Basic EFI Needs

7/29/2019 Basic EFI Needs

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For EFI to work properly, several different pieces of information have be knownand compared, so that an appropriate amount of fuel may be metered into the engine. This information includes the amount of air being pulled into the engine, the speed of the engine (rpm), the temperature of the air as it comes in (since cooler air is denser), the temperature of the engine (measured as coolant temp), the position of the throttle plate, and the amount of oxygen in the exhaust.

The cast of characters

The 22R-TE (and 22RE) EFI system consists of the following parts:air flow meter (AFM)temperature sensorsthrottle position sensor (TPS)oxygen sensorfuel injectorsfuel pressureelectronic control unit (ECU)

--------------------------------------------------------------------------------Air Flow MeterThe air flow meter, or AFM, is an analog device that measures the amount of air coming into the engine. On stock EFI setups, the AFM sits on top of the air filter

box. If you look inside the AFM, you see a big vane, also called the flapper door. Air presses against this vane as it comes into the engine. The greater the volume of air coming into the engine, the more this door opens. Attached to the top of this vane is an armature that also moves as the vane moves. The armature slides across a traceboard, or resistor ladder, that acts as a potentiometer, metering an electrical signal that gets sent to the EFI computer (ECU).

It may sound complicated, but in fact, this is a very simple device. The 22R-TEAFM sends a voltage signal to the ECU that varies from about 3v at idle to about8.5v at WOT. More air = more voltage. The rate at which this door swings open is controlled by a flat-wound spring. The tension on this spring is set via a black, plastic gear. When this gear is turned clockwise (CW), tension on the springis increased. More tension = harder to open = less signal for a given volume of

air = leaner operation. It is generally agreed that each tooth on these AFMs represents about a 2% change in the electrical signal sent by the AFM to the ECU,so, in theory, a change of 5 gear teeth would equal a 10% change in fuel delivery. It is not this linear a process, but this is the theory, anyway.

In addition to metering the incoming air, the AFM also contains an air temp sensor and a mechanical switch for the fuel pump relay.

For more info on AFMs, click here...

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Temp sensorsTemperature plays a critical role in EFI. Gasoline does not readily combust whenit is in liquid form, but it burns quite easily when it is aerated/vaporized. Heat is the primary control on vaporization. When the engine is cold, it takes alot more fuel get thing going. This is why our EFI engines have a cold start injector, so more fuel is made available when the engine is first started. Carbuerated engines have a choke for the same reason -- more fuel in the air/fuel mix. So keep this point in the back of your mind as you work through this text: cold sensors cause the ECU to meter in additional fuel.


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There are two temp sensors that play a role in determining fuel delivery. The first is the incoming air temp sensor, described above. There's not a lot to say about this: this is a variable resistor (thermistor) whose resistance varies in response to temperature.

Air temperature Resistance-4*F 10k-20k ohms32*F 4k-7k ohms68*F 2k-3k ohms104*F 0.9k-1.3k ohms140*F 0.4k - 0.7k ohms

The second temp sensor is a water temp sensor. EFI engines must be warmed up tofunction properly. There is an inverse relationship between engine heat and fueldemand, and cold engines need significantly more fuel than warm ones. Obviously, if an engine gets too hot, the fuel mix combusts TOO readily, and detonation occurs. Consistent temperature is critical to EFI engine operation! Which is whyyou should never remove your thermostat to attempt to cure an overheating problem: an engine that never warms up properly will use too much fuel, and there area lot of problems, like accelerated ring and bearing wear, that may result.

Some people will try to trick the engine into thinking it is colder than it actually is by placing a potentiometer in series with water temperature sender signa

l. Since the computer is expecting a 0.5v-to-2.5v signal from this sensor, it'spossible to make the computer meter more fuel if the resistance of this input ischanged. In my opinion, this is a band-aid approach to engine tuning; don't usethis method to coax more fuel from your ECU.

