This is a fuel injection help page for those who think the now old technology is too complicated to be serviced by the shadetree mechanic. Its not as complicated as you think.



TBI is a unit that looks like a carburator with one or two fuel injectors mounted directly in the bores of the TBI's carb looking unit. The injectors spray downward into the throttle body to the intake manifold.

Multi-Port or Port and CFI:

The Port system is when the fuel injector is mounted in the intake manifold in such a way that the fuel spray is directed just before the intake valve.


Common Injection Engine Failures...Quick Failure...: (No Start)

The EGR System Can Drive You Nuts...: See how it works...

To See a Little Bit of Info on Sensors...: go here...


The injector(s) work by opening and closing. The computer sends an electrical signal to the injector to open it (lift the pintel up) allowing a quick mist to spray into the intake. After the injector sprays a shot of fuel, the computer then stops the electrical signal to the injector. A spring is what closes the pintel.

On TBI injection the injectors are "side" feed meaning the fuel enters the injector from the side. They also have 2 o-rings. One large o-ring at the top/edge and one small one on the bottom.

The injectors themselves "rarely" go bad.

Lets follow the fuel on a journey from the fuel tank, to the injectors, to the regulator then back to the fuel tank.


This is a typical Ford system_______________

The fuel in the tank is picked up by a low pressure or hi pressure pump. Some vehicles have different pump set ups. Some have 1 pump in the tank. Another type has 1 pump in the tank and 1 on the car chassis near the fuel tank. On the 2 pump systems the pump in the fuel tank is a low pressure feed pump that feeds the hi pressure pump(It seems redundant but the high pressure pumps cannot prime (suck) the fuel on there own)

After the fuel goes thru the pump(s)the fuel then is pumped to the fuel filter.

After going thru the pump(s) then filter, the fuel goes to a tube shaped manifold which the top of the injectors are connected to. The bottom of injectors are connected the intake manifold on the engine. The top of the injector is pushed into the tube with an o-ring to seal the fuel from leaking. The bottom of the injector is pushed into the intake with an o-ring to seal fuel and vacuum.

While the fuel is in the manifold tube the computer is quickly opening and closing the injectors. Some fuel will go into the engine thru the injectors but most will bypass thru the pressure regulator where after passing thru, the fuel goes back to the tank to start over again.

"Regulator" On the opposite end of the manifold (metal rod), you will see a small can which has a black vacuum hose that goes from the top of the can to the engine (intake manifold).

This can is the fuel pressure regulator.

The fuel pressure is created by the fuel pump pumping the fuel between the pump and the regulator (can). The fuel manifold tube with injectors is in between the pump and the regulator The regulator is where the pressure is controled. It acts like you putting your thumb over a water hose trying to hold the water back that is basically how the regulator creates pressure.

***On the TBI type systems there is no tube. It does have the fuel pressure regulator after the injectors.

Now the injectors on a Ford open and close at one speed that never changes.

So to get more fuel to the engine when the injectors are only opening and closing at a certain speed you have to change the amount of fuel pressure in the manifold (rod).

The way its done is by the variable pressure regulator (can) sensing the vacuum in the intake manifold. The vacuum changes by pushing the throttle, that vacuum change either makes the fuel pressure rise (lower vacuum)as when the throttle is opening to speed up. Or the fuel pressure to drop (high vacuum) such as when its idling or decelerating.

When you push on the accelerator the vacuum goes to about 0" vacuum, almost closing off the flow in the regulator (can) so the pressure rises in the fuel line over the injectors thus more fuel flows through the injectors to the engine..... VVVRROOOOMM!

The electronic pulses that open and close the injectors come from the computer, the computer also controls the ignition timing by way of information (voltage, and resistance) from the input sensors.

Below is a very simple diagram of the fuel section without the filter


GM Fuel Injection______________________

Below is a typical GM fuel injection system. GM does things a little different than Ford

The main difference is instead of the fuel pressure going up and down, while the injectors keep a steady pulse like ford. the GM fuel pressure stays constant while the injectors are open longer.

