Elusive vacuum leaks can create confusion when you're trying to diagnose your car. Here are symptoms to look for, plus how to find the leak.
Time Under an hour
ComplexityIntermediate
CostVaries
Introduction
When I was in college, my old Subaru started sputtering and stalling out when I'd stop at traffic lights. At the time, I was not too wrench-savvy, but also too broke to pay a mechanic. So I asked a handy friend what he thought the problem was. He said, "Maybe a vacuum leak. But those are tricky to find."
"Baloney," I thought. I opened the hood and found the problem within a matter of minutes. It was a cracked hose. I was lucky, though, because sometimes it's not quite that simple.
If your car is running rough, not accelerating well, hissing or has thrown an engine code, here's how to figure out if it's a vacuum leak and where it's leaking.
What Is a Vacuum Leak?
Vehicle engines create a vacuum, which depending on your vehicle, controls systems like assisting power brakes or helping with the car's heating and cooling. A vacuum leak is a hole or crack in a hose, seal or elsewhere in that system, which allows unmetered air to bypass the air filter and enter the engine.
"This disrupts the optimal air-fuel ratio needed for efficient combustion," says Paul Knoll, an automotive expert with American Muscle. "As a result, vacuum leaks can lead to engine performance issues and impact the performance of different components, like the engine control unit (ECU) and various sensors." 

What Are the Signs of a Vacuum Leak?
Common signs of a vacuum leak include:
- Hissing noise in the engine bay
- Rough idle, either uneven or running at a higher or lower RPM
- Check engine light due to an imbalance in the air-fuel mixture
- Hesitation and lack of power when accelerating
- Engine stalling, especially at idle or when coming to a stop
- Decreased fuel efficiency due to inefficient combustion
- Issues with the heating and cooling system, such as being unable to switch the direction of the AC from blowing at the window to blowing out your vents
- Greater resistance from your power brakes. "The vacuum assist is supposed to make the pedal easy to push, so if you're facing difficulty braking, a leak could be to blame," says Duane "Doc" Watson, a technical trainer at Bosch Mobility Aftermarket.
Safety Precautions
Identifying and fixing a vacuum leak can expose you to unhealthy exhaust fumes, so be sure to work in a well-ventilated area and wear appropriate safety gear, including gloves and eye protection. Also, diagnosing it might involve using sprays such as carburetor cleaner, which are extremely combustible, so avoid any open flames and keep a fire extinguisher nearby, as even a hot exhaust manifold can cause them to catch fire.
"Since the engine will be running while you're making this diagnosis, I always recommend abiding by what we call the 'electrician's rule,'" says Watson. "Keep one hand behind your back while working on anything electrical. This way, you won't be tempted to rest your hand on an alternator or any other belt-operated parts of the engine that are spinning."
Tools Required
- Automotive stethoscope (optional)
- Scan tool (optional)
- Screwdrivers
- Smoke machine (optional)
- Vacuum gauge (optional)
- Wrenches
Materials Required
- Carburetor or brake cleaner spray and/or soapy water
- Replacement hoses or clams and/or seals
Project step-by-step (4)
Step 1
How to identify a vacuum leak
There are several ways to find out if you have a vacuum leak. First, check under the hood to see if you have a label there that depicts the hose routings, as that will streamline the process of locating the vacuum hoses, says Watson. Then, depending on what tools you have access to:
- Visually inspect hoses, connections and the intake manifold for cracks, breaks, loose or broken seals and disconnected parts.
- Listen for hissing sounds while the engine is running.
- Use an automotive stethoscope. “It includes a wand that you move across the top of the engine while it’s running, and when it identifies a leak, it will amplify the hissing sound the leak is causing,” says Watson.
- Use a smoke machine to put smoke into the intake system to see where it escapes (do this with the engine off).
- Spray carburetor cleaner or soapy water around suspected areas while the engine is running. When there’s a change in RPM, that could indicate the location of the leak.
“This can be dangerous, though,” says Chris “Moose” Pyle, a master certified technician with Just Answer. “If you have bad spark plug wires, there will be a possible flame.” As such, he suggests using brake cleaner, which evaporates quickly, and spraying it in small bursts.
Knoll also says, “Ideally, working in a quiet area and taking a systematic approach, checking one area at a time, will help you identify the vacuum leak quicker.”
Step 2
How to fix a vacuum leak
Once you know where the leak is, then you’ll need to fix the problem. If it’s the result of a rotten or cracked vacuum line, the repair will be fairly simple. “Just head to your local auto parts store, purchase the correct size of repair hose and make the switch according to the vehicle manufacturer’s instructions,” says Watson.
But, if the leak is due to an intake gasket leak or cracked intake manifold, the repair process will be far more complex, and could involve removing engine accessories and parts. “In this scenario, you’ll want to take your car into a shop and have a professional conduct the repair for you,” says Watson.
Step 3
FAQ
What causes vacuum leaks?
Knoll says common causes of vacuum leaks include aging hoses, loose or damaged connections, faulty gaskets, or accidental damage from previous repairs.
In Pyle’s experience, it’s often the latter. “For example, you were fixing something and did not do the job right, or you moved a brittle hose, causing it to crack, and now it leaks,” he says.
Will a vacuum leak throw a code?
Yes. If the leak is severe enough, a vacuum leak will likely throw a P0171 or P0174 diagnostic trouble code.
“This happens when air is entering the engine from beyond the throttle, and the computer doesn’t know how to compensate for the unmetered air,” says Watson. “This causes the engine to go lean, which can result in overheating or a worn-out oxygen sensor.”
How do you fix vacuum leaks?
First, figure out where the vacuum leak is coming from. Then, it’s a matter of replacing the damaged, broken or cracked part. That could be as easy as replacing a rotted hose or tightening a loose connection. If it’s a hard-to-access cracked seal or gasket, that could mean a major repair, like disassembling part of the engine.
How much does it cost for a mechanic to check for vacuum leaks?
A mechanic will probably charge you for an hour or so of labor to diagnose a vacuum leak, which will likely run between $100 and $150, depending on their hourly labor rates. To repair it will be more expensive, commonly between one and eight hours of labor. But the parts are generally inexpensive. “Plastic lines, gaskets, rubber hoses; parts like these could range from less than $10 to $40,” says Watson.
Step 4
About the Experts
- Duane “Doc” Watson is a technical trainer at Bosch Mobility Aftermarket. He has more than 45 years of experience in automotive repair, has trained thousands of technicians and earned many industry awards, including Chevrolet Technician of the Year and Buick Service Master of the Year.
- Paul Knoll is a dedicated car and truck enthusiast with a lifelong passion for vehicles and sharing his knowledge with the automotive community. He has worked his way up at AmericanMuscle.com and AmericanTrucks.com for more than a decade, and is currently marketing director.
- Chris “Moose” Pyle is a master-certified technician with 20-plus years of automotive experience. He specializes in gas and diesel engines, transmissions, steering, suspension, brakes and electrical diagnostics. He has also worked as an expert for JustAnswer.com since 2006.
Originally Published: November 27, 2018
