A vacuum leak is a leak anywhere between the engine and a mass air flow sensor. In most cars, a mass airflow sensor is installed at the air filter box. In the fuel injection system, the mass air flow sensor measures the air flow into the engine. The engine computer (Power Control Module) calculates how much fuel to inject based on the mass air flow sensor measurements. If there is a vacuum leak anywhere between the engine and the mass air flow sensor, it causes "unmetered" air to enter the system. This causes the actual air flow to be higher than the mass air flow sensor measures. As a result, the PCM miscalculates the amount of the injected fuel, and the engine runs "lean". The term "lean" means too much air and too little fuel. The effect of a vacuum leak is more noticeable at idle, when the air flow is lower.
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| Air induction system schematic |
Vacuum Leak Symptoms
Symptoms of a vacuum leak include the Check Engine light, rough idle, stalling and a hissing sound coming from the engine bay. The engine may run well at higher RPMs, but surges, runs rough and struggles to maintain stable RPMs at idle. Often, the engine stalls when stopping.
With a scan tool, one of the signs of a vacuum leak is the Long Term Fuel Trim (LTFT) going towards the plus (lean) side (e.g. over +15%) at idle, but returning close to zero at higher RPMs. This is an indication that the engine runs lean at idle. Always get a good scan tool where possible, it's better than guess work.
Common sources of vacuum leaks:
Cracked Intake snorkel: A rubber or plastic boot (snorkel) that connects the engine intake and the air filter box can rip or crack. It's a very common problem in older and high-mileage cars. The repair is easy, the snorkel must be replaced. In most cars, it's secured by worm gear clamps at both ends.
If an intake boot is not installed properly at an air filter box or at a throttle body, it can cause a vacuum leak too.
Leaking Intake Manifolds and Gaskets: An intake manifold is bolted to the engine head or plenum. With age, plastic intake manifolds warp. Gaskets and O-rings that seal the gaps harden and shrink. This causes vacuum leaks. This problem is very common in many cars. Plastic intake manifolds are especially prone to leaks.
To repair the problem, leaking gaskets and O-rings must be replaced. Gaskets and O-rings are cheap.
A plastic intake manifold can also crack, or one of the fittings can break off.
Cracked Plastic and Rubber Vacuum Hoses and Lines: With age, vacuum hoses become brittle and crack or rip. The hoses and lines that connect the PCV system to the intake manifold are especially prone to failures. This happens because crankcase vapours contain oil, that damages the rubber or plastic, causing hoses and lines to swell and crack. The repair is to replace a cracked vacuum hose or line.
Positive Crankcase Ventilation or PCV system: The PCV system removes blow-by gases and oil vapours from the engine crankcase. The PCV system collects the oil from vapours using an oil separator. After that the vapours are directed into the engine intake through the PCV valve.
In many cars, the plastic and rubber parts of the PCV system crack and fall apart at higher mileage, creating vacuum leaks. To repair the problem, failed parts must be replaced. Parts are not very expensive, but in many cars the components of the PCV system are installed under the intake manifold.
EGR Valve:
The EGR system directs a small amount of exhaust gases into the intake to lower the combustion temperature. An EGR Valve connects the intake manifold to the exhaust system. Normally, it opens only when the car is driven steady at higher speeds. Often, however, an EGR valve sticks in older cars due to carbon buildup. If an EGR valve is stuck open, it creates a vacuum leak, with the only difference that instead of air, it's the exhaust gases that enter the intake manifold.
A stuck-open EGR valve causes the same symptoms as vacuum leaks. Often, problems with an EGR valve that is stuck open are more noticeably after driving on the highway. For example, a car may stall (go off) when stopping after a long drive.
Brake Booster
A brake booster is installed between the brake pedal and the brake master cylinder. In most cars, it's connected to the engine intake and is operated by the engine vacuum. Inside a brake booster there is a vacuum diaphragm. When a brake booster leaks, it creates a vacuum leak. Symptoms of a leaking brake booster include a hissing noise coming from the brake pedal area and lack of brake assist.
A leaking brake booster is a safety concern and must be replaced. Thankfully, this problem is not common.
Diagnosing vacuum leaks is not always easy. If your car has a vacuum leak, there is a good chance someone already had the same problem in the same car. Try, for example, searching YouTube or Google for 'Toyota Corolla 2005 common vacuum leak' and you will see people posting the repair information. You can also search Youtube.
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