GM has the typical low pressure and high pressure pumps (sometimes only one pump). GM also uses a regulator BUT this regulator keeps the fuel at a constant pressure.

So to get more fuel to the engine the computer controls the amount of time the injectors are kept open (totally different from ford).

The computer gets the information on how much fuel to add to the engine by using the intake manifold vacuum.

The MAP sensor (manifold absolute pressure) is the sensor that tells the computer when the engine needs fuel. The MAP works by being connected to the intake manifold sensing vacuum. The computer sends the MAP sensor a 5 volt positive wire. This 5 volts is used by the MAP in this way. To send a variable positive voltage back to the computer thus telling the computer what the engines needs are. from 1 to the 5 volts are sent back to the computer from the MAP sensor. Again the MAP senser converts the 5 volt positive reference wire going to it from the computer to a voltage that varies from 1 to 5 volts. The voltage is constant to the vacuum in the engines intake manifold. For instance if 18 inches of vacuum equals 1.2 volts. Then any time the vacuum is -18 vacuum the voltage sent to the computer from the MAP will allways be the 1.2 volts.

So when you push the pedal toward the floor the voltage going back to the computer is at around 1 volt then as you push on the gas pedal the voltage goes up up up. Again from 1 volt at idle to 5 volts wide open throttle.

The MAP senser is not the only senser that informs the computer of whats going on BUT it is the most important senser for the air to fuel ratio that is used, the others such as the oxygen senser just fine tune the amount of fuel that is sent into the engine via the injectors.

Below is my diagram of the TBI injection (throttle body injection)

Below is a helpfull chart on sensors


Injector flow rates, color, part numbers from "WOLF"

Trouble shooting and the most common problems

*If you think you have a fuel problem and your vehicle is fuel injected most of the time the fuel pump is the culprit. the second most common problem is the filter will clog. Most of the time your problem may seem to be fuel related though it will end up being in the handfull of electric sensors that the computer uses to regulate fuel delivery.

**The computer itself can go bad. Usually the engine will run rough for a few days before the computer will fail. Sometimes the engine will run bad then run ok for a short period before failing. Remember your computer most likely will have a chip that must be removed from the old one which must be put into the new one.

*If your vehicle's engine just stops running without any indication of prior troubles you may want to check below.

*Check for spark, if there is no spark and its a Ford chances are the "TFI" brain that is attached to your distributer is bad. Or there is a pickup in the distributer they go out almost as often.

*GM engine with no spark check the "electronic module" in the distributer or underneath the distributer cap.

*Check for fuel, if there is no fuel flow check for fuel in the tank, fuel pump not working, fuel filter. Fuel pump relays rarely go bad.

*The o-rings can leak over time. There are 2 o-rings on injectors. On EFI, CFI, and Multi point injection the injector will be "top" feed, meaning the fuel enters the injector at the top of the injector. So there will be an o-ring at the top and the bottom of these type injectors.

* Most problems that cause your engine to run bad when the typical tune up parts are in good shape ie, plugs, wires, ect. are in the "control" sensors. The fuel injection itself rarely goes bad except for: Fuel pumps go bad :Fuel filters get clogged :Vacuum lines become cracked or stretch creating leak

* Another problem that occurs is after a sensor goes bad your engine will not have spark. So in turn you crank the engine 25-100 while trying to figure out what is the problem you un-beknowingly flood the spark plugs every time you try to start it. Doing so creates another problem. After finally getting the part that went bad replaced your vehicle still wont start. So you may replace sensor after sensor for no reason when all that is wrong is the plugs are splash fouled meaning the plugs will not spark enough to run the engine until the plugs are replaced.

* "Splash Fouling" occurs occassionally on fuel injection engines that are cranked over and over during trouble shooting. "Splash Fouling" means that raw fuel has been compressed but not fired creating a problem that is not fixable. I do not know exactly why the plugs are ruined by this but "MY HUNCH" is the raw gas gets compressed inside the plugs porcelain creating an area where the spark energy can go to ground thru the porcelain instead of thru the gap. Even if you clean your plugs until they shine, they are still useless when this occurs. I have seen this happen occasionally.


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