Which pollutants would increase if the purge control solenoid has been bypassed?

• A. CO and HC
• B. NOx and CO
• C. HC and NOx
• D. CO and NOx

Answer: (A)

The purge control solenoid is part of the evaporative emission system and meters fuel vapors from the charcoal canister into the intake to be burned in the engine, which lowers hydrocarbon emissions. If this solenoid is bypassed, that purge function is not properly controlling how vapors enter the engine. The result is more fuel vapor either escaping or entering the engine in an uncontrolled way, which tends to create a richer air-fuel mixture and more unburned hydrocarbons in the exhaust. That richer mixture also makes combustion less complete, pushing carbon monoxide up as well. Nitrogen oxides aren’t as directly affected by this vapor purge change, since they’re more tied to combustion temperature and excess air. So the pollutants that would increase are hydrocarbons and carbon monoxide.