The function of the idle cutoff valve is to make sure a hot engine shuts down properly when you turn the ignition key to off. Hot engines will sometimes “diesel” and keep running on poor gas, or in an overheated condition. On the other side of the coin, if your cutoff valve is bad, your engine won’t idle at all. To test your valve, turn on the ignition key without starting the engine. Go to the engine compartment and pull the wire off and on the switch terminal. If the switch is good, you will hear a metallic “clunk” as it cycles on and off. You can also bench test your switch by grounding the housing onto a battery minus terminal, and touching the actuating spade via a wire to the plus terminal.