What is long term and short term fuel trim? How does it work? Should bank 1 and 2 have around the same percentage? And what are normal levels for LT and ST FTRM? Or if someone could please direct me to the post that already talks about this that I couldn't find that would be much appreciated.

 

Cliffburton99:
As I understand it, the stock BLM max is 160 and stock BLM min is 108 and the PCM tries to do whatever is necessary to get them to 128. The other value can be somewhat higher or lower but supposedly the PCM can only add or subtract a max of 15% of the base (integer) injector pulse width when making corrections. I've seen the other value with a min in the 90s on rare occasion (never seen above >160). So I think 160's and <100 represent the plus or minus 15% max fuel corrections.
Some have said that these max and min values only indicate a problem if they are continuously near the max or min. Over time the PCM should "steer" the corrections towards 128 as it constructs it's own temporary table that is based on the O2 sensor readings.
Hopefully one of the experts will see this thread and post their comments.

 

Think of it this way.. if all the sensors and mechanical parts of the engine are working perfect the O2 sensors would detect a perfect fuel burn and the LT and ST trim (BLM's) would be equal to 128. BUT if something is off a little and the O2's see a rich or lean burn then the PCM steps in and corrects the fuel mixture. EG: If it sees the burn is rich and has to pull fuel to get the correct Air Fuel Ratio, it will indicate this by lowering the BLM values. Contrary if the O2's see a lean mix the PCM will add fuel and signal this by raising the BLM values. NOTE this is only in closed loop operation. In Open loop the PCM remembers the BLM's that were set in open loop, reads the old BLM values and uses then to control the engine till it warms up and the O2's are operational. One thing to keep in mind, even if the trims are a little off of 128, the engine is not really running rich or lean as the PCM is correcting the AFR. The BLM's higher or lower than 128 are only and indication that some sensor or the engine is not running perfect, which is normal operation. However the PCM can only correct for +/- 10-15% fueling errors, if the BLM's start to get get above 140 or below 116 then the PCM will not be able to correct fueling fast enough or at all. Everyone works towards achieving 128 and the feeling is engines run smoother and more efficient when close to 128. Also it's more critical to be close to 128 or lower just prior to entering WOT to assure correct Power Enrichment mixture is not to lean. Some reading here http://para.noid.org/~lj/PCM%20Tutorial/PCMtutorial.htm and scroll down to BLM's here on Solomon's site http://www.lt1pcmtuning.com/tips/

 

First things first, your pcm is set up so that a long term or short term fuel trim of 128 (stoichiometric) is perfect or no adjustment needed.

Now, when an adjustment is needed as determined by the o2 sensor. The first thing that happens is the short term fuel trims adjust very rapidly trying to find the stoichiometric ratio according to the o2 sensor. Now if the short terms have to get out of the range of 124-132 for a long enough period (which is very short in reality!) then the long term trims begin to move in the appropriate direction. The farther from 128 that the short term is the faster the long term fuel trim will change.

Now then lets say for example your o2 was reporting lean and your short term trims went up, then your long terms started to go up too. If things are working correctly, as your long term trim goes up the short term trim will go down and eventually back to near 128 once the long term trim is large enough. So what you end up with is a long term correction and no short term correction.

In other words if your short term correction is 128 or 0% then your long term correction is stable under that operating condition.

In theory both banks should be around the same amount of correction generally speaking. I would say usually they are within +-5% on most cars I have seen that are working properly. You cant trust any data while decelerating or rapid changes of the throttle. Only data from when your foot is on the pedal and steady or idling in gear fully warmed up.

Error can come from many sources: injectors problems, o2 sensors, wiring harness issues, leaky exhaust, worn valves/seats, vacuum leaks, ignition issues, tuning and on and on.

 

I have read two posts started by cliffburton99. He is using the term "long term fuel trim" (LTFT). That expression is only for OBDII cars, as far as I know. I deal with LTFTs on my 1999 Camaro SS. The BLM at 128 being stoic is OBDI stuff. I deal with BLMs on my '87 Camaro IROCZ. Looks like Cliff has 1999 car with an OBDII PCM. Therefore, stoic is 0% for the LTFT and STFT. For a new person to tuning, it must be very confusing when terminology switches back and forth between OBDI and OBDII.

 

LTFT's are used in all years. Earlier models call them "Block Learn Multipliers" (BLM's), but they are the same thing, and based on the same 128 baseline, at least in the OBD-I (93-95) and OBD-II (96/97) LT1 engines. Ditto with the STFT..... earlier years called the "integrators" (INT's)

A +17% LTFT would correspond to a 150 BLM. Maybe the LS1 PCM is different - if it is, let me know.

I suggested in his other thread that he needs to add a signature. Its sort of pointless to spend a lot of time speculating on what he owns.

 

Injuneer: you are correct about having a signature. It would help alot if the person making a post and asking for help would give us as much detail concerning the car as possible, instead of us guessing.

For me, I have an '87 OBDI car, and the terminology has always been BLM for the (and I hate to say it this way) long term fuel trimming which is stored in memory even after the engine is shut off. However, my 1999 sites and books have always used the phrase "long term fuel trim" for just that, OBDII stuff. I got plenty of books on this topic. I am simply following the namimg convention of others. And it could be that between the years of my experience the terminology was mixed. But I have always thought that there was a clear distinction between the OBDI and OBDII terminology.
Anyway nuff said on the topic.

 

I'm not arguing with you.... just pointing out that it is the same thing, whether you call it BLM, or LTFT. Its still based on 128 being the divisor, and the BLM (or LTFT) being the dividend in the injector pulse width equation. The fact that he cited a percentage indicates he's working in OBD-II. But a scan in OBD-II will pull up the fact that his LTFT is 150 when expressed as the number used in the pulse width equation.