Stft, Ltft, Wtf!?!

[niky]

"140 hp" FSDE here...

Been futzing around with this problem for quite a while... car starts overheating at odd times. Apparently, my car starts running ultra-lean then starts to overheat. So... checked EGR. Ok. Checked MAF... apparently okay. Checked Unichip. Okay. Removed unichip... problem still there... Clamp O2, same... return O2, same... agh.

So... finally got an OBDI scanner that hooks up to the weird box in the Laser/Lynx.

STFT = 50%

LTFT =.... -20% !?!?!

WTF???

I was under the impression that whatever goes into STFT eventually gets copied into LTFT. So... considering the car has been modified with the exhaust, cams and p&p for a few months, if it takes +50% to get it to even run with that... why hasn't that been copied into LTFT?

AgggggghhHH!!!!

 

[Fudgie]

Sounds to me like you have 2 different problems. I would start by verifying that your cooling system is up to par: No leaks (external and/or internal), both fans working properly and make sure the front of your radiator/condenser are not plugged up with s***. Under what circumstances does it begin to overheat? Do the fans come on? Have you verified actual engine temp w/ a scantool? Maybe a raytek on the upper hose? Any trouble codes??

 

[niky]

Haven't tried to pull codes... just found a scanner tool that will fit now (damn obscure OBDI box... grrr...)

Fans okay. They're going full blast when the car starts to edge into high temps. Radiator cap and thermostat replaced. Water pump okay. No leaks anywhere.

Overheating... well, first few times, track runs, then park it to cool it... and it starts overheating at idle. Fixed that when we checked the cams... turns out, exhaust was one tooth off. Have lumpy cams now that probably do something similar to the car, but have had no luck figuring out why. Sometimes it gets hot after a long run followed by idling... sometimes it just gets hot while idling.

Here's a clarification... it gets hot... but it doesn't overheat. Turn the car off (after a few minutes idle) and the coolant starts to boil over. Turn it on again and the temps stay ungodly high even after the fans go into overdrive.

I'm pretty sure the weird lean issue is causing some of the heat. But we're still stumped as to why the fuel trims (which seem to be causing it) are so far off from each other.

 

[niky]

Hmmm... turns out my exhaust flows too well... cold air at the muffler has been making the O2 sensor read strangely compared to other cars... we've got the scanner in and the Unichip and the O2 sensor plugged in midpipe, and it turns out our tuning was off, forcing the car to adapt. *sigh* Gonna try again... hopefully we can get the LPG kit working within the day...

180 bhp? On LPG? I hope!

 

[kura808]

hey niky sorry to thread jack but i saw ur car domain the other day. i must say, ur car looks awesome!! hehe. pretty kool how similar the lynx and the protege are. except urs is a bit better than our proteges

 

[TRXSandRacer02]

well let me answer some things to clear up some obvious confusion. STFT means short term fuel trim which is your upstream oxygen sensor. It it is a positive number it means it is adding fuel and negative means it is dropping fuel ( ___% or - __% ) to the calibration to make it run either more rich or mroe lean. Your LTFT means long term fuel trim. It is the calculated oxygen to fuel ratio (lambda 1:1 ie 14.7:1 oxygen to fuel ratio achevied) that is left after the catalytic. so it woudl seem you are dumping fuel to correct for something and your cat is obviously doing its job for the time being until you dump enough fuel in to destroy it

 

[TRXSandRacer02]

I would say if you do not know enough about fuel and emissions components maybe you shoudnt be adding things to your car that would effect them. Common sense

 

[niky]

well let me answer some things to clear up some obvious confusion. STFT means short term fuel trim which is your upstream oxygen sensor. It it is a positive number it means it is adding fuel and negative means it is dropping fuel ( ___% or - __% ) to the calibration to make it run either more rich or mroe lean. Your LTFT means long term fuel trim. It is the calculated oxygen to fuel ratio (lambda 1:1 ie 14.7:1 oxygen to fuel ratio achevied) that is left after the catalytic. so it woudl seem you are dumping fuel to correct for something and your cat is obviously doing its job for the time being until you dump enough fuel in to destroy it

Don't have a cat. We've finally got a few other FSDEs in the shop and it turns out the LTFT stays pretty constant at -8 to -10%. What was happening was that we unplugged the primary O2, which set LTFT to a constant based on an old set-up, and were tuning AFRs to what the wideband was reading at the tailpipe... which was wrong due to the stupid fast flow of my exhaust. Apparently, it got rich enough due to this to trigger the secondary O2 sensor to cause abberant lean conditions which caused runaway heat build-up. Or something like that.

