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Discussion and questions related to the course Practical Reflash Tuning
Im trying to get a better understanding of the WHY behind a couple tables for my GM 3800 v6. I fully get what they do, and how they interact with the base maps, but i dont fully get the WHY and if I should be using/adjusting them.
PE Fuel adder vs RPM vs Time
Why is Stoic set to 14.3:1? (any change to this and STFT+LTFT deviate by the % change)
Lastly on a 100% stock vehicle with greater than 5% fuel trims, do you adjust the maf scale or the injector tables?
First question is, does that PE vs Fuel adder actually do anything? It's possible that the table is populated with values but there is some other function like a temperature condition that turns it off. Try adjusting the whole table up or down and see if it works. And do those negative number make it richer or leaner? With a longer time elapsed you would expect it to go richer to keep parts from getting too hot, but there is also parts protection algorithms (catalyst overheat). The whole table could be mislabeled.
Stoich might be set to 14.3:1 because whatever fuel they were using to calibrate it with was 14.3:1 . For example some certification fuels have stoichiometric ratio around 14.3:1 .
On a 100% stock vehicle IMO you just leave the VE and MAF tables alone.
Yes the time adder appears to add fuel to the commanded AFR. I've had times with enough through to have PE for a while (lugging a hill in a head wind) and the AFR eventually goes from 13:6 down to 10:1 if enough time elapses. This seems excessive. Yes negative numbers add to the current commanded afr. Cat over temp is a separate table.
I have spots in my ltft that exceed ±10%
Ok so the time adder is for steady state condition such as towing to prevent the cat from deteriorating at an accelerated rate. Also, back then the emission regulations were less strict, so the EPA didn't scrutinize dumping in fuel so much. If you change those tables over the life of the vehicle you will get a catalyst efficiency (failed cat) code sooner as the cat breaks down due to heat.
Your LTFT exceeds +/-10% probably due to normal factors associated with mileage and age, such as deteriorating injectors and carbon deposits. You could change the MAF scaling or the VE or even injector settings, but if neither long term nor short term trims are clipping, and the car is completely stock, it's purely a science experiment for you. Stock calibrations are robust and meant to tolerate that kind of deviation without damaging anything and still meeting the basic performance of the vehicle.
There are literally millions and millions of completely stock cars on the road with long term fuel trims exceeding 10%.