EvoTech

iTrader: (25)

Can anyone chime on the basic concept of long term and short term fuel trim? What values would be considered normal and how do you adjust the fuel trim. Yes I can borrow a OBD II Scanner.

l2r99gst

iTrader: (2)

Long term fuel trims are what are used by the ECU to keep your car at a stoich air:fuel ratio (14.7:1) while idling and cruising, or under low load conditions. The ECU has a certain range of correction, say +/- 12% to adjust the fuel to keep the car at stoich.

For example, DSMLink in my DSM allows you to log fuel trims. There is a LTFT Lo for idle and LTFT Mid for cruise. Basically, you want your fuel trims as close to 0 as possible, meaning the ECU isn't compensating or adjusting either way. If you fuel trim is very positive, like +10%, then the ECU is having to add 10% more fuel than expected to keep a stoich mixture and vice versa for a -10% fuel trim.

On a stock car, you can correct fuel trims by making sure that both your air metering and fuel systems are in good working order and no leaks, such as venting a BOV, etc. In modified cars, with bigger injectors, etc, you can use your engine management to adjust the global percentages for the injector compensation, and if your EMS allows, you can adjust the deadtime of the injectors to get different log term fuel trims in order.

Hope that explains a little. As far as short term, it's just that...another adjustment that the ECU can make over a very short period, basically on top of the long term fuel, but your ltfts are what you need to keep in order. STFT are just there for minor, quick adjustements.

Eric

tlcoll1

iTrader: (14)

What Eric said. But one thing to mention is that it's not super-critical that the trims be at 0% - just within a small few %. The purpose of the fuel trims is to compensate for varying injector flow over the life of the car. So if your injectors are slowly clogging, the stock ECU will use the fuel trim to compensate by turning on the injectors for a slightly longer time. At some point, the injectors will reach a clogged-ness point where the fuel trims can't compensate any more, and your engine won't run at stoich AFR. This'll give you a lousy running engine and poor mileage.

Tom

TurboBob

iTrader: (1)

I'll add a few comments.

Watching the short term trim helps get a tune to run smooth.

Here's my take. The Short Term trim chases the O2 sensor back and forth across 14.7:1. The system is setup to toggle the O2 sensor and you can see the short term trim waggling around to keep the O2 sensor toggling.

If the short term is waggling across 0% then the long terms will not change.

If the short term trim is waggling around +10%, then the long term will slowly decrement to bring the short term back to waggling around 0% again.

Now, in a mitsu there are usually 3 long term trims, you can't tell which one is being used, but one is mostly idle, one is for light throttle cruising, and the last one is moderate accel and higher.

The short term trim and the long term trim are always used to control fueling. When you are 'open loop' the trims are still used in the fuel delivery, but they are frozen (short term at 0%, long term at whatever it was at)

Now, for light throttle driveability tuning, you want to watch your short trim. As you lightly accelerate, watch the STFT as the airflow and rpms climb. if the STFT climbs rapidly, there is a lean spot in your map that the STFT is chasing.

The above are generalities, but a good start in understanding the trims. If the LTFT's are reasonable, and the STFT doesn't have to reach too far, your drivability will be best.

Bob

l2r99gst

iTrader: (2)

Tom and Bob both gave great additional information. Now, you can see why fuel trims are an important tuning tool.

Just to add a little to what Bob said: You can tell what fuel trim is in use depening on what software/hardware you are using to read them. For example, with DSMLink, I could tell exactly what long term fuel trim was being used...ie LTFT Lo, LTFT mid, LTFT Hi. This was good in seeing where the Lo and Mid trims changed over (For tuning purposes, I mainly used Lo and Mid long term and sometimes short term as described by Bob). Depending on your state of tune, your Lo trims may be affected by your mid and vice versa. I don't want to go into detail here, but, nonetheless, it is possible to know which fuel trim is in use by the ECU, which also helps in getting a great tune.

As far as my own experience, I agree with Tom that you don't need your LTFTs at 0. As a matter of fact, they will fluctuate quite a bit during a normal day. I'm not positive about the Evo, but the DSM has a -12.5 to +12.5 range for the long term trims. As long as you are in the adjustment range, you're fine. But, if you are approaching one extreme, there is some sort of air metering/fueling issue that should be dealt with. That is also why it is important to have an EMS/piggyback that can dial in injectors and control dead-time, global, etc.


Eric

muchilus

guys, i'm technician, already bought 4runner 2004 v8 4.7, the vehicle is in great condition. but i have few sympthon related to poor start acceleration and fuel economy. when i start acelerate the engine, from start driving between 1000rpm-2000rpm i feel the engine slappy and poor performance, but when i push hard the pedal the engine respond very well. when i connect my obd2 scanner and start reading the basic values of the injection i have the next values.

Long term fuel 1 (bench1)
-15% to 17.5%
Short term fuel 1
-2.3% - 3.0+ %
also applied the same value range with the bench2, with maybe and little difference of 1-2%. on both section , short and long term.

-throttle position 15.6%
-rpm range 680-800 with not load
-air intake temperature
45c
-air flow
6.5gs
-COOLANT TEMP
85C
this everything on idle.

i have inspect and change. the original oem filter with COLD INTAKE AIR from k&N, engine oil 5w30, spark plug toyota original by Denson iridiums. I clean up the throttle body, the MAF SENSOR. and nothing change it, the value still on the same range. i'm getting the worst MPG 9 miles, when is suppose to give between 13-15 MPG at city.

i have check any air leaks or vacuum leaks on the air intake system. without any good results.

HOW I CAN REDUCE THE LONG FUEL TRIM VALUE CLOSE TO "0"?


help me out!

EVOIXreno

iTrader: (18)

Did you look at the thread date...

muchilus

not really, ,

but the basic is the same. no matter when was. i guess.

despat

Hi, I'm in a similar situ, getting 6 mpliter (we go by liter in canada). My STFT are always around -14 to -27. Plus my car stalls when on idle (only when hot). It starts up again but have to keep the accelerator pressed a bit to keep from stalling.

Thanks