--------------------------------------------------------------------------------Throttle Position Sensor (TPS)The throttle position sensor, as the name implies, tells the ECU how open the throttle is. It's not a particularly magical device: a plate is attached to the throttle shaft, and as the shaft turns this plate turns, and resistance is varied onsome contacts. The ECU uses this to determine when the engine is idling, at par

t throttle, and at WOT.

TPS sensors can be pesky little things. On the 22RE/RTE, the bottom-most adjustment screw can be hard to reach with the radiator hose and thermostat waterneck installed. Also, these screws can become a real pain in the ass to loosen after 100k miles, even if you're using a right angled Phillips screw driver. I went andreplaced these Phillips screws with metric Allen head bolts. This makes them alittle easier to access and turn with everything in place, although you may haveto cut a wrench down to make it fit the space between the TPS and the waterneck.

In addition to being hard to adjust, these pieces can also fail, or partially fail. The book gives a procedure for testing them, which I won't rehash here (anot

her chide to buy the factory service manual if you haven't already -- it makes life A LOT easier). One aspect of the TPS that doesn't get a lot of press, but should be looked at whenever you are experiencing odd acceleration/decelleration behavior, is the return spring, visible on the backside of the TPS. Like all springs, this spring can lose tension over time. It should snap back after the TPS is twisted. If yours doesn't, it's probably time to replace it. These little puppies go for about $110 from the dealer...

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Oxygen sensorThe oxygen sensor measures the amount of oxygen in the exhaust gasses. The O2 sensor is a component intended to ask as a system check: it analyzes the gasses after they've left the engine, and informs the ECU is the mixture is too rich or lean. The ECU then makes some adjustments based on this information. Your O2 sensor is your friend: make sure that it working properly, and change it when it fails. And it WILL fail, especially if you begin to play with the air/fuel ratio.

On the engines using the stock CT20 turbo, the O2 sensor is a single wire sensorlocated on the turbo exhaust elbow. It is expensive -- as much as $130 -- but it is also very accurate, and pretty tough. If you are still using the stock CT20turbo, I would strongly consider keeping the stock sensor because of its accuracy and reliability.

However, if you're on a hard budget or have switched to an aftermarket turbo, you can switch this sensor if you are willing to do a little welding. There are many aftermarket O2 sensors, but not all are created the same. Personally, I thinkthe NTK sensors are very good and worth the few extra bucks. Get a good four wire NTK sensor -- Monarch Products has a good price on them -- and wire it up (the four wires represent two heater wires, a signal wire, and a signal ground.)

These sensors thread into a special bung, available from many muffler shops, which will need to be welded into your exhaust system before the catalytic converte

r. Even though these new sensors are heated, you should have the bung placed asclose to the turbo as it possible. You will need to run an ignition switched 12Vlead and a ground lead to the new sensor, and run its signal lead to the signallead that came from the old O2 sensor. If you are running the stock CT20, you will also need to leave the old sensor in place, unplugged, because of the way itattaches to the turbo outlet elbow. This change is well worth the effort if your old O2 sensor is bad...and these things do go bad with greater frequency thananyone would like. It is very easy to destroy an O2 sensor with a rich mixture,something that can happen in the course of tuning your motor.

There is a lot more to be said about O2 sensors. I would strongly encourage reading through the following web sites:

A good general primer on O2 sensors:

http://atlantis.austin.apple.com/people.pages/Jimbo/o2info.html

A good technical primer on O2 sensors, including wideband O2 sensors:

http://www.scuderiaciriani.com/rx7/O2_sensor.html

--------------------------------------------------------------------------------Fuel injectorsFuel injectors are really just simple solenoid devices that allow pressurized fu

el to be squirted into your intake manifold in a fine spray for a predeterminedperiod of time. Really -- that's all they do.

An injector's size is measured in how much volume of fuel it can flow in a givenperiod of time. US Domestic injectors are usually measured in pounds per hour (lbs/hr), while injectors for import engines are usually measured in cubic centimeters per minute (cc/min). You can convert between the two:


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To convert cc/min into lbs/hr, divide cc/min by 10.5. For example, the stock 295cc/min injectors works out to 28 lbs/hr.

