So what is difference on closed loop and open if you enable short term trims.

 

if you have any corrections (aka fuel trims in this case) it is NOT an open loop system.

long trims are there to register the long term changes: wear and tear, loss of compression, that sort of thing. short term trims are to register temporary changes that cannot be accredited, or accounted for with regular sensors, like cold petrol.

 

So why do I have option to turn on short term in the open loop section on hptuners?

 

because they havent labled a lot of things in the software correctly

 

So what is the correct label for this option?

 

it should be called 'short trim-only fuel correction' or something to that effect, and it should not be under open loop, as the definition of open loop is 'no corrections'

 

So still what does it do and what difference from this and closed loop with short trim?

 

If "OL STFT" is enabled, then when not in CL, the PCM uses STFT to trim to the stoich cells in the OLFA table...

the PCM does not trim any cells in the OLFA table that are not stoich.

 

There seems to be a fundamental misunderstanding of open vs. closed loop operation.

Open Loop: The PCM commands fueling based on whatever you told it to do. If there are narrow-band O2 sensors in the exhaust, their input is completely ignored. The phrase "open loop" means there is no feedback loop to alter the fueling...your car could be running rich or lean, but the PCM has no way of knowing that and will just keep on delivering the fuel it was told to deliver (based on the engine RPM and MAP readings).

Closed Loop: The PCM commands fueling based on a look-up table here as well, but the fueling can be trimmed based on readings from the narrow band O2 sensors in the exhaust. If you are running lean in certain areas (RPM vs. MAP), it can command extra fuel, and vice versa. The phrase "closed loop" means that the PCM is receiving feedback from the O2 sensors and will take that information to help it get the fueling back in line. "Back in line" is the key phrase here: since the PCM can only accept narrow band O2 sensor feedback, and narrow band O2 sensors can only tell you whether or not the engine is running at stoich, the trims can only add or subtract fuel to get the engine back to stoichiometric operation. At WOT, you don't want a stoich A/F ratio (since that would be way lean under those operating conditions), so the PCM automatically goes into open loop operation at WOT - it is a known shortcoming of narrow band O2 sensors, but considering how little time over its entire life the engine spends operating at WOT, it was a compromise that was probably pretty easy to make from GM's standpoint.

Short vs. Long term fuel trims: fuel trims fall into two basic categories - short and long term. Both are based on feedback from the O2 sensors. The short term trims handle the moment-to-moment changes that affect fueling; a good example was given above with cold vs. warm gas. The long term trims look at the average of those short term trims over time, and trim the fueling based on historical averages. If the O2 sensors keep telling the PCM that the engine is running too lean in a certain area, it corrects this first by altering the short term trims. After a few mintues, the engine could be running too rich in that same area, so it corrects it again by altering the short term fuel trims.

Now, if the lean condition was caused by something a little more permanent so that the O2's keep reporting a lean condition in that same area over a long period of time, the PCM figures that it must be something that requires a consistent correction, so that becomes a long term fuel trim. How long does it take before a long term fuel trim is set? I have no idea...

I'm pretty sure that if you command open loop operation, the last values used for STFTs and LTFTs are saved, and are still used to correct the fueling until they are reset back to 0. Regardless, the trims are set by narrow band O2 sensors, so if there have been a lot of changes (environmental and/or otherwise) since the O2 sensor feedback started to be ignored, you could be trimming the fueling based on outdated, incorrect information.

I'm a bit rusty on STFTs vs. LTFTs, so if I've stated something incorrectly, please point that out. I haven't run in closed loop operation for close to 5 years at this point.

 

I run in a pseudo Semi Open Loop mode. In that mode when ever you command 14.63:1 the o2 sensors will apply feedback but only in Short Term trims. ie there is no saved data, its only correcting while at that particular 14.63 state. Once you leave 14.63 the trims are forgotten. Then if I go to any other AFR, its full open loop.