The engine in your car is essentially a big, powerful vacuum pump. The up-and-down motion of its pistons creates a vacuum, drawing air into the engine to mix with fuel and produce energy.
The air the engine uses is carefully metered and measured by computerized sensors, and that data is used by the engine’s computer to figure out exactly how much fuel is needed for maximum power and efficiency.
When unwanted, unmetered air gets into the engine, it’s called a vacuum leak. These leaks can create a lot of problems for your vehicle. Let’s take a look at some symptoms of a vacuum leak, or you can jump ahead to some possible causes and solutions.
The check engine light is on: Your engine’s computer can detect vacuum leaks by comparing data from various sensors. If the data from one sensor doesn’t match up with what the other sensors are reporting, the computer knows something’s wrong. It’ll log a trouble code that is retrievable with a scanning tool, and it’ll turn on the check engine light on your dashboard.
The car’s idle is high or sporadic: The amount of air that’s supposed to get into your engine is limited by a throttle body. This is a butterfly valve that opens as you press the accelerator pedal. The more you press, the larger the opening gets, allowing more air into the engine and increasing its RPMs. Since a vacuum leak similarly lets air into the engine, the engine will respond by idling faster. When this happens, the car’s computer will try to compensate, typically creating a sporadic or fluctuating idle speed.
The engine stalls: In some cases, a vacuum leak can cause your engine to die or stall out. If the unmetered air is excessive, the engine’s sensors may not be able to report data back to the computer correctly. This can can cause the engine to stall or only stay running when you press the throttle, and it can make it hard to start the car.
You hear a squealing or sucking sound from the engine: A vacuum leak can sound just like the end of the tube on your home vacuum cleaner. If you hear a sucking noise from under your hood, you likely have a vacuum leak.
Get it diagnosed by a professional
Broken vacuum tube or hose: The most common cause of a vacuum leak is a broken, torn or disconnected vacuum hose or tube. Because of the extreme heat generated by an internal combustion engine, these plastic and rubber tubes can deteriorate or break and cause a vacuum leak. There are a lot of fittings that connect the tubes and hoses that can break, as well.
Leaking actuators, solenoids, valves and regulators: The parts that the vacuum hoses and tubes lead to can also leak. Engines made in the last two to three decades often have exhaust gas recirculation (EGR) valves, secondary air injection valves, fuel pressure regulators and purge valves that require a vacuum source to operate. If these parts crack or fail, they can cause a vacuum leak. In addition, some older cars have vacuum-operated actuators for air conditioning, heat and even door locks that can leak and cause issues.
Leaking gaskets and seals: Your engine uses several gaskets and seals to contain things like oil and coolant, but these are also used to make airtight seals for parts like the intake manifold and throttle bodies. Seals can dry up over time and begin to leak, and this can cause vacuum leaks that can be more difficult to detect.
Leaking brake booster: Cars that use a brake booster in the power braking system can experience a vacuum leak if the diaphragm in the booster fails. The first sign of this will be a brake pedal that’s hard to press. The check engine light also typically will come on. Before replacing the brake booster, make sure the line going to it is connected and not damaged.
» LEARN MORE: Get an estimate for your vacuum leak diagnosis
Start with a visual inspection: If you suspect your car has a vacuum leak, the first step in diagnosis is taking a good look at the engine. Because the most common cause is leaking tubes and hoses, open the hood and look for anything that seems out of place. Some cars have a vacuum hose routing diagram sticker that you can use for reference. If you find a broken or torn hose, try to track down a factory replacement part. If the factory tube or hose isn’t available, you can try to repair it with generic vacuum hoses or tubing from an auto parts store.
Do a smoke test: Using smoke is a safe and effective way to find vacuum leaks. It’s possible to do this yourself, but if you aren’t sure how your engine’s intake works or just don’t feel comfortable with it, a mechanic can do the inspection instead.
Most shops will use a professional testing machine that generates smoke from a special liquid similar to the “fog” used at concerts or parties. It’s injected into the engine’s air intake system with the engine off — from there, you just look for smoke leaking from a defective part. If smoke is seen coming from any valve, solenoid, actuator or regulator, new parts are typically needed. If smoke is found leaking from where the intake meets the cylinder head, usually a new gasket will fix the situation.
Don’t use flammable liquids: Some people use products such as starting fluid or brake cleaner to find vacuum leaks by spraying them on and around the running engine. The idea is that if a leak is present, the highly flammable chemical will be sucked into the engine and burned as fuel, causing an increase in RPM and indicating where the leak is. This is highly dangerous for obvious reasons and should be avoided. With the availability of pressurized smoke cans, spraying dangerous chemicals on a hot engine is just not necessary.
Although vacuum leaks can be a pain, most of them are preventable. Periodically look over your vacuum lines. On older cars especially, they will start to get hard and brittle. At this point you know it’s time for replacement.
It’s also important to be gentle with these tubes when performing other car maintenance. Vacuum leaks can be caused during general maintenance like changing out spark plugs. Carefully remove and replace these lines, and you can avoid a trip to the repair shop.