A broken vacuum hose during a TSI smog check will typically cause which pollutant to rise at idle?

• A. HC would increase at idle.
• B. CO would increase at idle.
• C. NOx would increase at idle.
• D. NOx would decrease at idle.

Answer: (A)

A broken vacuum hose lets unmetered air into the intake, creating a lean air-fuel mixture at idle. The engine management system may try to compensate by adding more fuel, but at idle the mixture often remains lean enough that combustion can be incomplete in some cylinders, allowing unburned fuel to escape as hydrocarbons. Those hydrocarbons show up as higher HC emissions in the exhaust. The extra oxygen in a lean condition tends to help oxidize CO, so CO is not typically increased and may even drop. NOx formation relies on high combustion temperatures, which aren’t reached at idle, so NOx doesn’t usually rise from a vacuum leak. Therefore, hydrocarbon emissions rise.