STFT and LTFT question

Hi guys, I would like to know where to start learning about STFT and LTFT? I think a proper tool lime Durametric will read the values as FRA and TRA plus have the option the read the voltage values for the O2 sensors. To make a long story short my friend gave me an OBDII reader he doesnt need anymore. The model is an INNOVA 3100 ABS. Not Porsche specific but it help me clearing the P0491 and P0492 I get once in while ( I get it once twice per month maybe, still have to sort this out) My question is about the other value reading Im trying to understand mostly the STFT and LTFT values. I have read some documentation so far like this page: http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info But I would like to know what means the informations Im reading for my application. The reading were taken and data frozen at 3440rpms so this is not live monitoring. STFT Bank 1: -5.5% LTFT Bank 1: -0.8% STFT Bank 2: -2.3% LTFT Bank 2: -3.9% Calculated load: 9.4% ECT: 177.8 DegF First I would like to know if you see something wrong with the readings of the STFT/LTFT. As the reading are not even close Im wondering which one I should rely on. What would be the reason for one bank to be so far appart from the other? Secondly do I need a better tool to diagnose a possible problem? I am inclined towards preventive maintenance so a Durametric could be a wise investment but I also have to buy a portable CPU ect... I have never took this kind of reading on idle at operating temperature but I will start monitoring the mesurements and take notes when I take the car out of storage this spring. If I understand well the mesurements at idle at operating temp. should be close to 1.0% if they are in the minus side the engine run too rich and if its on the plus side engine is running lean. Is there specific tolerances values for the M96 3.6 engine? Just to let know I cleaned the MAF sensor with CRC a couple weeks after the readings were taken but havent had another code issue and I didnt monitor any more infos.

The answers to your questions are kinda complex. simply, no, there is no issue with the data you provided. Typically fuel trims are expected to stay as close to zero as possible. Most car manufacturers put a limit at 10% +/-. If they go beyond that it will set a check engine light (po171 p0174) for rich or lean condition. So, anything within 10% +/- is within specs. You should NEVER expect them to stay at 0 once the vehicle is at operating temps. Imagine it as a delicate balancing act between the 02 sensors, MAF and Fuel and Injector on time to make sure the proper fuel ratio is in the engine. Short term trims are quick changes to instant needs. Long term trims are over a longer time frame. They work together to make everyone happy. For example: If your car was running perfect and you were watching the trims and 02 readings, disconnect a small vacuum line - You see the 02 sensor start reading lean, the "fuel trims" react by adding fuel to compensate for the problem. First the short term would react to correct, then the long term would, as the problem is corrected by the trims you would see the long term stabilize and the short run back to zero. then when the vacuum leak is fixed you would see the opposite. 02 would run rich, short term would pull back fuel, the long term would until everyone is happy. Below is a good video to help explain it better. You Tube

Typically, you would like to see fuel trims + or - 10% in an ideal world. Some manufacturers will allow up to 50% total adaption to base fueling which is an enormous amount of variety. Yours is taking out 6.2-6.3% on both sides. What that means is the base programming in the computer is providing 6% more fuel than the O2 sensors deem that it needs. That's perfectly fine. You also want to make sure bank 1 and bank 2 are close to each other. It doesn't get much closer than what you have. It is impossible to program a computer that is spot on all of the time. There are so many things that can affect fuel trims, O2 sensor age and degradation, amount of ethanol in the fuel, vacuum leaks, MAF sensor condition, fuel pressure drift from spec, base engine health (blowby, oil consumption, etc) that there is no perfect number. What you have there is just fine though. If you start to see an issue where one side is different than the other, it can be time to do some digging to see why that is or if you add STFT and LTFT numbers together and see it is combined to be more than 10-15% variation plus or minus.

well ... you have two types of ltft - idle and part load (full load runs open loop - o2 sensors not working because ecu adds more gas to keep things cool - so ecu can't set trims). not sure how your obd reader works, but a porsche tool (pst2, piwis - both very expensive) calls the ltft for idle 'rkat' and the ltft for part load 'fra'. the acceptable idle range before the ecu throws a code is +/- 4% (where 0% is considered no trims). the acceptable part-load range is 0.7 to 1.32 (where 1 is considered no trims). not sure if durametric uses the same nomenclature. so, assuming the ltft you are reading is part load, and converting your % to actuals, you get bank 1 at 0.992 and band 2 at 0.961 - both well within acceptable limit.

Hi perfect guys really good explanation here. I wasnt too sure where to start from and I think you gave me some good basics here. I will defenately keep an eye on theses values as I want to make sure the O2 sensors are in good conditions and check for faulty injectors, vacuum leak and other problems that may arise. Thanks again everyone Hugo