Vacuum Leak

The intake manifold runs at lower pressure than outside air, so any crack in a hose, gasket or valve lets air in. That air is not measured, and the control unit does not add fuel for it at first. Typical sources are cracked rubber hoses, intake gaskets, crankcase ventilation valves and brake booster lines.

Typical leak points are cracked hoses, intake gaskets, brake booster hoses and crankcase ventilation parts. The intake manifold of a petrol engine runs at lower pressure than outside air whenever the throttle is partly closed, so any opening draws air in. Leaks are usually most noticeable at idle, where they make up a larger share of total air flow, and less noticeable at higher speeds.

Fuel trims rise as the control unit corrects the lean mixture. The effect is largest at idle, when intake vacuum is highest and air flow is smallest. Air that enters after the mass air flow sensor is not measured, so the mixture becomes lean and the control unit adds fuel to compensate.

Vacuum leaks cause a hissing sound, rough or high idle, hesitation and lean codes P0171 or P0174. A smoke test is the quickest way for a workshop to find them. A vacuum leak can cause rough idle, hissing, lean codes such as P0171 and high fuel trims. A workshop can find leaks quickly with a smoke test.