How, EXACTLY, does the ECU work?

[gnlw]

I've read that the ECU adjusts based upon a person's driving style. How exactly does it adjust? Does it only adjust the air-fuel mixture? Does it also adjust the ignition timing? Does it only remember the past copule of hundred miles, or does adjust based upon all cumulative data input from day one? Does it adjust the transmission's shifting characterisitcs? (I.E shift points and how hard it shifts) How does one's driving style really affect the adjustments? Will driving the truck hard mean that the ECU will then calibrate to generate the most horsepower and torque when the truck is driven hard? Does resetting the ECU after a mod really accomplish anything in the long run, or will the results be the same without resetting after a few hundred miles anyway? Sorry for all the questions, but the workings of the ECU are a mystery to me. Thanks in advance for any replies.

[mustang67408]

The ECU "learns" about your engine as you drive the car. The "learning" is actually a process that the ECU uses to track the tolerance changes of the sensors and actuators on the engine. Two examples of these values are the position of the idle-air bypass valve (automatic choke) at idle with the A/C on, and the trim gain on the injectors. The ECU stores these "learned" values in battery backed-up RAM so that it doesn't have to start from scratch the next time you turn your engine over.
"Resetting the ECU" is the process of clearing all the long term memory from the ECU's memory. These variables trim idle speed, fuel, spark, and more. The ECU will also store trouble codes for diagnostic capability. By resetting the ECU, the trim values will be set to some neutral default value and all of the trouble codes will be cleared.
How-To

To "reset" the ECU, all you have to do is remove the source of backup power. To keep the memory alive while the car is off, the ECU has a direct line, through a fuse, that goes right to the battery. This is usually the same line and fuse that supplies backup power to your radio so it doesn't forget all of its settings. Your ECU has now lost all of its long term memory values and will "reset" them all to defaults when it next powers on. By the way, the factory radio's memory lasts for a minute or two, so you probably will not lose your saved stations.
Myth

There is quite a bit of mythology revolving around resetting the ECU. Many people mistakenly believe that they need to reset the ECU after installing major engine modifications. Others believe resetting will resolve engine problems. Neither are correct.
Reality

The ECU is always tracking the engine's sensors and actuators. Resetting the ECU does not cause the ECU to learn engine changes any faster. The ECU is always tracking the gradual changes in the engine sensors and actuators. The ECU never gets satisfied with a setting, and is always updating the trim values. It will discover whatever change you made to your engine and make the appropriate changes. In fact, unplugging the ECU will most likely increase the learning curve of the new part.
There are only two reasons why you would want to reset your ECU. One is to clear a CHECK ENGINE light or error code after fixing the problem that caused the error. The other is to reset the spark advance trim value. The advance trim controls how much the timing is retarded from what the factory engineers considered optimum. This particular trim value is the only one the ECU "resets" to a non-neutral MAX value. If you are driving on low grade gas, the pinging picked up by the knock sensor causes this trim value to be lowered, and your timing to be retarded. If you go to the track and put good gas in the tank, you might not want to wait for the ECU to learn about the new gas and advance the timing. So resetting the ECU in this case makes sense. However, in the process, you will lose your idle and fuel trims (probably not too important for drag racing, though).
Questions

Won't clearing the trouble codes make the car run better? If the ECU notices a problem with a sensor, it will flag a code and possibly turn on the CHECK ENGINE light. Should the problem go away, the ECU will turn off the light and start using the sensor as normal once again. The code will be stored inside for later diagnostic retrieval. But that code being stored does not affect the ECU operation in any way.
I reset the ECU, and now the car runs rough! Why? The engine may run rough or idle poorly right after ECU reset. The ECU now has default numbers in its trim memory - it has to adjust them to match the tune of your engine and its sensors and actuators. It might take a day or two of driving in traffic before the engine will settle down again. Now you know why the ECU remembers these values in the first place!

