Symptoms & Solutions of a Vacuum Leak in a Car

I hear a loud hissing from within my car. Could it be a vacuum leak? 

How do you know that your car has a vacuum leak issue? Save your car from secondary loss with these easy and cost-effective troubleshooting tips. 

Before anything else, what is a vacuum leak?

A Vacuum Leak Spells Trouble to Your Car

It’s a critical issue that involves a leakage in the engine’s vacuum system. It can be anywhere between the engine and the mass airflow (MAF) sensor or air flow metre (AFM), which is located at the air filter box. A leak along this network can spell trouble for you because it will reduce your car’s overall performance and may cause secondary problems. 

You can read more about air flow metre and its functions in our previous post. For now, it will suffice to mention that an AFM (or MAF) sensor measures the mass of air intake and sends this info to the ECM or PCM, which will then control the correct amount of fuel to be injected based on this measurement.

In a vacuum leak situation, the amount of air actually entering the engine is not measured correctly by the AFM, causing the PCM to miscalculate. This results in too little fuel and too much air, which is commonly referred to as an engine running lean. 

What Are the Symptoms of a Vacuum Leak?

Too much air into the engine can slow down your vehicle. That’s one symptom, but there are more. Please take note that other mechanical issues can also cause some of these symptoms. In other words, please diagnose and troubleshoot with caution.

Hissing Sound

This is a common symptom of a vacuum leak in a car. If caused by a vacuum leak, the hissing sound should come from the engine area when the engine is running. It’s like a quiet purr and difficult to hear normally, but experienced mechanics will have no problem detecting it. Maybe you’ll take some time to pick up this skill, but if you listen intently, you’ll also catch the sound. Broken vacuum hoses create this high pitch sound as they suck air through the leak. If you follow the hissing sound, it will lead you to the leak.

High Idle RPM

If you encounter a large vacuum leak, you will often experience rough and high idle RPM. A rough or bumpy idling happens because the throttle body tries to stabilise by opening and closing. 


Engine misfires are indicative of a vacuum leak. Your car cannot fire the cylinder properly due to the extra air intake, which disrupts the correct air-fuel proportion of fuel to the engine. This becomes noticeable when hard accelerating, such as when climbing hills or pulling away quickly from red lights.

Engine Check Light

The easiest way to tell is through that ‘Check Engine’ light indicator. If you see this light activated, along with other symptoms that we have described in this article, then you can include vacuum leaks in your list of potential causes.

High or Slow Acceleration

If you find any problem when doing high or slow acceleration in your car, then you can narrow down the possible causes to include a vacuum leak between the engine and airflow area. The vehicle could be reacting roughly to an incorrect air-fuel ratio caused by a vacuum leak. 

Brake Pedal Press Issues

Heavy diesel engines improve brake quality and overall performance by using a vacuum pump booster. It helps to engage the right suction for accurate brakes. When there’s a vacuum leak issue, you will find the brake pedal hard to press but will work if you apply a bit more force.

Poor Fuel Consumption 

You will notice poor or high fuel consumption when there’s a vacuum leak. If your car is fuel-efficient on average but suddenly guzzles more diesel than usual, you better start looking for a leak. 

Oil Leakag

Vacuum pumps are located either on the right or left side of your vehicle’s engine. If you notice an oil leak in the vacuum pumps’ area, the culprit could be a vacuum leak. You should check around the pumps to sort out this issue before they start causing other faults.

Black Exhaust

Excessive black smoke from the exhaust, shrinking gaskets along with O rings, weak gears shifting transmission points, and cracked rubber hoses of the vacuum are also symptoms of a vacuum leak. They can lead to serious secondary headaches if left unattended, so you need to sort out the vacuum leak as early as possible.

How to Locate a Vacuum Leak

Once you confirm a vacuum leak, you should repair it immediately. Locating the vacuum leak is the hardest part of this process. I am describing some methods below to detect the leak and get the necessary repairs.

By Visual Inspection

The first step is to detect a vacuum leak through keen visual inspection. Search for burnt hoses and damaged plastic or rubber connections. This is the simplest process of inspecting. Each car model has a unique hose system, so you should pay attention to the clamps that join them together.

Using Soapy Water to Detect Vacuum Leaks

This is the most commonly used for detecting vacuum leaks and also the most affordable and easy to DIY. It will show you the leakage spot through bubbles. You will need a spray bottle filled with soapy water. First, turn the engine on and leave it idling. Then spray the suspected area – possibly the vacuum hoses or connectors – with the soapy solution. 

In other words, this method should work with the first one – visual inspection – so that you don’t spray randomly. Look out for bubbles in the area applied with soapy water because these are dead giveaways of a vacuum leak.

Using a Smoke Machine for Vacuum Leaks Detection

An automotive smoke machine is typically used by professionals to locate a vacuum leak. It always provides accurate results. 

Using WD40 to Locate a Vacuum Leak

You can safely use a can of WD40 to locate a vacuum leak. To do it, start the engine and leave it idle. Spray on the hoses and around the inlets. If you find the engine’s speed increasing when you spray a particular area, that’s the likely location of your leak.

So once you’ve located the vacuum leak, what happens next? 

How to Fix a Vacuum Leak

Of course, you don’t locate a leak and leave it there. That would be folly.

The final step is fixing it. Below are some quick and dirty tips for troubleshooting a vacuum leak.

Sealing the Holes

If the holes are found on the hoses, you can use a sealant to shut them. In this case, rubber sealants work perfectly and are the best replacement for gorilla duct tapes. Just make sure that the hoses are not near any hot surfaces. Hot surfaces can melt the sealant and cause more damage to the unit.

Replacing the Hoses

For massive damage to the hoses, patching won’t work. Replacing the hoses is the only feasible solution. Replacing hoses is easy and affordable.

Seek Professional Service

If the leak does not occur in the hoses, it can be in the intake manifold section. It is a very complex area to deal with if you don’t have the skills. Don’t compromise your car’s health and your safety. It’s best to consult the Pro!

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