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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.

@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…

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?)

If there is a mechanical link (most newer cars have purely electronic throttle, so this isn’t the case), then yes, pressing down on the pedal opens the butterfly valve in the throttle body.

However, as you guessed, that isn’t the only thing that happens. The throttle position sensor (either on the throttle body or in the pedal assy) tells the computer that you’re wanting more power, and the computer then tells the injectors to do their thing…

So you get more air AND more gas… and depending on how hard you’ve pushed the pedal down, you’ll get a given amount of power… push a little lighter, and it gives just a bit more gas and air… mash the pedal and it dumps in tons more…

When you open the “flap” (butterfly valve) with a carb more air is drawn through the carb, and the carb responds by spraying more fuel in with the air.
The carb has a venturi, which is simply a part of the passage way that narrows down then opens back up.
In this narrow area the air velocity increases and the pressure drops, similar to the way an airplane wing gets lift.
The more the air flow the more the pressure drops.
There’s a little nozzle in there and gas gets sucked into the airstream.
Some carbs have a variable venturi with a plunger that can make the narrow section bigger or smaller.

The Buicks made before 1961 also combined the starter with the accelerator. You turned the key from either the lock position or the off position to on and stepped down on the accelerator. This closed the choke and the accelerator pump shot a stream of gas from the carburetor down into the intake manifold. You pushed the accelerator about 1/3 of the way down. When the engine started, I think a vacuum switch disengaged the starter motor. If the car was “flooded”, you pushed the pedal to the floor which then opened the automatic choke. The Packards, at least through the 1950 model had this arrangement and I believe the Chevrolets until 1937 may have had the same starting arrangement.

Two different systems. On the carb, the bowl holds fuel. When you pump the gas, the pump in the carb squirts fuel into the intake manifold to suck into the combustion chambers. On fuel injected, the accelerator just operates a variable switch on the valve and doesn’t do anything except tell the computer where the throttle is. The fuel rail containing the injectors is also under pressure, so the computer decides when to open the injectors to send fuel to the combustion chambers. So pushing on the throttle doesn’t do anything until the computer decides. Opening the throttle all the way is a default telling the computer to shut off all fuel to the engine to clear a flooded condition. Its like the difference of a microwave versus a gas stove.

Some people are saying the fuel injected gas pedal does nothing but tell the computer you want more throttle. However, others are saying there is a mechanical linkage as well, namely opening the throttle body flap for more air (AND telling the sensor you want more gas)

Thanks, I love learning all this. I was living in a cave for 20 years.

Depends on the EFI system, some have the direct connection with a cable, some just register the position of the gas pedal and electronically open the throttle valve with a small stepping electric motor.

It has been 50 years, but I believe my '36 Chevrolet had a large metal foot pedal just for the starter. Not the throttle.

Interesting discussion. Things are so easy today. Just turn the key.

I can remember getting a ride to school in the 70s with a friend’s mother. She drove a black, 63 Impala convertible - white top, red leather, 327 cubes - a car which, in later years, we all coveted long after it had gone to the bone yard. Anyway my friend and I used to laugh watching his mom’s foot pumping the accelerator before starting on a cold morning. The car was not particularly well maintained and the foot pumping routine was almost as unique as the secret handshake to get into the Masonic Lodge - get it wrong and no entry or start. I had an old Caddy with a big Rochester four barrel that was likewise finicky to start when cold - probably because a certain do-it-yourselfer had been messing with linkages, air idle screws etc. My wife’s first car was an old four door straight six Nova that needed manual assistance with the carb butterfly valve (a ruler or a screw driver) before it would start on a cold day. Fond memories when there was a more visceral connection with one’s transportation and even the mechanically disinclined could usually get something to run (albeit often poorly) with a few basic tools and what they had picked up from shop class, dad or grand dad.

@irlandes–You may be correct about the foot pedal starter on your 1936 Chevrolet. I know that my dad’s 1939 Chevrolet had the foot pedal starter and this system was used through the 1948 Chevrolet models. I do remember a 1930s Chevrolet using the starting system that I described for the Buicks. It must have been earlier than 1936.