How can you tell if the gas cap or fuel is bad on a 1997 Ford Expedition when check engine light comes and AutoZone said valve sticking?

While a sticking valve can trigger a check engine light, it is unlikely to be caused by a bad gas cap or fuel. To determine the exact cause of the check engine light, you will need to have the trouble codes read from the vehicle's computer. AutoZone or other auto parts stores typically offer this service for free. Once you have the trouble codes, you can research them to get a better understanding of the issue.

That being said, here are some symptoms that may indicate a bad gas cap or fuel:

Bad Gas Cap:

* Difficulty starting the vehicle

* Engine stalling or running rough

* Check engine light illuminated

* Strong fuel odor

* Poor fuel economy

Bad Fuel:

* Engine knocking or pinging

* Reduced power and acceleration

* Check engine light illuminated

* Difficulty starting the vehicle

* Stalling or rough idling

If you suspect that the gas cap or fuel may be the cause of the check engine light, you can try replacing the gas cap first. If that does not resolve the issue, you may need to have the fuel system inspected by a qualified mechanic.