Q: My truck has trouble starting, and when it does start, it feels like it's choking or struggling to stay on.

The EVAP system prevents fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP system collects and temporarily stores the fuel vapors in the charcoal canister. The charcoal canister is filled with activated carbon pellets that can absorb the fuel vapors. When the engine is running, the fuel vapors are purged from the canister and burned in the engine.

The vent control valve (solenoid) controls the flow of outside air in and out of the charcoal canister. One side of the vent valve is connected to the charcoal canister. The other side is connected to the vent hose that has a filter or screen at the end and is attached to the car body or frame. In some cars, the vent valve is attached to the canister; in others, it’s installed separately near the canister. The vent valve is controlled by the engine computer (PCM). Normally, the vent valve is open. It closes when the engine computer tests the EVAP system for leaks.

If a leak in the EVAP system is detected, the Check Engine Light will illuminate on the dash and the trouble code related to the problem will be stored in the engine computer. When an air leak is present in the EVAP system, the MAP sensor will not be able to determine the air density, and the MAF sensor will not be able to determine the volume of air entering the engine correctly. This will lead to an over-fueling situation or an under-fueling situation depending on the size of the air leak and engine operating speeds and load. This may eventually lead to the destruction of the catalytic converter as well.

I would recommend having an expert, such as one from YourMechanic, come to your home to take a look at your car to determine why the Check Engine Light is on and if your EVAP purge solenoid needs to be replaced.