Vacuum leaks are one of the most common driveability faults — rough idle, hesitation, raised idle rpm — and one of the easiest to identify on a 5-gas analyzer once you know the signature.
The signature
An unmetered air leak (intake manifold gasket, vacuum hose, throttle shaft seal, brake booster line) adds air the ECU doesn't account for. The exhaust goes lean:
Why CO₂ drops
The extra air both dilutes the exhaust and leans the combustion, reducing CO₂ output. A genuine vacuum leak typically suppresses CO + CO₂ below 12%, so pair the signature with a probe-placement sanity check first.
Confirmation tests
1 · Propane enrichment
With the engine idling, release a small stream of propane around suspected leak points. If rpm rises, that area is drawing unmetered air — propane is being burned where the air is sneaking in.
2 · Smoke machine
Pressurise the intake with smoke. Leaks are visible directly — the canonical confirmation test.
3 · Holy Grail graph
Plot λ vs rpm. Vacuum leaks dominate at idle and their effect fades as rpm rises (the natural airflow eclipses the leak). You'll see λ fall from >1.3 at idle toward 1.0 at 2500+ rpm.
4 · Visual inspection
Cracked rubber hoses, brittle PCV connections, missing or loose clamps, split brake booster lines.
Distinguishing from other lean causes
- MAF under-reporting: ECU still tries to add fuel based on MAF values; pattern is messier, fuel trims tell a different story.
- Fuel delivery failure: typically sets explicit trim codes; HC may be elevated in a different way.
- Secondary Air Injection (SAI): normal during cold start; confirm engine is warm and SAI isn't active.
Case study
// case · 2012 focus 1.6
Complaint: rough idle, hesitation. Idle gases: CO 0.2%, CO₂ 12.0%, HC 60 ppm, O₂ 6.5%, λ 1.42. Signature: vacuum leak. Smoke test showed a split at the intake manifold gasket. Replaced; idle smoothed, O₂ dropped to 0.3%.