Fuel Injection Basics

This is a page that should help to take some of the mystery out of fuel injection (FI) on late model cars and trucks. There are three basic areas of a fuel injection system, feedback, processing, and output. The Electronic Control Module (ECM) needs to know what's going on via sensors in order to make the motor run most efficiently, and with the least amount of emissions produced. Different manufacturers will have slightly different systems, and this is in no way meant to be the definitive EFI web page! The two types of fuel injection systems are as follows:

Speed Density
Speed density uses manifold vacuum and tables within the computer to figure out how much fuel to put in the motor. The main benefit of a speed density system vs a Mass Air Flow (MAF) is the omission of a Mass Air Flow Sensor (MAFS). The Manifold Air Pressure (MAP) Sensor will sense the vacuum from the intake, and use that to determine load, etc.

Mass Air Flow (MAF)
While MAF is possibly the most popular FI system, it certainly has pros and cons. MAF is more forgiving to engine (performance) modifications because it uses a formula to determine load and fuel trim, but you need a MAFS. A MAFS is a big, round sensor that measures the amount of air going through the intake into the throttle body. Unfortunately, you need to keep this dry and reduce vibrations as to keep the wires inside happy. If your MAFS breaks, you will notice drive-ability issues. MAFS are also expensive. A Ford MAFS will run $250 from the dealer, and GM ones, I've read, are even more than that.

It should be noted that some systems are MAF with a speed density backup - quite possibly the best of both worlds. This has been used in early 90s GM performance applications, and probably others.

Closed loop and open loop are often used when describing FI systems. When the system is in closed loop, it is accepting all information possible, especially the O2 sensor, to trim the fuel curve to the motor's needs. When the system is in open loop, it's running based on a few of the sensors in the engine compartment, but more or less just winging it based on history to determine fuel requirements. An ECM will go start out in open loop, and as the engine, and more importantly O2 sensor, reach their normal operating temperature (NOT), it will changed to closed loop. The O2 sensor doesn't work until it hits 600 F, so without that occurring, the system will stay in open loop.

This section will detail out some of the sensors that FI uses to determine what's going on with the engine's fuel requirements, etc. I'll be sure to mention different manufacturers as I see fit for the purpose of this web page.

Crank Position Sensor (CPS)
Crank position sensors are used to help the computer determine such things as RPMs and make sure the distributor is firing correctly. I've primarily heard about these on Jeep vehicles, and so far they're the only ones I know that actually have them. The CPS resides in a boss in the bell-housing and reads the crank position based on the teeth of the flex plate or flywheel. There's a magnet in the CPS that's pulled towards the flex plate or flywheel as a tooth goes by, and that's how the computer gets its information. When installing the CPS, it has to be extremely close to the teeth, but not touching. If you have an automatic, you should receive a piece of paper with a replacement CPS. This goes between the CPS and the teeth during installation and ensures the proper distance. The paper is then torn up when you try and start it for the first time. Manual trannys use bolts to space the sensor. If the Chrysler ECM doesn't get a valid signal from the CPS, it will fire the fuel pump relay for only a second, then kick off. A bad CPS leads to a non-running vehicle very quickly.

Coolant Temp Sensor (CTS)
In order for the ECM to know how hot the motor is running, it needs to probe the temperature of the coolant. This is vital in a few areas, one of them being electric fan activation. The computer often controls the electric fan relays based on the temperature reading it gets from this particular sensor. The sensor will also have a hand in on whether the system is in closed loop or open loop. If the coolant is under a 100 F, then the system may run the injectors a little rich or even activate a ninth injector (on an 8 cylinder motor) to allow for a rich condition.

Intake Air Temp (IAT)
The IAT lets the computer try and calculate how dense the air is that's coming into the motor. It can measure the flow, but without knowing the temperature, it can't determine the density. This could also help the computer determine if it should run in a slightly rich condition and even be in open/closed loop. Certainly one of the least important sensors.

Knock Sensor
There's literally a listening device that screws into a boss in one of the coolant passages that can actually hear if an engine is knocking. If the engine is knocking, it can go ahead and change the timing in the distributor ad-hoc. This is why many computer controlled engines do not have adjustable timing, the computer does it all.

Manifold Air Pressure (MAP)
On speed density fuel injections systems, this determines the load on the engine via the vacuum drawn on the sensor. This could also contribute to controlling an automatic transmission.

Mass Air Flow Sensor (MAFS)
The MAFS is a large cast aluminum ring that sits in the air intake path and lets the ECM determine just how much air is going into the motor. They sometimes use a 'hot wire' technology where the wire has voltage applied to it, and as it's cooled, the wire draws more electricity. The computer can measure this load, or draw, and determine how much air is passing over, and cooling, the wire.

