Vacuum leaks: problems, symptoms, repairs
What is the definition of a vacuum leak? Anywhere between the engine and a mass air flow sensor, there’s a leak. A mass airflow sensor is positioned at the air filter box in most autos, as shown in the diagram.
The mass air flow sensor in the fuel injection system detects the air flow into the engine. Based on the mass air flow sensor values, the engine computer (PCM) calculates how much fuel to inject.
If a vacuum leak exists between the engine and the mass air flow sensor, “unmetered” air will enter the system. As a result, the actual air flow is more than what the mass air flow sensor detects. As a result, the PCM incorrectly calculates the amount of injected fuel, causing the engine to run “lean.” The phrase “lean” refers to a situation in which there is too much air and not enough fuel. At idle, when the air flow is smaller, the effect of a vacuum leak is more obvious.
Vacuum leak symptoms
The Check Engine light, a rough idle, stalling, and a hissing sound emanating from the engine bay are all signs of a vacuum leak. At higher RPMs, the engine performs well, but at idle, it surges, runs rough, and struggles to maintain steady RPMs. When stopped, the engine frequently stalls. The Long Term Fuel Trim (LTFT) going towards the plus (lean) side (e.g. above +15 percent) at idle but returning near to zero at higher RPMs is one of the symptoms of a vacuum leak, according to a scan tool. This means that the engine is running lean at idle.
Common sources of vacuum leaks:
Intake snorkel: Rips or cracks can occur in the rubber or plastic duct or boot (snorkel) that connects the engine intake and the air filter box. It’s a typical issue in older, high-mileage vehicles. The only thing that needs to be replaced is the rubber boot. Worm gear clamps are used on both ends in most automobiles. The part ranges in price from $25 to $65. A vacuum leak can also occur if an intake boot is not correctly placed at an air filter box (as shown in this photo) or at a throttle body.
Intake manifolds and gaskets: The engine head or plenum is fastened to the intake manifold. Plastic intake manifolds deteriorate with age.
Gaskets and O-rings used to seal gaps harden and shrink with time. Vacuum leaks arise as a result of this. Many cars, including Ford, Toyota, Chrysler, and General Motors, have this issue. Leaks are especially common in plastic intake manifolds. More information on the intake manifold can be found here.
Leaking gaskets and O-rings must be changed to fix the problem. Gaskets and O-rings are inexpensive, but removing the intake manifold is time-consuming. Parts (gasket kit) cost $20-$60, with labor costing $150-$450.
A plastic intake manifold might crack or one of the fittings can break off in some autos. Intake manifold cracks, for example, were rather prevalent in some early Ford engines. In this instance, an intake manifold ($90-$320) must be changed.