorated vacuum hoses can lead to leaks in the system. Faulty intake manifold gaskets are another common cause of vacuum leaks. A damaged throttle body gasket or air intake boot after the MAF sensor can result in a vacuum leak as well.

Leaks can also occur due to a bad crankcase ventilation valve, or “breather box.” This is very common on BMW, AUDI, and Volvos, but can happen on any car.

How are Vacuum System Leaks Diagnosed?

The best method is smoke testing, which helps identify the presence of leaks by introducing smoke into the system and observing where it escapes.

The smoke machine injects smoke at very low pressure, over time the entire intake fills with smoke and leaks can be seen. Remember, vacuum and pressure are different, there are times when things seem to seal themselves when being pulled by a vacuum rather than being pushed out by pressure.

Another way to test the crankcase breather system is by using a CEM meter, this meter will measure the crankcase vacuum in millibar. If the breather’s diaphragm tears vacuum becomes uncontrollable in the engine crankcase resulting in a very lean condition.

Conclusion

In conclusion, it is crucial to address vacuum leaks promptly as they can negatively impact your vehicle’s performance and fuel efficiency. If you are experiencing poor fuel mileage or rough running, Ignoring the symptoms or delaying repairs can lead to further damage and costly repairs down the line.

For techs, If you suspect a vacuum leak in your customer’s vehicle be sure to check and record the fuel trim data before clearing any codes. Use the smoke machine to check for leaks and always measure the crankcase pressure and compare it to the vehicle’s specifications.

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