Oxygen Sensor (O2 Sensor)
O2 sensors are probably the most common subject when talking EFI. The O2 sensor monitors the exhaust for the presence of oxygen, and from that determines if the engine is running rich or lean, and adjusts the amount of fuel that goes into a motor (fuel trim). This sampling happens many times a second and directly contributes to the 'short term fuel trim' which is the instant-by-instant fuel ratio changing that occurs by varying the 'pulse width' of the injectors. (The pulse width is the term used for the duration that an injector injects fuel for that particular combustion cycle.) The O2 sensor has an operating temperature of about 600 F, so the sensor must be fairly close to the engine or 'heated'. Heated O2 sensors are generally more expensive, and have a third wire (+12v). With the advent of OBDII, there are now TWO O2 sensors PER catalytic converter. This will help the computer determine if the converter is functioning properly by measuring the exhaust both pre- and post converter. If there's more than one catalytic converter, you'll have what's known as bank 1 and bank 2. Bank 1 being the driver's side, and bank 2 being the passenger side.

Throttle Position Sensor (TPS)
Serving as one of the most important sensor's, the TPS lets the ECM know just how much throttle you (as the driver) are giving the engine. This will come into play for many ECM functions, such as load, fuel trim (long and short term) etc. When a TPS goes bad, expect erratic things to happen. The best thing about a TPS is that it can easily be tested, even without an expensive scan tool. The Ohm reading on a TPS is 0 to 5 Ohms, 5 being wide open throttle (WOT).

Vehicle Speed Sensor (VSS)
The computer likes to know how fast the vehicle is traveling. It uses this measurement to activate emissions equipment such as exhaust gas recirculation (EGR) valves and determine load and fuel trim. The ECM may display a fault code if no VSS is present, however, there's likely to be no drive-ability issues without one. The sensor goes on the speedometer output on your transfer case or transmission, and converts the mechanical signal into an electronic one.

In this section, we'll talk about how the computer can use the information it's gained to change the operating conditions in and around the motor. The computer has to be able to change how the engine operates, or else the feedback mechanisms described in the first part would be moot.

Idle Air Control Motor (IAC)
This motor controls how the motor idles. It's essentially a choke that's controlled by the ECM, allowing a certain amount of air in the motor to raise or lower the RPMs at idle. The IAC may have to be calibrated when installed - check your owner's manual if you're replacing yours. This motor is obviously very important to proper engine operation at idle.

Ignition Module
The ignition module is only being mentioned because it's a representative of a computer controlled distributor. Quite simply put, and electronically controlled distributor can be advanced or retarded as need be, per the input from the sensors on the motor.

Obviously, the whole system is based around 'injectors'. Injectors can precisely inject fuel into the intake runner of throttle body and are usually electronically controlled. There are two basic types of injector setups, throttle body injection (TBI) or multi-port injection (MPI). The TBI uses one or two injectors at the throttle body to provide a constant stream of fuel for all the cylinders of the engine. The MPI uses an injector for each individual cylinder, and they're usually aimed right at the back of the valves to provide a direct shot into the combustion chamber. MPI is known as a 'dry' runner or intake because the intake manifold runners only contain air, and the fuel is atomized and introduced to the air right before it enters the combustion chamber. The two types of MPI systems are called 'batch fire' and 'sequential' port. Batch fire systems fire all or a group of injectors all at once, and sequential injectors are fired only when that cylinder is at a particular point in its stroke. Sequential apparently doesn't add any performance, but supposedly adds economy and reduced emissions (as well as a level of complexity).

Fuel Pump Relay
The fuel pump relay is controlled directly by the ECM. When you first turn the key from off to the run position, the ECM will run the fuel pump for roughly 3 seconds. This builds pressure in the fuel system, assuming your next move it to try and start the motor. The most important reason the fuel pump is computer controlled is so that the fuel pump can be automatically shut off in the event of a collision. If, after the collision, the motor isn't running, then the fuel pump should be shut down. This is to prevent the fuel pump from pumping, even though the motor can't use the fuel. If a fuel line is ruptured as a result of the collision, this could lead to a rather large fuel spill, and the obvious danger associated with that. The three domestic manufacturers handle fuel pump operation in a different way:

  • GM uses an oil pressure sensor to make sure that oil pressure exists before letting the fuel pump run. This is also beneficial during the loss of an oil pump, as it can save your motor
  • Fords have a vehicle impact sensor that detects shock on the vehicle and will kill the fuel pump based on that. This is somewhat misleading because you could have a fairly minor collision, and the fuel pump could still shut off.
  • Chrysler vehicles simply use a spark detector to ensure that the engine is actually running, or trying to run, before introducing fuel. Again, this could be misleading if you're trying to troubleshoot and don't have spark.

In 1996, the federal government mandated that cars must conform to the OBDII standard. Some of the key things to note about this standard are that the scanner hookup must be within 3 ft of the driver, and that some manufacturers adopted this as early as 1994. These early implementations of OBDII did not have to fully meet the fed requirement, but were still based on OBDII. The best thing for troubleshooting is a code scanner for fuel injected engines. Chevy's have a diagnostic port that can be used without a scanner, and Chrysler's owners can get codes with just a simple key trick. OBDII owners are fairly dependent on the code scanner, and if the malfunction indicator lamp (MIL) comes on, a diagnostic device is required to clear the codes. The law says that you must have to perform two actions on the device in order for the light to go off - sort of like an acknowledgment action. This page is not to help you troubleshoot direct problems, but is to help you understand your EFI so that you can better understand information gained from diagnostic procedures. I don't claim to be an EFI expert, but this will help people understand the basic concepts. (I hope)