To convert lbs/hr to cc/min, multiply pounds per hour by 10.5.

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Injectors come in two types: low impedance (typically 12 ohms), and two subtypes: "peak and hold", and "saturated."

The 22RTE has low impedance (1.7 ohms) peak and hold injectors.

You should never, ever mix "peak and hold" injectors with "saturated" injectorson the same circuit, or you will burn out your injector driver. However, it is worth noting that A peak and hold injector driver is capable of driving saturatedinjectors of the same impedance, as long as they are the only type of injectorson the circuit.

The length of time an injector is open and squirting fuel is called the "pulse width," and it is measured in milliseconds (MS). As rpms increase, an injector can only be held open for so long before it needs to be held open again for the next engine revolution -- this is called its "duty cycle." Even though a fuel injector's flow rate is measured at its maximum duty cycle (100%), fuel injectors should never be operated at 100% duty cycle. Instead, a typical maximum duty cycleis around 80%.

--------------------------------------------------------------------------------Fuel pressureFuel pressure plays a big role in the operation of a fuel injection system. Beca

use the injector is essentially a gate valve for fuel delivery, increasing fuelpressure can allow you to cram more fuel into the intake tract for a given injector pulse width. Typical fuel pressure for a stock 22RTE engine is 43 psi. The maximum fuel pressure that the stock injectors can handle is about 70 psi -- above that and the injector begins to fight the fuel pressure to get its valve open,and fuel deliver actually decreases in volume.

Fuel pressure is determined by both the fuel pump and the fuel pressure regulator. The stock fuel pressure regulator has a hose that connects it to intake manifold vacuum/pressure, so that as manifold pressure rises, fuel pressure also rises, typically by a 1:1 ratio. The pump determines the overall volume of fuel thatthe system is capable of delivering to the injectors. The size of the fuel lines can also be a limiting factor if fuel demand is significantly greater than sto

ck.

--------------------------------------------------------------------------------The Electronic Control Unit (ECU)The ECU is the brains of the system, and the place where all of the input from the AFM, TPS, O2 sensor, and coolant temp sensor signals come together. The stock22RTE computer is a digital TCCS type (Toyota Computer Controlled System), which means that it uses a number of internal spark and fuel maps to determine how l


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ong to pulse the injectors, and how much spark advance to give to the ignition system.

The general operation of the ECU is pretty straightforward: the ECU gets input from the AFM, distributor, TPS, O2, and coolant temp sensor. It parses all of this data and chunks it down to a number, and then compares this number to a valuein a table (or several numbers to values in several tables...) and then pulses the injectors and fires the ignitor appropriately. Under certain circumstances, the ECU uses feedback from the O2 sensor to bring the air/fuel ratio back to ~14.7:1.

Just to make sure the engine doesn't inadvertently self-destruct, the ECU has afew safety parameters built in, including an "over boost" fuel cut parameter and(reportedly) a top speed parameter. I've never hit the speed parameter (doing 112+ miles per hour in a pickup truck never seemed like a good idea to me, although it might have when I was younger ;-), but I've hit the over boost fuel cut before, and it's no fun for you or the engine (suddenly unloading the pistons at 5000 rpm is kinda tough on 'em). The secret to the fuel cut is that it's determined by TPS angle and injector pulse width (how long each injector needs to fire).If you use bigger injectors and fire them less often, you can avoid fuel cut...but you may need a larger AFM to pull this off.

The TCCS system was fairly advanced for Toyota in the mid '80's (by contrast, early EFI Celicas used an analog EFI system that is less precise but, oddly enough

, more tunable...), a necessary step for them to meet their emissions and fuel economy goals. For the enthusiast, it is something of a stumbling block, but it is still capable of supporting an engine capable of making about 250 hp. Most importantly, though, is that it is very easy to live with on a daily basis -- something that can't be said for a poorly tuned aftermarket computer. Before you decide that the stock ECU has to go, be sure you are capable of tuning the box you intend to replace it with. If you can't tune what you intend to use yourself, youjust might be better off sticking with what you already have!

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Basic EFI Needs