How to Find a Vacuum Leak Without Guessing
Vacuum leaks can cause lean codes, rough idle, high idle, stalling, and misfires. A visual inspection and smoke test are safer and more reliable than spraying flammable chemicals around a running engine.
Visual inspection
Look for cracked hoses, loose clamps, disconnected fittings, damaged intake ducting, and seals disturbed during recent repairs.
Use fuel trims
High positive fuel trims at idle that improve as RPM rises can support the possibility of a vacuum leak, though scan data must be interpreted in context.
Smoke testing
A low-pressure automotive smoke machine can reveal leaks at hoses, gaskets, PCV plumbing, and intake connections. Follow the tool manufacturer's instructions.
After the repair
Clear codes only after documenting them, then verify fuel trims and idle behavior after the repair.
Bottom line
Diagnose the cause before ordering parts. Vehicle configurations vary, so verify specifications and procedures for your exact year, engine, and model.