Here is another good site for more detailed INFO
http://auto.howstuffworks.com/car-computer1.htm
Hope this helps

[gnlw]

Thanks for the explanation mustang 67408. As a follow up, I recently put a Magnaflow muffler on (no other mods). I drove the truck for about 100 miles before resetting the ECU. I could swear that my truck pulled harder at full throttle after the reset. If in fact the truck does perform better after the reset, does this mean that the default settings are providing more power than my learned settings? Will my settings eventually adjust back to the "pre-reset" settings anyway? If so, after how many miles will this occur? Thanks in advance for any replies.

[mustang67408]

Originally Posted by gnlw

Thanks for the explanation mustang 67408. As a follow up, I recently put a Magnaflow muffler on (no other mods). I drove the truck for about 100 miles before resetting the ECU. I could swear that my truck pulled harder at full throttle after the reset. If in fact the truck does perform better after the reset, does this mean that the default settings are providing more power than my learned settings? Will my settings eventually adjust back to the "pre-reset" settings anyway? If so, after how many miles will this occur? Thanks in advance for any replies.

My own personal opinion is "YES" the ECU will revert back to its pre-set settings after a week or so of driving. The initial pulling your felt may diminish after some time.
You havent changed anything electrcial about the ECU so it doesnt know about the muffler. The ECU has perameters progammed into it for a specific exh size designed by Toyota, if you changed this things may feel better for a while, but the ECU is going to try to adjust everything back to its Programmed perameters.
Many people have tried adding RESISTORS to the MAF to "TRICK" the ECU into thinking the truck is running cooler than it really is, this is supposed to richen the mixture and produce more power, but from what I have heard and read, the ECU is smarter than this and after a week or so relearns things and makes everything go back to its initial programming and targets.
I think you need to have another ECU that is programmable or adjustable to actually make more HP and TQ.
Toyota did their homework on these trucks and personnally I dont feel there is much more to extract from them. They are great just the way they are.
S/Cing is a whole nother ballgame.
Just my opinion
Others may know more and share with you.

[gnlw]

Thanks again mustang67408. I've also heard about long term maps and short term maps. Can you explain how they work? How long does it really take for the ECU to create a long term map, etc. Thanks in advance for any replies. Any other responses from the other tech experts would also be appreciated.

[mustang67408]

Feedback from the 02 or A/F sensor(s) influence short term fuel trim and then in turn short term fuel trim influences long term fuel trim.
Short term values are not stored in memory and are lost when the ignition is turned off. Long term values are stored in memory, because they are part of the basic injection duration. Long term values affect injection duration in OPEN and CLOSED loop because they are used to calculate basic injection duration
It is important to remember that ACTUAL fuel trim is opposite of the actual DTC code stored.
A system to lean with code of P0171 will mean the ecu is making a RICH correction.

FUEL TRIM:
As things change over time the ECU needs a way to adjust the injection duration and improve drivability and emmisions performance. Fuel Trim is a program in the ECU designed to compensate for these changes.
Fuel trim allows the ECU to LEARN and adjust the injection time quickly back to "NORMAL"
SHORT TERM FUEL: Instant responce and correction
SHort FT is based on the 02/-A/F sensors and therefore only functions in closed loop. SHort FT responds rapidly to changes in the O2-A/F. If Short FT is varying close to 0% little or no correction is needed. When SHORT FT is positive, the ECU has added fuel by increasing injection duration. A negative% means it is subtracting fuel by decreasing duration.
The SHORT FT value is a TEMPORARY value and not stored.
SHORT TERM FT is used to modify LONG TERM FUEL TRIM. When the SHORT FT remiains HIGHER or LOWER than expected, the ECU will add or subtract this from the LONG TERM VALUE.
SHORT TERM FT is a temporary addition or subtraction of fuel to basic injection duration


LONG TERM FUEL:
Long FT is stored in memory because it is part of the basic injection duration calculation. The ECU uses the SHORT FT to modify the LONG FT
Long FT does not react rapidly to sudden changes, it only changes when the ECU decides to use the SHORT FT value to modify it. It is run in open and closed loop and is stored in memory.
A positive % indicates the ECU is adding fuel and a negative% indicates its removing fuel.
LONG TERM FT is part of basic fuel trim and stored in memory as a guideline.

[gnlw]

Thanks again for the info mustang67408.

[mustang67408]

Your Welcome and HAPPY HOLIDAYS