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Remember pumping the gas pedal before starting? Carburetor vs. fuel injection?

I am assuming we no longer need to do this b/c of carburetors vs. fuel injectors.
Can someone explain the difference? As in, why does pumping the pedal before starting not have any effect on a fuel injected engine?
Do we not need to do this b/c the fuel lines are now constantly pressurized?



  • With a carb there's something call the 'accelerator pump' that would squirt extra gas when you pushed the gas pedal to help (you guessed it) to accelerate the car. I does this engine running or not, so it also helps richen the mixture when you're starting the car. With fuel injection the computer take care of giving the car the extra gas it needs.
  • Hitting the gas pedal before starting would also activate the automatic choke if the car had one.
  • On the old carbureted engines, you pressed the gas pedal before starting to set the choke position and give a squirt of fuel into the engine via the accelerator pump. This was necessary because the carb does not feed fuel properly at low (starting) RPM. A cold engine also needs a little extra fuel to compensate for questionable vaporization at low temperatures.

    The fuel injection system does nothing until the engine is turning (starting or running), so pressing the pedal before you start will make no difference. The fuel injectors work at any RPM, so a choke is not necessary. The computer simply feeds a little extra gas through the fuel injectors to start the engine. Cold temperature fuel delivery is also managed by the computer which checks the mixture with oxygen sensors.
  • Also on a carb'ed car hitting the gas pedal will set the choke plate closed on most cars. This was need to start the car especially in cold weather.
  • I have a 2001 Lincoln LS that runs rich and no one can solve the problem. The fuel sys pressure tests fine. First start in morning is normal but after that I MUST give it gas to get it to turn over.

    Also, car has a fuel smell. Any ideas?
  • bh, is the check engine light on? If so have the codes checked and post back with the code numbers.
  • edited April 2012
    @bh. You should start a separate thread for your issue. When you do that provide complete info on the car - mileage, past maintenance, etc. Also fill in the blanks under "no one can solve the problem." Well, some "someones" have done something. So fill us in.

    As for the general question in this thread, I'd simply add that the "gas" pedal is not really a gas pedal once you're on fuel injection. Its really an "air" pedal and "signaler." The pedal opens the throttle plate and lets more air in. A throttle position sensor and air measuring sensor(s) - most often a mass airflow sensor - tell the computer what's going on with that and that's part of how the computer decides on fuel flow to the injectors. For starting & idling there is also a computer controlled idle air control valve. So the computer does do everything you used to do with your foot. At this point, pumping the "air" pedal before you turn the key does nothing. You might as well wave chicken bones over the ignition. Trying to "give it gas" when you turn the key to start it is actually messing with the computer's "plans" for getting it running - because you're allowing in more air then is supposed to go.

  • markmast has a good answer - I'll add this simple bit here... On most fuel injected engines (ok, every last one I'm familiar with), the throttle is either electronic, in which case pumping prior to turning the car on does nothing as the circuit is dead, or the throttle is connected via a cable just to the throttle body. When you step on the gas, all you're doing is opening and closing the throttle plate, which is simply regulating air flow, not fuel flow. Fuel flow is controlled by the computer, which uses a throttle position sensor along with other sensors to determine how much gas to add. When the car is off, that circuit is dead, so the only thing that happens by pumping the gas is that you're moving that butterfly valve in the throttle body... nothing else...
  • edited April 2012
    Pushing the pedal did three things:
    1) it "primed" the engine by using the accelerator pump to spray some fuel in
    2) it set the automatic choke
    3) it set the idle to high, via the high idle cam.

    Fuel injected systems control the engine's starting fuel needs by computer.
    Eraser gave an excellent description of a majot difference, being that pumping the pedak before turning the key does nothing on a fuel injected car but open and close the throttle plate. The computer in an injected car takes a signal from the engine temp sensor, and based on that combined with the other sensor inputs it
    1) richens the mix (makes the injector pulsewidths longer) and
    2) by passes the oxygen sensor circuit, to allow the engine to run rich,

    In short, in a carburated engine everything is operated manually by linkages. Pumping the pedal sprays gas into the engine and sets the chok & idle for cold operation.

    In an injected engine, all of its fuel comes from the injectors. The injectors stay closed when static and only open when a signal tells them to. Thus, when the kay is "OFF", there's no signal, the injectors don;t open, and nothing happens.
  • Thanks! Just a related question regarding the gas pedal working the throttle body flap.
    Does pressing down on the gas open the flap?

    What does this serve?
    In a carb, this would make the air/fuel mix leaner, thereby reducing power.
    This doesn't make sense, however, since pressing the gas increases power.
    So, pressing the gas opens the flap, which allows more airflow, which increases power?
    (And the injectors compensate for the increased airflow with more fuel?)
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