So the LTFT is attached to the secondary O2, not the primary? Hmmm... will pull plugs later and see what it does. Maybe that's the solution we've been looking for, because we assumed that STFT gets copied into LTFT over time as it seems to do on the Hondas and the secondary O2 sensor was just to detect that the cat was working.

Because, normally, the primary O2 sensor checks the AFRs before the catalytic, assures that it's exactly 14.7 in closed loop (under 3000 rpm) to keep the cats happy and the secondary O2 sensor just checks that the cat is working and triggers a CEL if it doesn't.

---

We know enough about fuel and emissions to modify the car, unfortunately, we don't know enough about this particular car to modify it without running into some glitches.

Add to that fact that the electronics are insanely sensitive to voltage changes... on every single FSDE we plug the Scangauge into, the idle reacts differently. Some idle lower, some idle higher. Same with chipping. Some idle higher, some idle lower, some don't idle. Damn unshielded wiring!

 

[Gmac03]

I searched on LTFT's and there wasn't much on this site for it, but it seemed this thread was the best place to ask the question.

Why do so many people care about the LTFT's in there car? All I hear in the MS3 crowd is my LTFT are high or low or out of parameters. From reading what I can, it doesn't seem like these are an acurate parameter to gauge whether or not your car is running well or not.

Please explain.

Gmac

 

[Gmac03]

Any takers?

Gmac

 

[jamesk]

"140 hp" FSDE here...

Been futzing around with this problem for quite a while... car starts overheating at odd times. Apparently, my car starts running ultra-lean then starts to overheat. So... checked EGR. Ok. Checked MAF... apparently okay. Checked Unichip. Okay. Removed unichip... problem still there... Clamp O2, same... return O2, same... agh.

So... finally got an OBDI scanner that hooks up to the weird box in the Laser/Lynx.

STFT = 50%

LTFT =.... -20% !?!?!

WTF???

I was under the impression that whatever goes into STFT eventually gets copied into LTFT. So... considering the car has been modified with the exhaust, cams and p&p for a few months, if it takes +50% to get it to even run with that... why hasn't that been copied into LTFT?

AgggggghhHH!!!!

Having a ST of 50 and a LT of -20 will give you a total of 30% fuel trim. That means the car is trying to add fuel because the o2 sensors are reading the afr's are too lean. When tuning, you want the short and long fuel trims to add up to zero, so since the the car is at a 30%, you want to add fuel accordingly to make those fuel trims equal zero. each o2 sensor has its own short and long fuel trim. In the protege, the primary sensor does all the work.

Yes Niky, you are correct with the long trims will be copied from the short trims, but that does not happen quickly. It takes the car to be driven almost 100 miles for the car to record the short trims into the long trims. In your case, the car sees a big change in what it was used to, and the short fuel trim is trying to add alot of fuel bringing it up all the way to 50%, while the LTFT is trying to take away fuel from previous recordings. So because of this, your goal as a tuner is to make the o2 sensors to do the smallest amount of work as possible when tuning, so that is why you want both fuel trims to equal to zero. Later on, when both fuel trims are equal to zero, the Short fuel trims will very little work and be close to zero, as the LTFT will adjust and be close to zero as well. But for the time being, you need to add fuel to lower your overall fuel trim to zero.

If you are still confused I will try to explain it another way, but it is much easier to see it happening in real life. It is alot easier to get a better understanding of it.

 

[niky]

Haha... what a grave-dig... car's running fine right now... only issue is the blown 3rd gear... and that job's been done for a week, just don't have time to pick the car up.

---

@Gmac: LTFT is the world. Having LTFT zeroed means you won't have to much about so much when tuning, as jamesk has said.

 

[Gmac03]

http://www.c5forum.com/diy/ltft.php

This is one of the pieces that i've read. Is it accurate?

I'm probably just confusing things, but it seems like it is contradictory to certain things i've read on here.

Gmac