well, ive been going through my truck testing and inspecting sensors for the last month now, trying to figure out why this thing runs like dog when its at operating temperature. I was looking at short term and long term fuel trims and my question is, the scan tool (snap-on modis ) says my Long term fuel trim is -12 to -16, why would it want to take away that much fuel when the short term is normal, +3 to -2? I dont know if this will make any sense to anyone....

 

Well with colder temps, the air is more dense and the fuel strategies are changed due to this. Ford considers the normal long term -10 to +10, with yours being a little higher, I would reccommend cleaning the MAF. As far as the short term goes, It will vary continuosly depending on throttle application. It does very little to help diagnosing. The long term finds a level where it can keep short term the closest to 0, if you have been doing alot of WOT runs, like at a drag strip, the long terms will be out of range due to the high demand of fuel. But take alook at your BARO reading, give that to me(will be in HZ) and your relative altitude and I can tell you if the MAF is contaminated.

 

If the MAF is contaminated, it's easy enough to clean. I don't even remove it from the duct, I hit it with a few shots of MAF cleaner and let it air dry for 15 minutes.

 

First off, short term fuel trim is very helpful for diagnosing fuel control issues and tells what the computer is diong now, Long term tells where it averages at over the "long term". They will not be "out of range" because of WOT runs, rather, shows where it averages.


97f150kid, what is you problem? Codes? When you say runs bad, what exactly is it doing?

 

the truck is guzzling fuel, thats my problem, and when i change something out to see if it does anything ( fuel filter, plugs, new coils, coil boots, have new upstream o2s just have to put them in, cold air, cleaned IAC, cleaned MAF and IAT ) the mileage just gets worse. Plus every now and then when i go to hit the gas of a stop sign it will want to kill, ill put my foot into it more and the RPMS will drop below 500, then all of a sudden it will spring back to life and act as nothing ever happened, its done that twice in the last 2 months. When i bought the truck, it didnt have any of the previously started things changed, and it ran like ***, now, i changed all those things out, and im getting 8 if im lucky, all city, no highway at all. im hoping that the o2s are to blame ( 136K originals ) but im not sure. Also, when the computer is in closed loop, it runs awesome! its an animal, but the second it goes to open loop, it runs like ***. have to put your foot into it a little more to go anywhere, compared to when its cold.

 

Well then I guess FOMOCO is teaching their techs wrong. The STFT's tell what the PCM is doing at the that certain time, but the PCM is changing the STFT constantly, and if most scan tools don't have a fast enough data rate to monitor this, just like O2's. A +3 and a -2 doesn't tell me anything, but that the PCM is satisfied with the current LTFT's. The STFT's are used to keep the A/F around stoich, once the PCM see's the O2's switching again it changes the LTFT. It doesn't take a long time for LTFT's to adjust, I have seen it change every 25-30secs. The whole purpose of STFT is to keep the O2 switching, and with the fast rate of newer adaptive fuel strategies, the STFT is looked at very little. So by his LTFT's being -16 & -12, they are out of "range", in the sense that the "range" being -10 to +10, which is what Ford considers normal.

 

joshmac4.6, then what do u think i should look at for what i said in the previous post?

 

Well the PCM only uses the O2's in closed loop, so If it runs best then, they are not likely the problem. You say it runs good in CL, but how does it run when at WOT, b/c the PCM commands OL-drive then ignores the O2's again?

 

when im driving and i go WOT, it kicks down, the rpms rev up for a second, back off , then it starts pullin, shifts into the next gear and slowly pulls thru that one.

 

Does it feel normal, does it pull and accelerate normally, or does it feel like it's being held back at WOT.

 

it feels held back, but i dont hold it back

 

Do you still have that modis? If so you could do a recording while driving down the road, start while cruising and then kick it, this will help because I can see the what is happening at the time it goes into OL.

 

I dont have access to it for another week or so.

"can see the what is happening at the time it goes into OL" < how am i suppose to send the recording to you?

 

Don't have to, just scroll through and record certain pid values at certain times, and post 'em.

 

If You are a FoMoCo tech then you misunderstood what they are trying to teach you.