A vacuum leak lets unmetered air into the engine after the mass airflow sensor, so the mixture runs lean. Typical symptoms are a high or rough idle, a hissing sound, hesitation, misfires and lean codes such as P0171 and P0174. Cracked vacuum hoses, intake manifold gaskets, the PCV system and the brake booster hose are the usual suspects, and a smoke test finds the leak reliably.
Vacuum leak symptoms
- High, rough or hunting idle
- Hissing or whistling from the engine bay
- Hesitation and misfires, especially at idle; see engine misfire
- Lean codes on one or both banks, and high positive fuel trims at idle that drop as engine speed rises
- A hard brake pedal if the brake booster hose or booster is leaking
Common sources
- Cracked or disconnected vacuum hoses and fittings
- Intake manifold and throttle body gaskets; see intake manifold gasket symptoms
- PCV valve and hoses; see PCV valve
- The brake booster and its hose
- Intake duct leaks between the MAF sensor and throttle body
How to find a vacuum leak
- Listen for hissing with the engine idling.
- Inspect hoses and fittings for cracks, softness and loose connections.
- Smoke test: a smoke machine fills the intake with visible smoke that escapes at the leak; it is the most reliable method.
- Read fuel trims: high positive trims at idle that fall at higher engine speed suggest a vacuum leak rather than a fuel delivery problem.
Spraying flammable cleaners around a running engine to find leaks is a fire risk; a smoke test is the safer choice.
Why the symptoms are worst at idle
At idle the engine draws little air, so a small leak is a large share of the total and upsets the mixture most. At higher engine speeds the measured airflow is much larger and the same leak matters less, which is why rough idle with smooth cruising is a classic vacuum leak pattern.
Can a vacuum leak damage the engine?
A small leak mostly costs drivability and fuel economy, but a large one causes lean misfires, and repeated misfires can overheat the catalytic converter. Running lean under heavy load is also harder on the engine because lean mixtures burn hotter. Fix leaks promptly, especially on boosted engines.
Fixing common leaks
Replace cracked hoses and fittings, re-seat loose connections, and replace leaking gaskets. After the repair, clear the codes and check that idle and fuel trims return to normal.
Vacuum leaks and boost
On supercharged and turbocharged engines, the same leaks can also leak boost under load, costing power; see what does an intercooler do? and turbo failure symptoms.
Related: MAF sensor · throttle body.
More guides: rough idle · MAP sensor · fuel trim · idle air control valve · car hesitates when accelerating.
Trouble codes: P0171 System Too Lean Bank 1 · P0174 System Too Lean Bank 2 · P0106 MAP Sensor Range/Performance · P0507 Idle RPM Higher Than Expected · P0108 MAP Sensor Circuit High · P2187 System Too Lean at Idle · all OBD2 codes.
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