Vacuum leaks: problems, symptoms, repairs
What is a vacuum leak? It's 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.
Engine air induction system diagram
The engine computer (PCM) 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.
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.
Common sources of vacuum leaks:
Intake manifolds and gaskets:
Positive Crankcase Ventilation or PCV system: The PCV system removes blow-by gases and oil vapors from the engine crankcase. The PCV system collects the oil from vapors using an oil separator. After that the vapors are directed into the engine intake through the PCV valve.
In many BMW, Volvo and other European models, the plastic and rubber parts of the PCV system crack and fall apart at higher mileage, creating vacuum leaks. In some GM cars, a PCV valve has a rubber diagram inside that is very common to fail. 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. Removing an intake manifold is a labor-intensive job.
Diagnosing vacuum leaks is not always easy. Mechanics use a scan tool and other testing equipment. One way is to look for common problems in your make model and year. 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. We also found many Youtube videos on diagnosing vacuum leaks. Checking the technical service bulletins can also help. We posted several links where you can get an access to a factory repair manual for a subscription fee in this article.
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