How to Restart a Car That Has Run Out of Gas

by Richard Rowe

    They used to call it "driver's education:" It was that 20 minutes you spent on the side of the road with a wrench, a cup full of gasoline and the sincere terror that the engine might backfire while you were pouring the gas under the hood. Run out of gas one time in a car with a mechanical fuel pump and the ensuing experience will teach you all about the importance of keeping fuel in your tank. Restarting a modern car with an electric fuel pump is a far simpler and less dangerous affair than restarting an older auto -- which is kind of a shame, when you think about it.

    Before electronic fuel injection became mainstream in the 1970s, automobiles required a mechanical pump to pour fuel into the carburetor's fuel bowls. Fluid pressure was important but not as important as fluid flow volume. So most engines used engine-driven fuel pumps that sucked fuel from the tank and fed it to the carburetor. But fuel injection systems require a great deal more pressure, and electronic fuel pumps came into use. Unlike earlier pumps -- which required the engine to run in order to pump fuel from the tank -- electric pumps could move fuel with or without the engine running.

    The first step to getting your car running is to put gas in the tank -- as much as possible. Electric fuel pumps have anti-drainback valves to maintain pressure for a little while after the pump shuts down, but those valves aren't perfect. Usually the fuel pump mechanism sits higher than the pump pickup in the bottom of the tank: Running the fuel level below the pickup means running out of gas and stalling. When you refill your tank with a gallon or 2 of gas, you're just barely covering the pickup and forcing the dry pump internals to suck fuel upward. Filling your gas tank completely full will gravity-force fuel into the pump's internals, priming it more quickly and helping to get the fuel to your engine without undue drain on the battery.

    You may have noticed when your engine stalled for lack of gas that it bucked and coughed several times before stalling; this happens because just before running completely dry, the fuel pump inhaled little bubbles of air. That air worked its way to your engine in the form of bubbles, which got bigger and bigger until the motor died from lack of fuel pressure. Modern cars often have provisions to shut down the fuel pump using a standalone circuit breaker. This breaker is designed to trip in an accident, shutting down fuel flow and preventing fire. In late-model autos, the engine computer may trip the breaker when it detects a loss of fuel pressure. For this reason, you may need to press the pump rest button after running out of gas. You'll often find it above the driver's foot well, under the carpet in your trunk, in the glove compartment, under the center console or under the front seats near the air bag's inertia sensor. Check your car's owner manual for the device's exact location.

    Once you have the gas tank filled and the pump switch reset, turn your ignition key to the "on" position and listen for the pump; you should hear it very distinctly at first, buzzing or humming, but it will quickly drop to a quiet hum as it fills up with fuel. Leave the ignition on for three to five seconds, then turn it off; then turn it on for three to five seconds again, then off again. On some cars, the lack of fuel pressure will trigger another pump shutdown, so you don't want to let the pump whir away for more than a few seconds at a time. Repeat three to four times to pressurize the system, then attempt to start the car. It should fire up almost immediately, though there may be a bit of lag or cough as air clears from the injector nozzles.

    If your check-engine light comes on after you've restarted the car, don't just assume it's a hangover from the stalling and your computer's just harmlessly waiting for you to clear the codes. Once you get the car started, immediately drive it to a mechanic, dealer or auto parts store to have its diagnostic trouble codes cleared and the check-engine light reset. Even with a protective shutdown switch, running out of gasoline and running the fuel pump dry can result in damage to the pump. Your computer will detect fluctuations in fuel pressure and trigger the check-engine light to warn you -- and it might be doing just that when you start the car. But you won't know unless you clear the existing codes to see if the light comes on again. If it does, have the codes checked; if the code for low fuel pressure has been triggered, then you're likely to soon be either buying a new fuel pump or getting a bit more education in walking.

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